Improve internal server connection management. (#1232)

Build/MQTTnet.nuspec

@@ -13,6 +13,10 @@
     <description>MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker) and supports v3.1.0, v3.1.1 and v5.0.0 of the MQTT protocol.</description>
     <releaseNotes>
 * [ManagedClient] Extended ReconnectAsync (thanks to @nvsnkv, #1202). 
+* [Server] Fixed a memory/performance leak when using QoS Level 1.
+* [Server] Exposed connection timestamp in client status. 
+* [Server] Refactored connection management code. 
+* [Server] Exposed more details in _MqttServerClientConnectedEventArgs_.
 * [MQTTnet.Server] Moved server project to a dedicated GitHub repository.
 * [MQTTnet, MQTTnet.Extensions.ManagedClient] Fixed bug that allowed invalid subscriptions (Thanks to @marcelwinh).
 Git commit: $gitCommit

Source/MQTTnet/Client/MqttClient.cs

@@ -363,9 +363,9 @@ namespace MQTTnet.Client
 
                 await Task.WhenAll(receiverTask, publishPacketReceiverTask, keepAliveTask).ConfigureAwait(false);
             }
-            catch (Exception e)
+            catch (Exception innerException)
             {
-                _logger.Warning(e, "Error while waiting for internal tasks.");
+                _logger.Warning(innerException, "Error while waiting for internal tasks.");
             }
             finally
             {
@@ -439,7 +439,7 @@ namespace MQTTnet.Client
                 {
                     if (exception is MqttCommunicationTimedOutException)
                     {
-                        _logger.Warning(null, "Timeout while waiting for response packet ({0}).", typeof(TResponsePacket).Name);
+                        _logger.Warning("Timeout while waiting for response packet ({0}).", typeof(TResponsePacket).Name);
                     }
 
                     throw;

Source/MQTTnet/Client/Options/MqttClientOptionsBuilder.cs

@@ -133,7 +133,7 @@ namespace MQTTnet.Client.Options
             return this;
         }
 
-        public MqttClientOptionsBuilder WithCredentials(string username, string password = null)
+        public MqttClientOptionsBuilder WithCredentials(string username, string password)
         {
             byte[] passwordBuffer = null;
 
@@ -145,7 +145,7 @@ namespace MQTTnet.Client.Options
             return WithCredentials(username, passwordBuffer);
         }
 
-        public MqttClientOptionsBuilder WithCredentials(string username, byte[] password = null)
+        public MqttClientOptionsBuilder WithCredentials(string username, byte[] password)
         {
             _options.Credentials = new MqttClientCredentials
             {
@@ -155,6 +155,16 @@ namespace MQTTnet.Client.Options
 
             return this;
         }
+        
+        public MqttClientOptionsBuilder WithCredentials(string username)
+        {
+            _options.Credentials = new MqttClientCredentials
+            {
+                Username = username
+            };
+
+            return this;
+        }
 
         public MqttClientOptionsBuilder WithCredentials(IMqttClientCredentials credentials)
         {

Source/MQTTnet/Client/Receiving/MqttApplicationMessageReceivedHandlerDelegate.cs

@@ -1,11 +1,12 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Client.Receiving
 {
-    public class MqttApplicationMessageReceivedHandlerDelegate : IMqttApplicationMessageReceivedHandler
+    public sealed class MqttApplicationMessageReceivedHandlerDelegate : IMqttApplicationMessageReceivedHandler
     {
-        private readonly Func<MqttApplicationMessageReceivedEventArgs, Task> _handler;
+        readonly Func<MqttApplicationMessageReceivedEventArgs, Task> _handler;
 
         public MqttApplicationMessageReceivedHandlerDelegate(Action<MqttApplicationMessageReceivedEventArgs> handler)
         {
@@ -14,7 +15,7 @@ namespace MQTTnet.Client.Receiving
             _handler = context =>
             {
                 handler(context);
-                return Task.FromResult(0);
+                return PlatformAbstractionLayer.CompletedTask;
             };
         }
 

Source/MQTTnet/Formatter/IMqttDataConverter.cs

@@ -8,7 +8,6 @@ using MQTTnet.Client.Unsubscribing;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
-using MqttClientSubscribeResult = MQTTnet.Client.Subscribing.MqttClientSubscribeResult;
 
 namespace MQTTnet.Formatter
 {
@@ -22,7 +21,7 @@ namespace MQTTnet.Formatter
 
         MqttClientPublishResult CreateClientPublishResult(MqttPubRecPacket pubRecPacket, MqttPubCompPacket pubCompPacket);
         
-        MqttClientSubscribeResult CreateClientSubscribeResult(MqttSubscribePacket subscribePacket, MqttSubAckPacket subAckPacket);
+        Client.Subscribing.MqttClientSubscribeResult CreateClientSubscribeResult(MqttSubscribePacket subscribePacket, MqttSubAckPacket subAckPacket);
         
         MqttClientUnsubscribeResult CreateClientUnsubscribeResult(MqttUnsubscribePacket unsubscribePacket, MqttUnsubAckPacket unsubAckPacket);
 

Source/MQTTnet/MqttTopicFilter.cs

@@ -8,6 +8,7 @@ namespace MQTTnet
     {
     }
 
+    // TODO: Consider using struct instead.
     public class MqttTopicFilter
     {
         /// <summary>

Source/MQTTnet/Protocol/MqttRetainHandling.cs

@@ -3,7 +3,9 @@
     public enum MqttRetainHandling
     {
         SendAtSubscribe = 0,
+        
         SendAtSubscribeIfNewSubscriptionOnly = 1,
+        
         DoNotSendOnSubscribe = 2
     }
 }

Source/MQTTnet/Server/Internal/CheckSubscriptionsResult.cs

@@ -1,6 +1,6 @@
 using MQTTnet.Protocol;
 
-namespace MQTTnet.Server
+namespace MQTTnet.Server.Internal
 {
     public struct CheckSubscriptionsResult
     {

Source/MQTTnet/Server/Internal/MqttClientConnection.cs

@@ -1,94 +1,82 @@
-using MQTTnet.Adapter;
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using MQTTnet.Adapter;
 using MQTTnet.Client;
+using MQTTnet.Client.Disconnecting;
 using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
-using MQTTnet.Formatter;
 using MQTTnet.Implementations;
 using MQTTnet.Internal;
 using MQTTnet.PacketDispatcher;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server.Status;
-using System;
-using System.Collections.Generic;
-using System.Threading;
-using System.Threading.Tasks;
-using MQTTnet.Client.Disconnecting;
 
-namespace MQTTnet.Server
+namespace MQTTnet.Server.Internal
 {
     public sealed class MqttClientConnection : IDisposable
     {
         readonly Dictionary<ushort, string> _topicAlias = new Dictionary<ushort, string>();
         readonly MqttPacketIdentifierProvider _packetIdentifierProvider = new MqttPacketIdentifierProvider();
         readonly MqttPacketDispatcher _packetDispatcher = new MqttPacketDispatcher();
-        readonly CancellationTokenSource _cancellationToken = new CancellationTokenSource();
+        
-
         readonly IMqttRetainedMessagesManager _retainedMessagesManager;
         readonly MqttClientSessionsManager _sessionsManager;
 
+        readonly IMqttChannelAdapter _channelAdapter;
         readonly IMqttNetScopedLogger _logger;
         readonly IMqttServerOptions _serverOptions;
 
-        readonly IMqttChannelAdapter _channelAdapter;
-        readonly MqttConnectionValidatorContext _connectionValidatorContext;
-        readonly IMqttDataConverter _dataConverter;
         readonly string _endpoint;
-        readonly DateTime _connectedTimestamp;
 
-        volatile Task _packageReceiverTask;
+        CancellationTokenSource _cancellationToken;
-        DateTime _lastNonKeepAlivePacketReceivedTimestamp;
 
-        long _receivedPacketsCount;
+        public MqttClientConnection(
-        long _sentPacketsCount = 1; // Start with 1 because the CONNECT packet is not counted anywhere.
+            MqttConnectPacket connectPacket,
-        long _receivedApplicationMessagesCount;
-        long _sentApplicationMessagesCount;
-        MqttClientDisconnectReason _disconnectReason;
-
-        public MqttClientConnection(MqttConnectPacket connectPacket,
             IMqttChannelAdapter channelAdapter,
             MqttClientSession session,
-            MqttConnectionValidatorContext connectionValidatorContext,
             IMqttServerOptions serverOptions,
             MqttClientSessionsManager sessionsManager,
             IMqttRetainedMessagesManager retainedMessagesManager,
             IMqttNetLogger logger)
         {
-            Session = session ?? throw new ArgumentNullException(nameof(session));
             _serverOptions = serverOptions ?? throw new ArgumentNullException(nameof(serverOptions));
             _sessionsManager = sessionsManager ?? throw new ArgumentNullException(nameof(sessionsManager));
             _retainedMessagesManager = retainedMessagesManager ?? throw new ArgumentNullException(nameof(retainedMessagesManager));
 
             _channelAdapter = channelAdapter ?? throw new ArgumentNullException(nameof(channelAdapter));
-            _connectionValidatorContext = connectionValidatorContext ?? throw new ArgumentNullException(nameof(connectionValidatorContext));
+            _endpoint = channelAdapter.Endpoint;
-            _dataConverter = _channelAdapter.PacketFormatterAdapter.DataConverter;
+            
-            _endpoint = _channelAdapter.Endpoint;
+            Session = session ?? throw new ArgumentNullException(nameof(session));
             ConnectPacket = connectPacket ?? throw new ArgumentNullException(nameof(connectPacket));
 
             if (logger == null) throw new ArgumentNullException(nameof(logger));
             _logger = logger.CreateScopedLogger(nameof(MqttClientConnection));
-
-            _connectedTimestamp = DateTime.UtcNow;
-            LastPacketReceivedTimestamp = _connectedTimestamp;
-            _lastNonKeepAlivePacketReceivedTimestamp = LastPacketReceivedTimestamp;
         }
 
-        public MqttClientConnectionStatus Status { get; private set; } = MqttClientConnectionStatus.Initializing;
-
-        public MqttConnectPacket ConnectPacket { get; }
-
         public string ClientId => ConnectPacket.ClientId;
 
-        public bool IsReadingPacket => _channelAdapter.IsReadingPacket;
+        public string Endpoint => _endpoint;
 
-        public DateTime LastPacketReceivedTimestamp { get; private set; }
+        public MqttClientConnectionStatistics Statistics { get; } = new MqttClientConnectionStatistics();
+
+        public bool IsRunning { get; private set; }
+
+        public MqttConnectPacket ConnectPacket { get; }
+
+        public bool IsReadingPacket => _channelAdapter.IsReadingPacket;
 
         public MqttClientSession Session { get; }
 
+        public bool IsTakenOver { get; set; }
+
+        public bool IsCleanDisconnect { get; private set; }
+
         public async Task StopAsync(MqttClientDisconnectReason reason)
         {
-            Status = MqttClientConnectionStatus.Finalizing;
+            IsRunning = false;
-            _disconnectReason = reason;
 
             if (reason == MqttClientDisconnectReason.SessionTakenOver || reason == MqttClientDisconnectReason.KeepAliveTimeout)
             {
@@ -96,30 +84,10 @@ namespace MQTTnet.Server
                 // token because the entire connection is closed (disposed) as soon as the cancellation
                 // token is cancelled. To there is no chance that the DISCONNECT packet will ever arrive
                 // at the client!
-                try
+                await TrySendDisconnectPacket(reason).ConfigureAwait(false);
-                {
-                    var disconnectOptions = new MqttClientDisconnectOptions
-                    {
-                        ReasonCode = reason,
-                        ReasonString = reason.ToString()
-                    };
-                    
-                    var disconnectPacket = _channelAdapter.PacketFormatterAdapter.DataConverter.CreateDisconnectPacket(disconnectOptions);
-
-                    using (var timeout = new CancellationTokenSource(_serverOptions.DefaultCommunicationTimeout))
-                    {
-                        await _channelAdapter.SendPacketAsync(disconnectPacket, timeout.Token).ConfigureAwait(false);
-                    }
-                }
-                catch (Exception exception)
-                {
-                    _logger.Warning(exception, "Client '{0}': Error while sending DISCONNECT packet after takeover.", ClientId);
-                }
             }
 
             StopInternal();
-
-            await (_packageReceiverTask ?? PlatformAbstractionLayer.CompletedTask);
         }
 
         public void ResetStatistics()
@@ -127,24 +95,16 @@ namespace MQTTnet.Server
             _channelAdapter.ResetStatistics();
         }
 
-        public void FillStatus(MqttClientStatus status)
+        public void FillClientStatus(MqttClientStatus clientStatus)
         {
-            status.ClientId = ClientId;
+            clientStatus.ClientId = ClientId;
-            status.Endpoint = _endpoint;
+            clientStatus.Endpoint = _endpoint;
-            status.ProtocolVersion = _channelAdapter.PacketFormatterAdapter.ProtocolVersion;
 
-            status.ReceivedApplicationMessagesCount = Interlocked.Read(ref _receivedApplicationMessagesCount);
+            clientStatus.ProtocolVersion = _channelAdapter.PacketFormatterAdapter.ProtocolVersion;
-            status.SentApplicationMessagesCount = Interlocked.Read(ref _sentApplicationMessagesCount);
+            clientStatus.BytesSent = _channelAdapter.BytesSent;
+            clientStatus.BytesReceived = _channelAdapter.BytesReceived;
 
-            status.ReceivedPacketsCount = Interlocked.Read(ref _receivedPacketsCount);
+            Statistics.FillClientStatus(clientStatus);
-            status.SentPacketsCount = Interlocked.Read(ref _sentPacketsCount);
-
-            status.ConnectedTimestamp = _connectedTimestamp;
-            status.LastPacketReceivedTimestamp = LastPacketReceivedTimestamp;
-            status.LastNonKeepAlivePacketReceivedTimestamp = _lastNonKeepAlivePacketReceivedTimestamp;
-
-            status.BytesSent = _channelAdapter.BytesSent;
-            status.BytesReceived = _channelAdapter.BytesReceived;
         }
 
         public void Dispose()
@@ -152,31 +112,60 @@ namespace MQTTnet.Server
             _cancellationToken.Dispose();
         }
 
-        public Task RunAsync()
+        public async Task RunAsync()
         {
-            _packageReceiverTask = RunInternalAsync(_cancellationToken.Token);
+            _logger.Info("Client '{0}': Session started.", ClientId);
-            return _packageReceiverTask;
+
+            Session.WillMessage = ConnectPacket.WillMessage;
+            
+            using (var cancellationToken = new CancellationTokenSource())
+            {
+                _cancellationToken = cancellationToken;
+
+                try
+                {
+                    Task.Run(() => SendPacketsLoop(cancellationToken.Token), cancellationToken.Token).RunInBackground(_logger);
+
+                    Session.IsCleanSession = false;
+
+                    IsRunning = true;
+
+                    await ReceivePackagesLoop(cancellationToken.Token).ConfigureAwait(false);
+                }
+                finally
+                {
+                    IsRunning = false;
+                    
+                    cancellationToken.Cancel();
+                    _cancellationToken = null;
+                }
+            }
+        
+            _packetDispatcher.CancelAll();
+
+            if (!IsTakenOver && !IsCleanDisconnect && Session.WillMessage != null)
+            {
+                _sessionsManager.DispatchApplicationMessage(Session.WillMessage, this);
+                Session.WillMessage = null;
+            }
+
+            _logger.Info("Client '{0}': Connection stopped.", ClientId);
         }
 
-        async Task RunInternalAsync(CancellationToken cancellationToken)
+        Task SendPacketAsync(MqttBasePacket packet, CancellationToken cancellationToken)
+        {
+            return _channelAdapter.SendPacketAsync(packet, cancellationToken).ContinueWith(task => { Statistics.HandleSentPacket(packet); }, cancellationToken);
+        }
+
+        async Task ReceivePackagesLoop(CancellationToken cancellationToken)
         {
-            var disconnectType = MqttClientDisconnectType.NotClean;
             try
             {
-                _logger.Info("Client '{0}': Session started.", ClientId);
+                // We do not listen for the cancellation token here because the internal buffer might still
-
+                // contain data to be read even if the TCP connection was already dropped. So we rely on an
-                Session.WillMessage = ConnectPacket.WillMessage;
+                // own exception in the reading loop!
-
-                await SendAsync(_channelAdapter.PacketFormatterAdapter.DataConverter.CreateConnAckPacket(_connectionValidatorContext), cancellationToken).ConfigureAwait(false);
-
-                Task.Run(() => SendPendingPacketsAsync(cancellationToken), cancellationToken).RunInBackground(_logger);
-
-                Session.IsCleanSession = false;
-
                 while (!cancellationToken.IsCancellationRequested)
                 {
-                    Status = MqttClientConnectionStatus.Running;
-
                     var packet = await _channelAdapter.ReceivePacketAsync(cancellationToken).ConfigureAwait(false);
                     if (packet == null)
                     {
@@ -184,13 +173,7 @@ namespace MQTTnet.Server
                         return;
                     }
 
-                    Interlocked.Increment(ref _sentPacketsCount);
+                    Statistics.HandleReceivedPacket(packet);
-                    LastPacketReceivedTimestamp = DateTime.UtcNow;
-
-                    if (!(packet is MqttPingReqPacket || packet is MqttPingRespPacket))
-                    {
-                        _lastNonKeepAlivePacketReceivedTimestamp = LastPacketReceivedTimestamp;
-                    }
 
                     if (packet is MqttPublishPacket publishPacket)
                     {
@@ -210,7 +193,8 @@ namespace MQTTnet.Server
                     }
                     else if (packet is MqttPingReqPacket)
                     {
-                        await SendAsync(MqttPingRespPacket.Instance, cancellationToken).ConfigureAwait(false);
+                        // See: The Server MUST send a PINGRESP packet in response to a PINGREQ packet [MQTT-3.12.4-1].
+                        await SendPacketAsync(MqttPingRespPacket.Instance, cancellationToken).ConfigureAwait(false);
                     }
                     else if (packet is MqttPingRespPacket)
                     {
@@ -218,10 +202,7 @@ namespace MQTTnet.Server
                     }
                     else if (packet is MqttDisconnectPacket)
                     {
-                        Session.WillMessage = null;
+                        IsCleanDisconnect = true;
-                        disconnectType = MqttClientDisconnectType.Clean;
-
-                        StopInternal();
                         return;
                     }
                     else
@@ -246,40 +227,111 @@ namespace MQTTnet.Server
                 {
                     _logger.Error(exception, "Client '{0}': Error while receiving client packets.", ClientId);
                 }
+            }
+        }
+
+        async Task SendPacketsLoop(CancellationToken cancellationToken)
+        {
+            MqttQueuedApplicationMessage queuedApplicationMessage = null;
+            MqttPublishPacket publishPacket = null;
+
+            try
+            {
+                while (!cancellationToken.IsCancellationRequested)
+                {
+                    queuedApplicationMessage = await Session.ApplicationMessagesQueue.DequeueAsync(cancellationToken).ConfigureAwait(false);
+                    if (queuedApplicationMessage == null)
+                    {
+                        return;
+                    }
+
+                    if (cancellationToken.IsCancellationRequested)
+                    {
+                        return;
+                    }
+
+                    publishPacket = _channelAdapter.PacketFormatterAdapter.DataConverter.CreatePublishPacket(queuedApplicationMessage.ApplicationMessage);
+                    publishPacket.QualityOfServiceLevel = queuedApplicationMessage.SubscriptionQualityOfServiceLevel;
+
+                    // Set the retain flag to true according to [MQTT-3.3.1-8] and [MQTT-3.3.1-9].
+                    publishPacket.Retain = queuedApplicationMessage.IsRetainedMessage;
+
+                    publishPacket = await InvokeClientMessageQueueInterceptor(publishPacket, queuedApplicationMessage).ConfigureAwait(false);
+                    if (publishPacket == null)
+                    {
+                        // The interceptor has decided that the message is not relevant and will be fully ignored.
+                        continue;
+                    }
+
+                    if (publishPacket.QualityOfServiceLevel > 0)
+                    {
+                        publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
+                    }
+
+                    if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtMostOnce)
+                    {
+                        await SendPacketAsync(publishPacket, cancellationToken).ConfigureAwait(false);
+                    }
+                    else if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtLeastOnce)
+                    {
+                        using (var awaiter = _packetDispatcher.AddAwaiter<MqttPubAckPacket>(publishPacket.PacketIdentifier))
+                        {
+                            await SendPacketAsync(publishPacket, cancellationToken).ConfigureAwait(false);
+                            await awaiter.WaitOneAsync(_serverOptions.DefaultCommunicationTimeout).ConfigureAwait(false);
+                        }
+                    }
+                    else if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.ExactlyOnce)
+                    {
+                        using (var awaiter1 = _packetDispatcher.AddAwaiter<MqttPubRecPacket>(publishPacket.PacketIdentifier))
+                        using (var awaiter2 = _packetDispatcher.AddAwaiter<MqttPubCompPacket>(publishPacket.PacketIdentifier))
+                        {
+                            await SendPacketAsync(publishPacket, cancellationToken).ConfigureAwait(false);
+                            var pubRecPacket = await awaiter1.WaitOneAsync(_serverOptions.DefaultCommunicationTimeout).ConfigureAwait(false);
+
+                            var pubRelPacket = _channelAdapter.PacketFormatterAdapter.DataConverter.CreatePubRelPacket(pubRecPacket, MqttApplicationMessageReceivedReasonCode.Success);
+                            await SendPacketAsync(pubRelPacket, cancellationToken).ConfigureAwait(false);
+
+                            await awaiter2.WaitOneAsync(_serverOptions.DefaultCommunicationTimeout).ConfigureAwait(false);
+                        }
+                    }
+
+                    _logger.Verbose("Client '{0}': Queued application message sent.", ClientId);
+                }
+            }
+            catch (OperationCanceledException)
+            {
+            }
+            catch (Exception exception)
+            {
+                if (exception is MqttCommunicationTimedOutException)
+                {
+                    _logger.Warning(exception, "Client '{0}': Sending publish packet failed: Timeout.", ClientId);
+                }
+                else if (exception is MqttCommunicationException)
+                {
+                    _logger.Warning(exception, "Client '{0}': Sending publish packet failed: Communication exception.", ClientId);
+                }
+                else
+                {
+                    _logger.Error(exception, "Client '{0}': Sending publish packet failed.", ClientId);
+                }
+
+                if (publishPacket?.QualityOfServiceLevel > MqttQualityOfServiceLevel.AtMostOnce)
+                {
+                    if (queuedApplicationMessage != null)
+                    {
+                        queuedApplicationMessage.IsDuplicate = true;
+                        Session.ApplicationMessagesQueue.Enqueue(queuedApplicationMessage);
+                    }
+                }
 
                 StopInternal();
             }
-            finally
-            {
-                if (_disconnectReason == MqttClientDisconnectReason.SessionTakenOver)
-                {
-                    disconnectType = MqttClientDisconnectType.Takeover;
-                }
-
-                if (Session.WillMessage != null)
-                {
-                    _sessionsManager.DispatchApplicationMessage(Session.WillMessage, this);
-                    Session.WillMessage = null;
-                }
-
-                _packetDispatcher.CancelAll();
-
-                _logger.Info("Client '{0}': Connection stopped.", ClientId);
-
-                try
-                {
-                    await _sessionsManager.CleanUpClient(ClientId, _channelAdapter, disconnectType);
-                }
-                catch (Exception e)
-                {
-                    _logger.Error(e, "Client '{0}': Error while cleaning up", ClientId);
-                }
-            }
         }
 
         void StopInternal()
         {
-            _cancellationToken.Cancel();
+            _cancellationToken?.Cancel();
         }
 
         async Task EnqueueSubscribedRetainedMessagesAsync(ICollection<MqttTopicFilter> topicFilters)
@@ -294,7 +346,7 @@ namespace MQTTnet.Server
         Task HandleIncomingPubRelPacketAsync(MqttPubRelPacket pubRelPacket, CancellationToken cancellationToken)
         {
             var pubCompPacket = _channelAdapter.PacketFormatterAdapter.DataConverter.CreatePubCompPacket(pubRelPacket, MqttApplicationMessageReceivedReasonCode.Success);
-            return SendAsync(pubCompPacket, cancellationToken);
+            return SendPacketAsync(pubCompPacket, cancellationToken);
         }
 
         async Task HandleIncomingSubscribePacketAsync(MqttSubscribePacket subscribePacket, CancellationToken cancellationToken)
@@ -302,7 +354,7 @@ namespace MQTTnet.Server
             var subscribeResult = await Session.SubscriptionsManager.SubscribeAsync(subscribePacket, ConnectPacket).ConfigureAwait(false);
             var subAckPacket = _channelAdapter.PacketFormatterAdapter.DataConverter.CreateSubAckPacket(subscribePacket, subscribeResult);
 
-            await SendAsync(subAckPacket, cancellationToken).ConfigureAwait(false);
+            await SendPacketAsync(subAckPacket, cancellationToken).ConfigureAwait(false);
 
             if (subscribeResult.CloseConnection)
             {
@@ -318,38 +370,36 @@ namespace MQTTnet.Server
             var reasonCodes = await Session.SubscriptionsManager.UnsubscribeAsync(unsubscribePacket).ConfigureAwait(false);
             var unsubAckPacket = _channelAdapter.PacketFormatterAdapter.DataConverter.CreateUnsubAckPacket(unsubscribePacket, reasonCodes);
 
-            await SendAsync(unsubAckPacket, cancellationToken).ConfigureAwait(false);
+            await SendPacketAsync(unsubAckPacket, cancellationToken).ConfigureAwait(false);
         }
 
         Task HandleIncomingPublishPacketAsync(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
         {
-            Interlocked.Increment(ref _sentApplicationMessagesCount);
-
             HandleTopicAlias(publishPacket);
 
-            var applicationMessage = _dataConverter.CreateApplicationMessage(publishPacket);
+            var applicationMessage = _channelAdapter.PacketFormatterAdapter.DataConverter.CreateApplicationMessage(publishPacket);
             _sessionsManager.DispatchApplicationMessage(applicationMessage, this);
 
             switch (publishPacket.QualityOfServiceLevel)
             {
                 case MqttQualityOfServiceLevel.AtMostOnce:
-                    {
+                {
-                        return PlatformAbstractionLayer.CompletedTask;
+                    return PlatformAbstractionLayer.CompletedTask;
-                    }
+                }
                 case MqttQualityOfServiceLevel.AtLeastOnce:
-                    {
+                {
-                        var pubAckPacket = _dataConverter.CreatePubAckPacket(publishPacket, MqttApplicationMessageReceivedReasonCode.Success);
+                    var pubAckPacket = _channelAdapter.PacketFormatterAdapter.DataConverter.CreatePubAckPacket(publishPacket, MqttApplicationMessageReceivedReasonCode.Success);
-                        return SendAsync(pubAckPacket, cancellationToken);
+                    return SendPacketAsync(pubAckPacket, cancellationToken);
-                    }
+                }
                 case MqttQualityOfServiceLevel.ExactlyOnce:
-                    {
+                {
-                        var pubRecPacket = _dataConverter.CreatePubRecPacket(publishPacket, MqttApplicationMessageReceivedReasonCode.Success);
+                    var pubRecPacket = _channelAdapter.PacketFormatterAdapter.DataConverter.CreatePubRecPacket(publishPacket, MqttApplicationMessageReceivedReasonCode.Success);
-                        return SendAsync(pubRecPacket, cancellationToken);
+                    return SendPacketAsync(pubRecPacket, cancellationToken);
-                    }
+                }
                 default:
-                    {
+                {
-                        throw new MqttCommunicationException("Received a not supported QoS level.");
+                    throw new MqttCommunicationException("Received a not supported QoS level.");
-                    }
+                }
             }
         }
 
@@ -376,134 +426,64 @@ namespace MQTTnet.Server
                     }
                     else
                     {
-
                     }
                 }
             }
         }
-
+        
-        async Task SendPendingPacketsAsync(CancellationToken cancellationToken)
+        async Task TrySendDisconnectPacket(MqttClientDisconnectReason reason)
         {
-            MqttQueuedApplicationMessage queuedApplicationMessage = null;
-            MqttPublishPacket publishPacket = null;
-
             try
             {
-                while (!cancellationToken.IsCancellationRequested)
+                var disconnectOptions = new MqttClientDisconnectOptions
                 {
-                    queuedApplicationMessage = await Session.ApplicationMessagesQueue.DequeueAsync(cancellationToken).ConfigureAwait(false);
+                    ReasonCode = reason,
-                    if (queuedApplicationMessage == null)
+                    ReasonString = reason.ToString()
-                    {
+                };
-                        return;
-                    }
 
-                    if (cancellationToken.IsCancellationRequested)
+                var disconnectPacket = _channelAdapter.PacketFormatterAdapter.DataConverter.CreateDisconnectPacket(disconnectOptions);
-                    {
-                        return;
-                    }
 
-                    publishPacket = _dataConverter.CreatePublishPacket(queuedApplicationMessage.ApplicationMessage);
+                using (var timeout = new CancellationTokenSource(_serverOptions.DefaultCommunicationTimeout))
-                    publishPacket.QualityOfServiceLevel = queuedApplicationMessage.SubscriptionQualityOfServiceLevel;
+                {
-
+                    await SendPacketAsync(disconnectPacket, timeout.Token).ConfigureAwait(false);
-                    // Set the retain flag to true according to [MQTT-3.3.1-8] and [MQTT-3.3.1-9].
-                    publishPacket.Retain = queuedApplicationMessage.IsRetainedMessage;
-
-                    if (_serverOptions.ClientMessageQueueInterceptor != null)
-                    {
-                        var context = new MqttClientMessageQueueInterceptorContext(
-                            queuedApplicationMessage.SenderClientId,
-                            ClientId,
-                            queuedApplicationMessage.ApplicationMessage,
-                            queuedApplicationMessage.SubscriptionQualityOfServiceLevel);
-
-                        if (_serverOptions.ClientMessageQueueInterceptor != null)
-                        {
-                            await _serverOptions.ClientMessageQueueInterceptor.InterceptClientMessageQueueEnqueueAsync(context).ConfigureAwait(false);
-                        }
-
-                        if (!context.AcceptEnqueue || context.ApplicationMessage == null)
-                        {
-                            return;
-                        }
-
-                        publishPacket.Topic = context.ApplicationMessage.Topic;
-                        publishPacket.Payload = context.ApplicationMessage.Payload;
-                        publishPacket.QualityOfServiceLevel = context.SubscriptionQualityOfServiceLevel;
-                    }
-
-                    if (publishPacket.QualityOfServiceLevel > 0)
-                    {
-                        publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNextPacketIdentifier();
-                    }
-
-                    if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtMostOnce)
-                    {
-                        await SendAsync(publishPacket, cancellationToken).ConfigureAwait(false);
-                    }
-                    else if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.AtLeastOnce)
-                    {
-                        var awaiter = _packetDispatcher.AddAwaiter<MqttPubAckPacket>(publishPacket.PacketIdentifier);
-                        await SendAsync(publishPacket, cancellationToken).ConfigureAwait(false);
-                        await awaiter.WaitOneAsync(_serverOptions.DefaultCommunicationTimeout).ConfigureAwait(false);
-                    }
-                    else if (publishPacket.QualityOfServiceLevel == MqttQualityOfServiceLevel.ExactlyOnce)
-                    {
-                        using (var awaiter1 = _packetDispatcher.AddAwaiter<MqttPubRecPacket>(publishPacket.PacketIdentifier))
-                        using (var awaiter2 = _packetDispatcher.AddAwaiter<MqttPubCompPacket>(publishPacket.PacketIdentifier))
-                        {
-                            await SendAsync(publishPacket, cancellationToken).ConfigureAwait(false);
-                            var pubRecPacket = await awaiter1.WaitOneAsync(_serverOptions.DefaultCommunicationTimeout).ConfigureAwait(false);
-
-                            var pubRelPacket = _channelAdapter.PacketFormatterAdapter.DataConverter.CreatePubRelPacket(pubRecPacket, MqttApplicationMessageReceivedReasonCode.Success);
-                            await SendAsync(pubRelPacket, cancellationToken).ConfigureAwait(false);
-
-                            await awaiter2.WaitOneAsync(_serverOptions.DefaultCommunicationTimeout).ConfigureAwait(false);
-                        }
-                    }
-
-                    _logger.Verbose("Client '{0}': Queued application message sent.", ClientId);
                 }
             }
             catch (Exception exception)
             {
-                if (exception is OperationCanceledException)
+                _logger.Warning(exception, "Client '{{0}}': Error while sending DISCONNECT packet (Reason = {1}).", ClientId, reason);
-                {
-                }
-                else if (exception is MqttCommunicationTimedOutException)
-                {
-                    _logger.Warning(exception, "Client '{0}': Sending publish packet failed: Timeout.", ClientId);
-                }
-                else if (exception is MqttCommunicationException)
-                {
-                    _logger.Warning(exception, "Client '{0}': Sending publish packet failed: Communication exception.", ClientId);
-                }
-                else
-                {
-                    _logger.Error(exception, "Client '{0}': Sending publish packet failed.", ClientId);
-                }
-
-                if (publishPacket?.QualityOfServiceLevel > MqttQualityOfServiceLevel.AtMostOnce)
-                {
-                    queuedApplicationMessage.IsDuplicate = true;
-
-                    Session.ApplicationMessagesQueue.Enqueue(queuedApplicationMessage);
-                }
-
-                StopInternal();
             }
         }
 
-        Task SendAsync(MqttBasePacket packet, CancellationToken cancellationToken)
+        async Task<MqttPublishPacket> InvokeClientMessageQueueInterceptor(MqttPublishPacket publishPacket, MqttQueuedApplicationMessage queuedApplicationMessage)
         {
-            return _channelAdapter.SendPacketAsync(packet, cancellationToken).ContinueWith(task =>
+            if (_serverOptions.ClientMessageQueueInterceptor == null)
             {
-                Interlocked.Increment(ref _receivedPacketsCount);
+                return publishPacket;
+            }
 
-                if (packet is MqttPublishPacket)
+            var context = new MqttClientMessageQueueInterceptorContext
-                {
+            {
-                    Interlocked.Increment(ref _receivedApplicationMessagesCount);
+                SenderClientId = queuedApplicationMessage.SenderClientId,
-                }
+                ReceiverClientId = ClientId,
-            }, cancellationToken);
+                ApplicationMessage = queuedApplicationMessage.ApplicationMessage,
+                SubscriptionQualityOfServiceLevel = queuedApplicationMessage.SubscriptionQualityOfServiceLevel
+            };
+
+            if (_serverOptions.ClientMessageQueueInterceptor != null)
+            {
+                await _serverOptions.ClientMessageQueueInterceptor.InterceptClientMessageQueueEnqueueAsync(context).ConfigureAwait(false);
+            }
+
+            if (!context.AcceptEnqueue || context.ApplicationMessage == null)
+            {
+                return null;
+            }
+
+            publishPacket.Topic = context.ApplicationMessage.Topic;
+            publishPacket.Payload = context.ApplicationMessage.Payload;
+            publishPacket.QualityOfServiceLevel = context.SubscriptionQualityOfServiceLevel;
+
+            return publishPacket;
         }
     }
-}
+}

Source/MQTTnet/Server/Internal/MqttClientConnectionStatistics.cs

@@ -0,0 +1,90 @@
+using System;
+using System.Threading;
+using MQTTnet.Packets;
+using MQTTnet.Server.Status;
+
+namespace MQTTnet.Server.Internal
+{
+    public sealed class MqttClientConnectionStatistics
+    {
+        readonly DateTime _connectedTimestamp;
+
+        DateTime _lastNonKeepAlivePacketReceivedTimestamp;
+        DateTime _lastPacketReceivedTimestamp;
+        DateTime _lastPacketSentTimestamp;
+
+        // Start with 1 because the CONNACK packet is not counted here.
+        long _receivedPacketsCount = 1;
+
+        // Start with 1 because the CONNECT packet is not counted here.
+        long _sentPacketsCount = 1;
+
+        long _receivedApplicationMessagesCount;
+        long _sentApplicationMessagesCount;
+
+        public MqttClientConnectionStatistics()
+        {
+            _connectedTimestamp = DateTime.UtcNow;
+
+            _lastPacketReceivedTimestamp = _connectedTimestamp;
+            _lastPacketSentTimestamp = _connectedTimestamp;
+
+            _lastNonKeepAlivePacketReceivedTimestamp = _connectedTimestamp;
+        }
+
+        public DateTime LastPacketReceivedTimestamp => _lastPacketReceivedTimestamp;
+
+        public void HandleReceivedPacket(MqttBasePacket packet)
+        {
+            if (packet == null) throw new ArgumentNullException(nameof(packet));
+            
+            // This class is tracking all values from Clients perspective!
+            _lastPacketSentTimestamp = DateTime.UtcNow;
+
+            Interlocked.Increment(ref _sentPacketsCount);
+
+            if (packet is MqttPublishPacket)
+            {
+                Interlocked.Increment(ref _sentApplicationMessagesCount);
+            }
+
+            if (!(packet is MqttPingReqPacket || packet is MqttPingRespPacket))
+            {
+                _lastNonKeepAlivePacketReceivedTimestamp = _lastPacketReceivedTimestamp;
+            }
+        }
+
+        public void HandleSentPacket(MqttBasePacket packet)
+        {
+            if (packet == null) throw new ArgumentNullException(nameof(packet));
+            
+            // This class is tracking all values from Clients perspective!
+            _lastPacketReceivedTimestamp = DateTime.UtcNow;
+
+            Interlocked.Increment(ref _receivedPacketsCount);
+
+            if (packet is MqttPublishPacket)
+            {
+                Interlocked.Increment(ref _receivedApplicationMessagesCount);
+            }
+        }
+
+        public void FillClientStatus(MqttClientStatus clientStatus)
+        {
+            if (clientStatus == null) throw new ArgumentNullException(nameof(clientStatus));
+            
+            clientStatus.ConnectedTimestamp = _connectedTimestamp;
+
+            clientStatus.ReceivedPacketsCount = Interlocked.Read(ref _receivedPacketsCount);
+            clientStatus.SentPacketsCount = Interlocked.Read(ref _sentPacketsCount);
+
+            clientStatus.ReceivedApplicationMessagesCount = Interlocked.Read(ref _receivedApplicationMessagesCount);
+            clientStatus.SentApplicationMessagesCount = Interlocked.Read(ref _sentApplicationMessagesCount);
+
+            clientStatus.LastPacketReceivedTimestamp = _lastPacketReceivedTimestamp;
+            clientStatus.LastPacketSentTimestamp = _lastPacketSentTimestamp;
+
+            clientStatus.LastNonKeepAlivePacketReceivedTimestamp = _lastNonKeepAlivePacketReceivedTimestamp;
+        }
+    }
+}

Source/MQTTnet/Server/Internal/MqttClientSession.cs

@@ -1,10 +1,10 @@
-using MQTTnet.Diagnostics;
+using System;
-using MQTTnet.Server.Status;
-using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;
+using MQTTnet.Diagnostics;
+using MQTTnet.Server.Status;
 
-namespace MQTTnet.Server
+namespace MQTTnet.Server.Internal
 {
     public sealed class MqttClientSession
     {
@@ -81,7 +81,7 @@ namespace MQTTnet.Server
             return SubscriptionsManager.UnsubscribeAsync(topicFilters);
         }
 
-        public void FillStatus(MqttSessionStatus status)
+        public void FillSessionStatus(MqttSessionStatus status)
         {
             status.ClientId = ClientId;
             status.CreatedTimestamp = _createdTimestamp;

Source/MQTTnet/Server/Internal/MqttClientSessionApplicationMessagesQueue.cs

@@ -1,10 +1,10 @@
-using MQTTnet.Internal;
+using System;
-using MQTTnet.Protocol;
-using System;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Internal;
+using MQTTnet.Protocol;
 
-namespace MQTTnet.Server
+namespace MQTTnet.Server.Internal
 {
     public sealed class MqttClientSessionApplicationMessagesQueue : IDisposable
     {

Source/MQTTnet/Server/Internal/MqttClientSessionsManager.cs

@@ -1,4 +1,11 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
 using MQTTnet.Adapter;
+using MQTTnet.Client.Disconnecting;
 using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
@@ -6,15 +13,8 @@ using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 using MQTTnet.Server.Status;
-using System;
-using System.Collections.Concurrent;
-using System.Collections.Generic;
-using System.Linq;
-using System.Threading;
-using System.Threading.Tasks;
-using MQTTnet.Client.Disconnecting;
 
-namespace MQTTnet.Server
+namespace MQTTnet.Server.Internal
 {
     public sealed class MqttClientSessionsManager : IDisposable
     {
@@ -53,53 +53,66 @@ namespace MQTTnet.Server
             Task.Run(() => TryProcessQueuedApplicationMessagesAsync(cancellationToken), cancellationToken).RunInBackground(_logger);
         }
 
-        public async Task HandleClientConnectionAsync(IMqttChannelAdapter channelAdapter, CancellationToken cancellationToken)
+        async Task<MqttConnectPacket> ReceiveConnectPacket(IMqttChannelAdapter channelAdapter, CancellationToken cancellationToken)
         {
             try
             {
-                MqttConnectPacket connectPacket;
+                using (var timeoutToken = new CancellationTokenSource(_options.DefaultCommunicationTimeout))
-                try
+                using (var effectiveCancellationToken = CancellationTokenSource.CreateLinkedTokenSource(timeoutToken.Token, cancellationToken))
                 {
-                    using (var timeoutToken = new CancellationTokenSource(_options.DefaultCommunicationTimeout))
+                    var firstPacket = await channelAdapter.ReceivePacketAsync(effectiveCancellationToken.Token).ConfigureAwait(false);
-                    {
-                        var firstPacket = await channelAdapter.ReceivePacketAsync(timeoutToken.Token).ConfigureAwait(false);
-                        connectPacket = firstPacket as MqttConnectPacket;
-                        if (connectPacket == null)
-                        {
-                            _logger.Warning(null,
-                                "Client '{0}': First received packet was no 'CONNECT' packet [MQTT-3.1.0-1].",
-                                channelAdapter.Endpoint);
 
-                            return;
+                    if (firstPacket is MqttConnectPacket connectPacket)
-                        }
+                    {
+                        return connectPacket;
                     }
                 }
-                catch (OperationCanceledException)
+            }
+            catch (OperationCanceledException)
+            {
+                _logger.Warning("Client '{0}': Connected but did not sent a CONNECT packet.", channelAdapter.Endpoint);
+            }
+            catch (MqttCommunicationTimedOutException)
+            {
+                _logger.Warning("Client '{0}': Connected but did not sent a CONNECT packet.", channelAdapter.Endpoint);
+            }
+
+            _logger.Warning("Client '{0}': First received packet was no 'CONNECT' packet [MQTT-3.1.0-1].", channelAdapter.Endpoint);
+            return null;
+        }
+        
+        public async Task HandleClientConnectionAsync(IMqttChannelAdapter channelAdapter, CancellationToken cancellationToken)
+        {
+            MqttClientConnection clientConnection = null;
+
+            try
+            {
+                var connectPacket = await ReceiveConnectPacket(channelAdapter, cancellationToken).ConfigureAwait(false);
+                if (connectPacket == null)
                 {
-                    _logger.Warning(null, "Client '{0}': Connected but did not sent a CONNECT packet.", channelAdapter.Endpoint);
+                    // Nothing was received in time etc.
-                    return;
-                }
-                catch (MqttCommunicationTimedOutException)
-                {
-                    _logger.Warning(null, "Client '{0}': Connected but did not sent a CONNECT packet.", channelAdapter.Endpoint);
                     return;
                 }
 
-                var connectionValidatorContext = await ValidateConnectionAsync(connectPacket, channelAdapter).ConfigureAwait(false);
+                MqttConnAckPacket connAckPacket;
-
+                
+                var connectionValidatorContext = await ValidateConnection(connectPacket, channelAdapter).ConfigureAwait(false);
                 if (connectionValidatorContext.ReasonCode != MqttConnectReasonCode.Success)
                 {
-                    // Send failure response here without preparing a session. The result for a successful connect
+                    // Send failure response here without preparing a session!
-                    // will be sent from the session itself.
+                    connAckPacket = channelAdapter.PacketFormatterAdapter.DataConverter.CreateConnAckPacket(connectionValidatorContext);
-                    var connAckPacket = channelAdapter.PacketFormatterAdapter.DataConverter.CreateConnAckPacket(connectionValidatorContext);
                     await channelAdapter.SendPacketAsync(connAckPacket, cancellationToken).ConfigureAwait(false);
-
                     return;
                 }
 
-                var connection = await CreateClientConnectionAsync(connectPacket, connectionValidatorContext, channelAdapter).ConfigureAwait(false);
+                clientConnection = await CreateClientConnection(connectPacket, channelAdapter, connectionValidatorContext.SessionItems).ConfigureAwait(false);
-                await _eventDispatcher.SafeNotifyClientConnectedAsync(connectPacket.ClientId).ConfigureAwait(false);
+
-                await connection.RunAsync().ConfigureAwait(false);
+                connAckPacket = channelAdapter.PacketFormatterAdapter.DataConverter.CreateConnAckPacket(connectionValidatorContext);
+                await channelAdapter.SendPacketAsync(connAckPacket, cancellationToken).ConfigureAwait(false);
+
+                await _eventDispatcher.SafeNotifyClientConnectedAsync(connectPacket, channelAdapter).ConfigureAwait(false);
+
+                await clientConnection.RunAsync().ConfigureAwait(false);
             }
             catch (OperationCanceledException)
             {
@@ -108,6 +121,41 @@ namespace MQTTnet.Server
             {
                 _logger.Error(exception, exception.Message);
             }
+            finally
+            {
+                if (clientConnection != null)
+                {
+                    if (clientConnection.ClientId != null)
+                    {
+                        // in case it is a takeover _connections already contains the new connection
+                        if (!clientConnection.IsTakenOver)
+                        {
+                            lock (_connections)
+                            {
+                                _connections.Remove(clientConnection.ClientId);
+                            }
+
+                            if (!_options.EnablePersistentSessions)
+                            {
+                                await DeleteSessionAsync(clientConnection.ClientId).ConfigureAwait(false);
+                            }
+                        }
+                    }
+
+                    var endpoint = clientConnection.Endpoint;
+
+                    if (clientConnection.ClientId != null && !clientConnection.IsTakenOver)
+                    {
+                        // The event is fired at a separate place in case of a handover!
+                        await _eventDispatcher.SafeNotifyClientDisconnectedAsync(
+                            clientConnection.ClientId,
+                            clientConnection.IsCleanDisconnect ? MqttClientDisconnectType.Clean : MqttClientDisconnectType.NotClean,
+                            endpoint).ConfigureAwait(false);
+                    }
+                }
+
+                await channelAdapter.DisconnectAsync(_options.DefaultCommunicationTimeout, CancellationToken.None).ConfigureAwait(false);
+            }
         }
 
         public async Task CloseAllConnectionsAsync()
@@ -132,7 +180,7 @@ namespace MQTTnet.Server
                 return _connections.Values.ToList();
             }
         }
-        
+
         public Task<IList<IMqttClientStatus>> GetClientStatusAsync()
         {
             var result = new List<IMqttClientStatus>();
@@ -142,10 +190,10 @@ namespace MQTTnet.Server
                 foreach (var connection in _connections.Values)
                 {
                     var clientStatus = new MqttClientStatus(connection);
-                    connection.FillStatus(clientStatus);
+                    connection.FillClientStatus(clientStatus);
 
                     var sessionStatus = new MqttSessionStatus(connection.Session, this);
-                    connection.Session.FillStatus(sessionStatus);
+                    connection.Session.FillSessionStatus(sessionStatus);
                     clientStatus.Session = sessionStatus;
 
                     result.Add(clientStatus);
@@ -164,7 +212,7 @@ namespace MQTTnet.Server
                 foreach (var session in _sessions.Values)
                 {
                     var sessionStatus = new MqttSessionStatus(session, this);
-                    session.FillStatus(sessionStatus);
+                    session.FillSessionStatus(sessionStatus);
 
                     result.Add(sessionStatus);
                 }
@@ -203,7 +251,7 @@ namespace MQTTnet.Server
             {
                 _connections.TryGetValue(clientId, out connection);
             }
-            
+
             lock (_sessions)
             {
                 _sessions.Remove(clientId);
@@ -213,39 +261,10 @@ namespace MQTTnet.Server
             {
                 await connection.StopAsync(MqttClientDisconnectReason.NormalDisconnection).ConfigureAwait(false);
             }
-            
+
             _logger.Verbose("Session for client '{0}' deleted.", clientId);
         }
 
-        public async Task CleanUpClient(string clientId, IMqttChannelAdapter channelAdapter, MqttClientDisconnectType disconnectType)
-        {
-            if (clientId != null)
-            {
-                // in case it is a takeover _connections already contains the new connection
-                if (disconnectType != MqttClientDisconnectType.Takeover)
-                {
-                    lock (_connections)
-                    {
-                        _connections.Remove(clientId);
-                    }
-
-                    if (!_options.EnablePersistentSessions)
-                    {
-                        await DeleteSessionAsync(clientId).ConfigureAwait(false);
-                    }
-                }
-            }
-
-            var endpoint = channelAdapter.Endpoint;
-
-            await SafeCleanupChannelAsync(channelAdapter).ConfigureAwait(false);
-
-            if (clientId != null)
-            {
-                await _eventDispatcher.SafeNotifyClientDisconnectedAsync(clientId, disconnectType, endpoint).ConfigureAwait(false);
-            }
-        }
-
         public void Dispose()
         {
             _messageQueue?.Dispose();
@@ -278,7 +297,7 @@ namespace MQTTnet.Server
                 MqttPendingApplicationMessage queuedApplicationMessage;
                 try
                 {
-                     queuedApplicationMessage = _messageQueue.Take(cancellationToken);
+                    queuedApplicationMessage = _messageQueue.Take(cancellationToken);
                 }
                 catch (ArgumentNullException)
                 {
@@ -315,7 +334,7 @@ namespace MQTTnet.Server
                         applicationMessage = interceptorContext.ApplicationMessage;
                     }
                 }
-                
+
                 await _eventDispatcher.SafeNotifyApplicationMessageReceivedAsync(senderClientId, applicationMessage).ConfigureAwait(false);
 
                 if (applicationMessage.Retain)
@@ -360,9 +379,12 @@ namespace MQTTnet.Server
             }
         }
 
-        async Task<MqttConnectionValidatorContext> ValidateConnectionAsync(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter)
+        async Task<MqttConnectionValidatorContext> ValidateConnection(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter)
         {
-            var context = new MqttConnectionValidatorContext(connectPacket, channelAdapter, new ConcurrentDictionary<object, object>());
+            var context = new MqttConnectionValidatorContext(connectPacket, channelAdapter)
+            {
+                SessionItems = new ConcurrentDictionary<object, object>()
+            };
 
             var connectionValidator = _options.ConnectionValidator;
 
@@ -388,9 +410,8 @@ namespace MQTTnet.Server
             return context;
         }
 
-        async Task<MqttClientConnection> CreateClientConnectionAsync(MqttConnectPacket connectPacket, MqttConnectionValidatorContext connectionValidatorContext, IMqttChannelAdapter channelAdapter)
+        async Task<MqttClientConnection> CreateClientConnection(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter, IDictionary<object, object> sessionItems)
         {
-            MqttClientConnection existingConnection;
             MqttClientConnection connection;
 
             using (await _createConnectionSyncRoot.WaitAsync(CancellationToken.None).ConfigureAwait(false))
@@ -401,14 +422,14 @@ namespace MQTTnet.Server
                     if (!_sessions.TryGetValue(connectPacket.ClientId, out session))
                     {
                         _logger.Verbose("Created a new session for client '{0}'.", connectPacket.ClientId);
-                        session = CreateSession(connectPacket.ClientId, connectionValidatorContext);
+                        session = CreateSession(connectPacket.ClientId, sessionItems);
                     }
                     else
                     {
                         if (connectPacket.CleanSession)
                         {
                             _logger.Verbose("Deleting existing session of client '{0}'.", connectPacket.ClientId);
-                            session = CreateSession(connectPacket.ClientId, connectionValidatorContext);
+                            session = CreateSession(connectPacket.ClientId, sessionItems);
                         }
                         else
                         {
@@ -419,16 +440,21 @@ namespace MQTTnet.Server
                     _sessions[connectPacket.ClientId] = session;
                 }
 
+                MqttClientConnection existingConnection;
+
                 lock (_connections)
                 {
                     _connections.TryGetValue(connectPacket.ClientId, out existingConnection);
-                    connection = CreateConnection(connectPacket, channelAdapter, session, connectionValidatorContext);
+                    connection = CreateConnection(connectPacket, channelAdapter, session);
 
                     _connections[connectPacket.ClientId] = connection;
                 }
 
                 if (existingConnection != null)
                 {
+                    await _eventDispatcher.SafeNotifyClientDisconnectedAsync(existingConnection.ClientId, MqttClientDisconnectType.Takeover, existingConnection.Endpoint);
+
+                    existingConnection.IsTakenOver = true;
                     await existingConnection.StopAsync(MqttClientDisconnectReason.SessionTakenOver).ConfigureAwait(false);
                 }
             }
@@ -436,7 +462,8 @@ namespace MQTTnet.Server
             return connection;
         }
 
-        async Task<MqttApplicationMessageInterceptorContext> InterceptApplicationMessageAsync(IMqttServerApplicationMessageInterceptor interceptor, MqttClientConnection clientConnection, MqttApplicationMessage applicationMessage)
+        async Task<MqttApplicationMessageInterceptorContext> InterceptApplicationMessageAsync(IMqttServerApplicationMessageInterceptor interceptor, MqttClientConnection clientConnection,
+            MqttApplicationMessage applicationMessage)
         {
             string senderClientId;
             IDictionary<object, object> sessionItems;
@@ -453,29 +480,20 @@ namespace MQTTnet.Server
                 sessionItems = clientConnection.Session.Items;
             }
 
-            var interceptorContext = new MqttApplicationMessageInterceptorContext(senderClientId, sessionItems, _logger)
+            var interceptorContext = new MqttApplicationMessageInterceptorContext
             {
+                ClientId = senderClientId,
+                SessionItems = sessionItems,
+                Logger = _logger,
                 AcceptPublish = true,
                 ApplicationMessage = applicationMessage,
-                CloseConnection =  false
+                CloseConnection = false
             };
 
             await interceptor.InterceptApplicationMessagePublishAsync(interceptorContext).ConfigureAwait(false);
             return interceptorContext;
         }
 
-        async Task SafeCleanupChannelAsync(IMqttChannelAdapter channelAdapter)
-        {
-            try
-            {
-                await channelAdapter.DisconnectAsync(_options.DefaultCommunicationTimeout, CancellationToken.None).ConfigureAwait(false);
-            }
-            catch (Exception exception)
-            {
-                _logger.Error(exception, "Error while disconnecting client channel.");
-            }
-        }
-
         MqttClientSession GetSession(string clientId)
         {
             lock (_sessions)
@@ -489,24 +507,23 @@ namespace MQTTnet.Server
             }
         }
 
-        MqttClientConnection CreateConnection(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter, MqttClientSession session, MqttConnectionValidatorContext connectionValidatorContext)
+        MqttClientConnection CreateConnection(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter, MqttClientSession session)
         {
             return new MqttClientConnection(
                 connectPacket,
                 channelAdapter,
                 session,
-                connectionValidatorContext,
                 _options,
                 this,
                 _retainedMessagesManager,
                 _rootLogger);
         }
 
-        MqttClientSession CreateSession(string clientId, MqttConnectionValidatorContext connectionValidatorContext)
+        MqttClientSession CreateSession(string clientId, IDictionary<object, object> sessionItems)
         {
             return new MqttClientSession(
                 clientId,
-                connectionValidatorContext.SessionItems,
+                sessionItems,
                 _eventDispatcher,
                 _options,
                 _retainedMessagesManager,

Source/MQTTnet/Server/Internal/MqttClientSubscriptionsManager.cs

@@ -1,12 +1,12 @@
-using MQTTnet.Packets;
+using System;
-using MQTTnet.Protocol;
-using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Packets;
+using MQTTnet.Protocol;
 
-namespace MQTTnet.Server
+namespace MQTTnet.Server.Internal
 {
     public sealed class MqttClientSubscriptionsManager
     {
@@ -37,9 +37,11 @@ namespace MQTTnet.Server
             {
                 var interceptorContext = await InterceptSubscribeAsync(originalTopicFilter).ConfigureAwait(false);
 
-                var finalTopicFilter = interceptorContext.TopicFilter;
+                var finalTopicFilter = interceptorContext?.TopicFilter ?? originalTopicFilter;
+                var acceptSubscription = interceptorContext?.AcceptSubscription ?? true;
+                var closeConnection = interceptorContext?.CloseConnection ?? false;
 
-                if (finalTopicFilter == null || string.IsNullOrEmpty(finalTopicFilter.Topic) || !interceptorContext.AcceptSubscription)
+                if (finalTopicFilter == null || string.IsNullOrEmpty(finalTopicFilter.Topic) || !acceptSubscription)
                 {
                     result.ReturnCodes.Add(MqttSubscribeReturnCode.Failure);
                     result.ReasonCodes.Add(MqttSubscribeReasonCode.UnspecifiedError);
@@ -50,12 +52,12 @@ namespace MQTTnet.Server
                     result.ReasonCodes.Add(ConvertToSubscribeReasonCode(finalTopicFilter.QualityOfServiceLevel));
                 }
 
-                if (interceptorContext.CloseConnection)
+                if (closeConnection)
                 {
                     result.CloseConnection = true;
                 }
 
-                if (!interceptorContext.AcceptSubscription || string.IsNullOrEmpty(finalTopicFilter?.Topic))
+                if (!acceptSubscription || string.IsNullOrEmpty(finalTopicFilter?.Topic))
                 {
                     continue;
                 }
@@ -83,12 +85,7 @@ namespace MQTTnet.Server
             foreach (var topicFilter in topicFilters)
             {
                 var interceptorContext = await InterceptSubscribeAsync(topicFilter).ConfigureAwait(false);
-                if (!interceptorContext.AcceptSubscription)
+                if (interceptorContext != null && !interceptorContext.AcceptSubscription)
-                {
-                    continue;
-                }
-
-                if (!interceptorContext.AcceptSubscription)
                 {
                     continue;
                 }
@@ -116,7 +113,7 @@ namespace MQTTnet.Server
             foreach (var topicFilter in unsubscribePacket.TopicFilters)
             {
                 var interceptorContext = await InterceptUnsubscribeAsync(topicFilter).ConfigureAwait(false);
-                if (!interceptorContext.AcceptUnsubscription)
+                if (interceptorContext != null && !interceptorContext.AcceptUnsubscription)
                 {
                     reasonCodes.Add(MqttUnsubscribeReasonCode.ImplementationSpecificError);
                     continue;
@@ -150,7 +147,7 @@ namespace MQTTnet.Server
             foreach (var topicFilter in topicFilters)
             {
                 var interceptorContext = await InterceptUnsubscribeAsync(topicFilter).ConfigureAwait(false);
-                if (!interceptorContext.AcceptUnsubscription)
+                if (interceptorContext != null && !interceptorContext.AcceptUnsubscription)
                 {
                     continue;
                 }
@@ -187,7 +184,7 @@ namespace MQTTnet.Server
                 {
                     continue;
                 }
-                
+
                 qosLevels.Add(subscription.Value.QualityOfServiceLevel);
             }
 
@@ -211,23 +208,41 @@ namespace MQTTnet.Server
 
         async Task<MqttSubscriptionInterceptorContext> InterceptSubscribeAsync(MqttTopicFilter topicFilter)
         {
-            var context = new MqttSubscriptionInterceptorContext(_clientSession.ClientId, topicFilter, _clientSession.Items);
+            var interceptor = _options.SubscriptionInterceptor;
-            if (_options.SubscriptionInterceptor != null)
+            if (interceptor == null)
             {
-                await _options.SubscriptionInterceptor.InterceptSubscriptionAsync(context).ConfigureAwait(false);
+                return null;
             }
 
+            var context = new MqttSubscriptionInterceptorContext
+            {
+                ClientId = _clientSession.ClientId,
+                TopicFilter = topicFilter,
+                SessionItems = _clientSession.Items
+            };
+
+            await interceptor.InterceptSubscriptionAsync(context).ConfigureAwait(false);
+
             return context;
         }
 
         async Task<MqttUnsubscriptionInterceptorContext> InterceptUnsubscribeAsync(string topicFilter)
         {
-            var context = new MqttUnsubscriptionInterceptorContext(_clientSession.ClientId, topicFilter, _clientSession.Items);
+            var interceptor = _options.UnsubscriptionInterceptor;
-            if (_options.UnsubscriptionInterceptor != null)
+            if (interceptor == null)
             {
-                await _options.UnsubscriptionInterceptor.InterceptUnsubscriptionAsync(context).ConfigureAwait(false);
+                return null;
             }
 
+            var context = new MqttUnsubscriptionInterceptorContext
+            {
+                ClientId = _clientSession.ClientId,
+                Topic = topicFilter,
+                SessionItems = _clientSession.Items
+            };
+
+            await interceptor.InterceptUnsubscriptionAsync(context).ConfigureAwait(false);
+
             return context;
         }
 
@@ -254,4 +269,4 @@ namespace MQTTnet.Server
             };
         }
     }
-}
+}

Source/MQTTnet/Server/Internal/MqttRetainedMessagesManager.cs

@@ -1,13 +1,13 @@
-using MQTTnet.Diagnostics;
+using System;
-using MQTTnet.Implementations;
-using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Diagnostics;
+using MQTTnet.Implementations;
 using MQTTnet.Internal;
 
-namespace MQTTnet.Server
+namespace MQTTnet.Server.Internal
 {
     public sealed class MqttRetainedMessagesManager : IMqttRetainedMessagesManager
     {

Source/MQTTnet/Server/Internal/MqttServerEventDispatcher.cs

@@ -1,9 +1,11 @@
-using MQTTnet.Client.Receiving;
-using MQTTnet.Diagnostics;
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Adapter;
+using MQTTnet.Client.Receiving;
+using MQTTnet.Diagnostics;
+using MQTTnet.Packets;
 
-namespace MQTTnet.Server
+namespace MQTTnet.Server.Internal
 {
     public sealed class MqttServerEventDispatcher
     {
@@ -26,7 +28,7 @@ namespace MQTTnet.Server
 
         public IMqttApplicationMessageReceivedHandler ApplicationMessageReceivedHandler { get; set; }
 
-        public async Task SafeNotifyClientConnectedAsync(string clientId)
+        public async Task SafeNotifyClientConnectedAsync(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter)
         {
             try
             {
@@ -36,7 +38,13 @@ namespace MQTTnet.Server
                     return;
                 }
 
-                await handler.HandleClientConnectedAsync(new MqttServerClientConnectedEventArgs(clientId)).ConfigureAwait(false);
+                await handler.HandleClientConnectedAsync(new MqttServerClientConnectedEventArgs
+                {
+                    ClientId = connectPacket.ClientId,
+                    UserName = connectPacket.Username,
+                    ProtocolVersion = channelAdapter.PacketFormatterAdapter.ProtocolVersion,
+                    Endpoint = channelAdapter.Endpoint
+                }).ConfigureAwait(false);
             }
             catch (Exception exception)
             {
@@ -54,7 +62,12 @@ namespace MQTTnet.Server
                     return;
                 }
 
-                await handler.HandleClientDisconnectedAsync(new MqttServerClientDisconnectedEventArgs(clientId, disconnectType, endpoint)).ConfigureAwait(false);
+                await handler.HandleClientDisconnectedAsync(new MqttServerClientDisconnectedEventArgs
+                {
+                    ClientId = clientId,
+                    DisconnectType = disconnectType,
+                    Endpoint = endpoint
+                }).ConfigureAwait(false);
             }
             catch (Exception exception)
             {
@@ -72,7 +85,11 @@ namespace MQTTnet.Server
                     return;
                 }
 
-                await handler.HandleClientSubscribedTopicAsync(new MqttServerClientSubscribedTopicEventArgs(clientId, topicFilter)).ConfigureAwait(false);
+                await handler.HandleClientSubscribedTopicAsync(new MqttServerClientSubscribedTopicEventArgs
+                {
+                    ClientId = clientId,
+                    TopicFilter = topicFilter
+                }).ConfigureAwait(false);
             }
             catch (Exception exception)
             {
@@ -90,7 +107,11 @@ namespace MQTTnet.Server
                     return;
                 }
 
-                await handler.HandleClientUnsubscribedTopicAsync(new MqttServerClientUnsubscribedTopicEventArgs(clientId, topicFilter)).ConfigureAwait(false);
+                await handler.HandleClientUnsubscribedTopicAsync(new MqttServerClientUnsubscribedTopicEventArgs
+                {
+                    ClientId = clientId,
+                    TopicFilter = topicFilter
+                }).ConfigureAwait(false);
             }
             catch (Exception exception)
             {

Source/MQTTnet/Server/Internal/MqttServerKeepAliveMonitor.cs

@@ -1,12 +1,12 @@
-using MQTTnet.Diagnostics;
+using System;
-using MQTTnet.Internal;
-using System;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Client.Disconnecting;
+using MQTTnet.Diagnostics;
 using MQTTnet.Implementations;
+using MQTTnet.Internal;
 
-namespace MQTTnet.Server
+namespace MQTTnet.Server.Internal
 {
     public sealed class MqttServerKeepAliveMonitor
     {
@@ -70,7 +70,7 @@ namespace MQTTnet.Server
         {
             try
             {
-                if (connection.Status != MqttClientConnectionStatus.Running)
+                if (!connection.IsRunning)
                 {
                     // The connection is already dead or just created so there is no need to check it.
                     return;
@@ -94,14 +94,14 @@ namespace MQTTnet.Server
                 // If the client sends 1 sec. the server will allow up to 1.5 seconds.
                 var maxDurationWithoutPacket = connection.ConnectPacket.KeepAlivePeriod * 1.5D;
 
-                var secondsWithoutPackage = (now - connection.LastPacketReceivedTimestamp).TotalSeconds;
+                var secondsWithoutPackage = (now - connection.Statistics.LastPacketReceivedTimestamp).TotalSeconds;
                 if (secondsWithoutPackage < maxDurationWithoutPacket)
                 {
                     // A packet was received before the timeout is affected.
                     return;
                 }
 
-                _logger.Warning(null, "Client '{0}': Did not receive any packet or keep alive signal.", connection.ClientId);
+                _logger.Warning("Client '{0}': Did not receive any packet or keep alive signal.", connection.ClientId);
 
                 // Execute the disconnection in background so that the keep alive monitor can continue
                 // with checking other connections.

Source/MQTTnet/Server/Internal/MqttTopicFilterComparer.cs

@@ -1,6 +1,6 @@
 using System;
 
-namespace MQTTnet.Server
+namespace MQTTnet.Server.Internal
 {
     public static class MqttTopicFilterComparer
     {

Source/MQTTnet/Server/MqttApplicationMessageInterceptorContext.cs

@@ -1,35 +1,27 @@
-using System;
+using System.Collections.Generic;
-using System.Collections.Generic;
 using MQTTnet.Diagnostics;
 
 namespace MQTTnet.Server
 {
     public sealed class MqttApplicationMessageInterceptorContext
     {
-        public MqttApplicationMessageInterceptorContext(string clientId, IDictionary<object, object> sessionItems, IMqttNetScopedLogger logger)
-        {
-            ClientId = clientId;
-            SessionItems = sessionItems;
-            Logger = logger ?? throw new ArgumentNullException(nameof(logger));
-        }
-
         /// <summary>
         /// Gets the currently used logger.
         /// </summary>
-        public IMqttNetScopedLogger Logger { get; }
+        public IMqttNetScopedLogger Logger { get; internal set; }
 
         /// <summary>
         /// Gets the client identifier.
         /// Hint: This identifier needs to be unique over all used clients / devices on the broker to avoid connection issues.
         /// </summary>
-        public string ClientId { get; }
+        public string ClientId { get; internal set; }
 
         public MqttApplicationMessage ApplicationMessage { get; set; }
 
         /// <summary>
         /// Gets or sets a key/value collection that can be used to share data within the scope of this session.
         /// </summary>
-        public IDictionary<object, object> SessionItems { get; }
+        public IDictionary<object, object> SessionItems { get; internal set; }
 
         public bool AcceptPublish { get; set; } = true;
 

Source/MQTTnet/Server/MqttClientConnectionStatus.cs

@@ -1,11 +0,0 @@
-namespace MQTTnet.Server
-{
-    public enum MqttClientConnectionStatus
-    {
-        Initializing,
-
-        Running,
-
-        Finalizing
-    }
-}

Source/MQTTnet/Server/MqttClientMessageQueueInterceptorContext.cs

@@ -2,19 +2,11 @@
 
 namespace MQTTnet.Server
 {
-    public class MqttClientMessageQueueInterceptorContext
+    public sealed class MqttClientMessageQueueInterceptorContext
     {
-        public MqttClientMessageQueueInterceptorContext(string senderClientId, string receiverClientId, MqttApplicationMessage applicationMessage, MqttQualityOfServiceLevel subscriptionQualityOfServiceLevel)
+        public string SenderClientId { get; internal set; }
-        {
-            SenderClientId = senderClientId;
-            ReceiverClientId = receiverClientId;
-            ApplicationMessage = applicationMessage;
-            SubscriptionQualityOfServiceLevel = subscriptionQualityOfServiceLevel;
-        }
 
-        public string SenderClientId { get; }
+        public string ReceiverClientId { get; internal set; }
-
-        public string ReceiverClientId { get; }
 
         public MqttApplicationMessage ApplicationMessage { get; set; }
 

Source/MQTTnet/Server/MqttClientMessageQueueInterceptorDelegate.cs

@@ -1,11 +1,12 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Server
 {
-    public class MqttClientMessageQueueInterceptorDelegate : IMqttServerClientMessageQueueInterceptor
+    public sealed class MqttClientMessageQueueInterceptorDelegate : IMqttServerClientMessageQueueInterceptor
     {
-        private readonly Func<MqttClientMessageQueueInterceptorContext, Task> _callback;
+        readonly Func<MqttClientMessageQueueInterceptorContext, Task> _callback;
 
         public MqttClientMessageQueueInterceptorDelegate(Action<MqttClientMessageQueueInterceptorContext> callback)
         {
@@ -14,7 +15,7 @@ namespace MQTTnet.Server
             _callback = context =>
             {
                 callback(context);
-                return Task.FromResult(0);
+                return PlatformAbstractionLayer.CompletedTask;
             };
         }
 

Source/MQTTnet/Server/MqttConnectionValidatorContext.cs

@@ -4,21 +4,21 @@ using System.Security.Cryptography.X509Certificates;
 using System.Text;
 using MQTTnet.Adapter;
 using MQTTnet.Formatter;
+using MQTTnet.Implementations;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
 namespace MQTTnet.Server
 {
-    public class MqttConnectionValidatorContext
+    public sealed class MqttConnectionValidatorContext
     {
-        private readonly MqttConnectPacket _connectPacket;
+        readonly MqttConnectPacket _connectPacket;
-        private readonly IMqttChannelAdapter _clientAdapter;
+        readonly IMqttChannelAdapter _clientAdapter;
 
-        public MqttConnectionValidatorContext(MqttConnectPacket connectPacket, IMqttChannelAdapter clientAdapter, IDictionary<object, object> sessionItems)
+        public MqttConnectionValidatorContext(MqttConnectPacket connectPacket, IMqttChannelAdapter clientAdapter)
         {
             _connectPacket = connectPacket;
             _clientAdapter = clientAdapter ?? throw new ArgumentNullException(nameof(clientAdapter));
-            SessionItems = sessionItems;
         }
 
         /// <summary>
@@ -39,7 +39,7 @@ namespace MQTTnet.Server
 
         public byte[] RawPassword => _connectPacket?.Password;
 
-        public string Password => Encoding.UTF8.GetString(RawPassword ?? new byte[0]);
+        public string Password => Encoding.UTF8.GetString(RawPassword ?? PlatformAbstractionLayer.EmptyByteArray);
 
         /// <summary>
         /// Gets or sets the will delay interval.
@@ -128,7 +128,7 @@ namespace MQTTnet.Server
         /// <summary>
         /// Gets or sets a key/value collection that can be used to share data within the scope of this session.
         /// </summary>
-        public IDictionary<object, object> SessionItems { get; }
+        public IDictionary<object, object> SessionItems { get; internal set; }
 
         /// <summary>
         /// This is used for MQTTv3 only.

Source/MQTTnet/Server/MqttPendingApplicationMessage.cs

@@ -1,4 +1,6 @@
-namespace MQTTnet.Server
+using MQTTnet.Server.Internal;
+
+namespace MQTTnet.Server
 {
     public sealed class MqttPendingApplicationMessage
     {

Source/MQTTnet/Server/MqttQueuedApplicationMessage.cs

@@ -3,7 +3,7 @@ using MQTTnet.Protocol;
 
 namespace MQTTnet.Server
 {
-    public class MqttQueuedApplicationMessage
+    public sealed class MqttQueuedApplicationMessage
     {
         public MqttApplicationMessage ApplicationMessage { get; set; }
 

Source/MQTTnet/Server/MqttServer.cs

@@ -12,6 +12,7 @@ using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Implementations;
 using MQTTnet.Internal;
+using MQTTnet.Server.Internal;
 
 namespace MQTTnet.Server
 {

Source/MQTTnet/Server/MqttServerApplicationMessageInterceptorDelegate.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Server
 {
@@ -14,7 +15,7 @@ namespace MQTTnet.Server
             _callback = context =>
             {
                 callback(context);
-                return Task.FromResult(0);
+                return PlatformAbstractionLayer.CompletedTask;
             };
         }
 

Source/MQTTnet/Server/MqttServerClientConnectedEventArgs.cs

@@ -1,18 +1,29 @@
 using System;
+using MQTTnet.Formatter;
 
 namespace MQTTnet.Server
 {
-    public class MqttServerClientConnectedEventArgs : EventArgs
+    public sealed class MqttServerClientConnectedEventArgs : EventArgs
     {
-        public MqttServerClientConnectedEventArgs(string clientId)
-        {
-            ClientId = clientId ?? throw new ArgumentNullException(nameof(clientId));
-        }
-
         /// <summary>
-        /// Gets the client identifier.
+        /// Gets the client identifier of the connected client.
         /// Hint: This identifier needs to be unique over all used clients / devices on the broker to avoid connection issues.
         /// </summary>
-        public string ClientId { get; }
+        public string ClientId { get; internal set; }
+
+        /// <summary>
+        /// Gets the user name of the connected client.
+        /// </summary>
+        public string UserName { get; internal set; }
+
+        /// <summary>
+        /// Gets the protocol version which is used by the connected client.
+        /// </summary>
+        public MqttProtocolVersion ProtocolVersion { get; internal set; }
+
+        /// <summary>
+        /// Gets the endpoint of the connected client.
+        /// </summary>
+        public string Endpoint { get; internal set; }
     }
 }

Source/MQTTnet/Server/MqttServerClientConnectedHandlerDelegate.cs

@@ -1,11 +1,12 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Server
 {
-    public class MqttServerClientConnectedHandlerDelegate : IMqttServerClientConnectedHandler
+    public sealed class MqttServerClientConnectedHandlerDelegate : IMqttServerClientConnectedHandler
     {
-        private readonly Func<MqttServerClientConnectedEventArgs, Task> _handler;
+        readonly Func<MqttServerClientConnectedEventArgs, Task> _handler;
 
         public MqttServerClientConnectedHandlerDelegate(Action<MqttServerClientConnectedEventArgs> handler)
         {
@@ -14,7 +15,7 @@ namespace MQTTnet.Server
             _handler = eventArgs =>
             {
                 handler(eventArgs);
-                return Task.FromResult(0);
+                return PlatformAbstractionLayer.CompletedTask;
             };
         }
 

Source/MQTTnet/Server/MqttServerClientDisconnectedEventArgs.cs

@@ -2,23 +2,16 @@ using System;
 
 namespace MQTTnet.Server
 {
-    public class MqttServerClientDisconnectedEventArgs : EventArgs
+    public sealed class MqttServerClientDisconnectedEventArgs : EventArgs
     {
-        public MqttServerClientDisconnectedEventArgs(string clientId, MqttClientDisconnectType disconnectType, string endpoint)
-        {
-            ClientId = clientId ?? throw new ArgumentNullException(nameof(clientId));
-            DisconnectType = disconnectType;
-            Endpoint = endpoint;
-        }
-
         /// <summary>
         /// Gets the client identifier.
         /// Hint: This identifier needs to be unique over all used clients / devices on the broker to avoid connection issues.
         /// </summary>
-        public string ClientId { get; }
+        public string ClientId { get; internal set; }
 
-        public MqttClientDisconnectType DisconnectType { get; }
+        public MqttClientDisconnectType DisconnectType { get; internal set; }
 
-        public string Endpoint { get; }
+        public string Endpoint { get; internal set; }
     }
 }

Source/MQTTnet/Server/MqttServerClientDisconnectedHandlerDelegate.cs

@@ -1,11 +1,12 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Server
 {
-    public class MqttServerClientDisconnectedHandlerDelegate : IMqttServerClientDisconnectedHandler
+    public sealed class MqttServerClientDisconnectedHandlerDelegate : IMqttServerClientDisconnectedHandler
     {
-        private readonly Func<MqttServerClientDisconnectedEventArgs, Task> _handler;
+        readonly Func<MqttServerClientDisconnectedEventArgs, Task> _handler;
 
         public MqttServerClientDisconnectedHandlerDelegate(Action<MqttServerClientDisconnectedEventArgs> handler)
         {
@@ -14,7 +15,7 @@ namespace MQTTnet.Server
             _handler = eventArgs =>
             {
                 handler(eventArgs);
-                return Task.FromResult(0);
+                return PlatformAbstractionLayer.CompletedTask;
             };
         }
 

Source/MQTTnet/Server/MqttServerClientSubscribedTopicEventArgs.cs

@@ -2,24 +2,18 @@
 
 namespace MQTTnet.Server
 {
-    public class MqttServerClientSubscribedTopicEventArgs : EventArgs
+    public sealed class MqttServerClientSubscribedTopicEventArgs : EventArgs
     {
-        public MqttServerClientSubscribedTopicEventArgs(string clientId, MqttTopicFilter topicFilter)
-        {
-            ClientId = clientId ?? throw new ArgumentNullException(nameof(clientId));
-            TopicFilter = topicFilter ?? throw new ArgumentNullException(nameof(topicFilter));
-        }
-
         /// <summary>
         /// Gets the client identifier.
         /// Hint: This identifier needs to be unique over all used clients / devices on the broker to avoid connection issues.
         /// </summary>
-        public string ClientId { get; }
+        public string ClientId { get; internal set; }
 
         /// <summary>
         /// Gets the topic filter.
         /// The topic filter can contain topics and wildcards.
         /// </summary>
-        public MqttTopicFilter TopicFilter { get; }
+        public MqttTopicFilter TopicFilter { get; internal set; }
     }
 }

Source/MQTTnet/Server/MqttServerClientSubscribedTopicHandlerDelegate.cs

@@ -1,24 +1,37 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Server
 {
-    public class MqttServerClientSubscribedHandlerDelegate : IMqttServerClientSubscribedTopicHandler
+    [Obsolete("Use MqttServerClientSubscribedTopicHandlerDelegate instead. This will be removed in a future version.")]
+    public sealed class MqttServerClientSubscribedHandlerDelegate : MqttServerClientSubscribedTopicHandlerDelegate
     {
-        private readonly Func<MqttServerClientSubscribedTopicEventArgs, Task> _handler;
+        public MqttServerClientSubscribedHandlerDelegate(Action<MqttServerClientSubscribedTopicEventArgs> handler) : base(handler)
+        {
+        }
 
-        public MqttServerClientSubscribedHandlerDelegate(Action<MqttServerClientSubscribedTopicEventArgs> handler)
+        public MqttServerClientSubscribedHandlerDelegate(Func<MqttServerClientSubscribedTopicEventArgs, Task> handler) : base(handler)
+        {
+        }
+    }
+    
+    public class MqttServerClientSubscribedTopicHandlerDelegate : IMqttServerClientSubscribedTopicHandler
+    {
+        readonly Func<MqttServerClientSubscribedTopicEventArgs, Task> _handler;
+
+        public MqttServerClientSubscribedTopicHandlerDelegate(Action<MqttServerClientSubscribedTopicEventArgs> handler)
         {
             if (handler == null) throw new ArgumentNullException(nameof(handler));
 
             _handler = eventArgs =>
             {
                 handler(eventArgs);
-                return Task.FromResult(0);
+                return PlatformAbstractionLayer.CompletedTask;
             };
         }
 
-        public MqttServerClientSubscribedHandlerDelegate(Func<MqttServerClientSubscribedTopicEventArgs, Task> handler)
+        public MqttServerClientSubscribedTopicHandlerDelegate(Func<MqttServerClientSubscribedTopicEventArgs, Task> handler)
         {
             _handler = handler ?? throw new ArgumentNullException(nameof(handler));
         }

Source/MQTTnet/Server/MqttServerClientUnsubscribedTopicEventArgs.cs

@@ -2,24 +2,18 @@
 
 namespace MQTTnet.Server
 {
-    public class MqttServerClientUnsubscribedTopicEventArgs : EventArgs
+    public sealed class MqttServerClientUnsubscribedTopicEventArgs : EventArgs
     {
-        public MqttServerClientUnsubscribedTopicEventArgs(string clientId, string topicFilter)
-        {
-            ClientId = clientId ?? throw new ArgumentNullException(nameof(clientId));
-            TopicFilter = topicFilter ?? throw new ArgumentNullException(nameof(topicFilter));
-        }
-
         /// <summary>
         /// Gets the client identifier.
         /// Hint: This identifier needs to be unique over all used clients / devices on the broker to avoid connection issues.
         /// </summary>
-        public string ClientId { get; }
+        public string ClientId { get; internal set; }
 
         /// <summary>
         /// Gets or sets the topic filter.
         /// The topic filter can contain topics and wildcards.
         /// </summary>
-        public string TopicFilter { get; }
+        public string TopicFilter { get; internal set; }
     }
 }

Source/MQTTnet/Server/MqttServerClientUnsubscribedTopicHandlerDelegate.cs

@@ -1,11 +1,12 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Server
 {
-    public class MqttServerClientUnsubscribedTopicHandlerDelegate : IMqttServerClientUnsubscribedTopicHandler
+    public sealed class MqttServerClientUnsubscribedTopicHandlerDelegate : IMqttServerClientUnsubscribedTopicHandler
     {
-        private readonly Func<MqttServerClientUnsubscribedTopicEventArgs, Task> _handler;
+        readonly Func<MqttServerClientUnsubscribedTopicEventArgs, Task> _handler;
 
         public MqttServerClientUnsubscribedTopicHandlerDelegate(Action<MqttServerClientUnsubscribedTopicEventArgs> handler)
         {
@@ -14,7 +15,7 @@ namespace MQTTnet.Server
             _handler = eventArgs =>
             {
                 handler(eventArgs);
-                return Task.FromResult(0);
+                return PlatformAbstractionLayer.CompletedTask;
             };
         }
 

Source/MQTTnet/Server/MqttServerConnectionValidatorDelegate.cs

@@ -1,11 +1,12 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Server
 {
-    public class MqttServerConnectionValidatorDelegate : IMqttServerConnectionValidator
+    public sealed class MqttServerConnectionValidatorDelegate : IMqttServerConnectionValidator
     {
-        private readonly Func<MqttConnectionValidatorContext, Task> _callback;
+        readonly Func<MqttConnectionValidatorContext, Task> _callback;
 
         public MqttServerConnectionValidatorDelegate(Action<MqttConnectionValidatorContext> callback)
         {
@@ -14,7 +15,7 @@ namespace MQTTnet.Server
             _callback = context =>
             {
                 callback(context);
-                return Task.FromResult(0);
+                return PlatformAbstractionLayer.CompletedTask;
             };
         }
 

Source/MQTTnet/Server/MqttServerMultiThreadedApplicationMessageInterceptorDelegate.cs

@@ -17,7 +17,7 @@ namespace MQTTnet.Server
             _callback = context =>
             {
                 callback(context);
-                return Task.FromResult(0);
+                return PlatformAbstractionLayer.CompletedTask;
             };
         }
 

Source/MQTTnet/Server/MqttServerOptions.cs

@@ -1,4 +1,5 @@
 using System;
+using MQTTnet.Server.Internal;
 
 namespace MQTTnet.Server
 {

Source/MQTTnet/Server/MqttServerStartedHandlerDelegate.cs

@@ -1,11 +1,12 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Server
 {
-    public class MqttServerStartedHandlerDelegate : IMqttServerStartedHandler
+    public sealed class MqttServerStartedHandlerDelegate : IMqttServerStartedHandler
     {
-        private readonly Func<EventArgs, Task> _handler;
+        readonly Func<EventArgs, Task> _handler;
 
         public MqttServerStartedHandlerDelegate(Action<EventArgs> handler)
         {
@@ -14,7 +15,7 @@ namespace MQTTnet.Server
             _handler = eventArgs =>
             {
                 handler(eventArgs);
-                return Task.FromResult(0);
+                return PlatformAbstractionLayer.CompletedTask;
             };
         }
 

Source/MQTTnet/Server/MqttServerStoppedHandlerDelegate.cs

@@ -1,11 +1,12 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Server
 {
-    public class MqttServerStoppedHandlerDelegate : IMqttServerStoppedHandler
+    public sealed class MqttServerStoppedHandlerDelegate : IMqttServerStoppedHandler
     {
-        private readonly Func<EventArgs, Task> _handler;
+        readonly Func<EventArgs, Task> _handler;
 
         public MqttServerStoppedHandlerDelegate(Action<EventArgs> handler)
         {
@@ -14,7 +15,7 @@ namespace MQTTnet.Server
             _handler = eventArgs =>
             {
                 handler(eventArgs);
-                return Task.FromResult(0);
+                return PlatformAbstractionLayer.CompletedTask;
             };
         }
 

Source/MQTTnet/Server/MqttServerSubscriptionInterceptorDelegate.cs

@@ -1,11 +1,12 @@
 using System;
 using System.Threading.Tasks;
+using MQTTnet.Implementations;
 
 namespace MQTTnet.Server
 {
-    public class MqttServerSubscriptionInterceptorDelegate : IMqttServerSubscriptionInterceptor
+    public sealed class MqttServerSubscriptionInterceptorDelegate : IMqttServerSubscriptionInterceptor
     {
-        private readonly Func<MqttSubscriptionInterceptorContext, Task> _callback;
+        readonly Func<MqttSubscriptionInterceptorContext, Task> _callback;
 
         public MqttServerSubscriptionInterceptorDelegate(Action<MqttSubscriptionInterceptorContext> callback)
         {
@@ -14,7 +15,7 @@ namespace MQTTnet.Server
             _callback = context =>
             {
                 callback(context);
-                return Task.FromResult(0);
+                return PlatformAbstractionLayer.CompletedTask;
             };
         }
 

Source/MQTTnet/Server/MqttSubscriptionInterceptorContext.cs

@@ -2,20 +2,13 @@
 
 namespace MQTTnet.Server
 {
-    public class MqttSubscriptionInterceptorContext
+    public sealed class MqttSubscriptionInterceptorContext
     {
-        public MqttSubscriptionInterceptorContext(string clientId, MqttTopicFilter topicFilter, IDictionary<object, object> sessionItems)
-        {
-            ClientId = clientId;
-            TopicFilter = topicFilter;
-            SessionItems = sessionItems;
-        }
-
         /// <summary>
         /// Gets the client identifier.
         /// Hint: This identifier needs to be unique over all used clients / devices on the broker to avoid connection issues.
         /// </summary>
-        public string ClientId { get; }
+        public string ClientId { get; internal set; }
 
         /// <summary>
         /// Gets or sets the topic filter.
@@ -26,7 +19,7 @@ namespace MQTTnet.Server
         /// <summary>
         /// Gets or sets a key/value collection that can be used to share data within the scope of this session.
         /// </summary>
-        public IDictionary<object, object> SessionItems { get; }
+        public IDictionary<object, object> SessionItems { get; internal set; }
 
         public bool AcceptSubscription { get; set; } = true;
 

Source/MQTTnet/Server/MqttUnsubscriptionInterceptorContext.cs

@@ -2,32 +2,25 @@
 
 namespace MQTTnet.Server
 {
-    public class MqttUnsubscriptionInterceptorContext
+    public sealed class MqttUnsubscriptionInterceptorContext
     {
-        public MqttUnsubscriptionInterceptorContext(string clientId, string topic, IDictionary<object, object> sessionItems)
-        {
-            ClientId = clientId;
-            Topic = topic;
-            SessionItems = sessionItems;
-        }
-
         /// <summary>
         /// Gets the client identifier.
         /// Hint: This identifier needs to be unique over all used clients / devices on the broker to avoid connection issues.
         /// </summary>
-        public string ClientId { get; }
+        public string ClientId { get; internal set; }
 
         /// <summary>
         /// Gets or sets the MQTT topic.
         /// In MQTT, the word topic refers to an UTF-8 string that the broker uses to filter messages for each connected client.
         /// The topic consists of one or more topic levels. Each topic level is separated by a forward slash (topic level separator). 
         /// </summary>
-        public string Topic { get; set; }
+        public string Topic { get; internal set; }
 
         /// <summary>
         /// Gets or sets a key/value collection that can be used to share data within the scope of this session.
         /// </summary>
-        public IDictionary<object, object> SessionItems { get; }
+        public IDictionary<object, object> SessionItems { get; internal set; }
 
         public bool AcceptUnsubscription { get; set; } = true;
 

Source/MQTTnet/Server/PrepareClientSessionResult.cs

@@ -1,9 +0,0 @@
-namespace MQTTnet.Server
-{
-    public class PrepareClientSessionResult
-    {
-        public bool IsExistingSession { get; set; }
-
-        public MqttClientConnection Session { get; set; }
-    }
-}

Source/MQTTnet/Server/Status/IMqttClientStatus.cs

@@ -16,8 +16,12 @@ namespace MQTTnet.Server.Status
 
         MqttProtocolVersion ProtocolVersion { get; }
 
+        DateTime ConnectedTimestamp { get; set; }
+        
         DateTime LastPacketReceivedTimestamp { get; }
 
+        DateTime LastPacketSentTimestamp { get; set; }
+        
         DateTime LastNonKeepAlivePacketReceivedTimestamp { get; }
 
         long ReceivedApplicationMessagesCount { get; }

Source/MQTTnet/Server/Status/IMqttSessionStatus.cs

@@ -11,6 +11,9 @@ namespace MQTTnet.Server.Status
         /// </summary>
         string ClientId { get; }
 
+        /// <summary>
+        /// Gets the count of messages which are not yet sent to the client but already queued.
+        /// </summary>
         long PendingApplicationMessagesCount { get; }
 
         IDictionary<object, object> Items { get; }

Source/MQTTnet/Server/Status/MqttClientStatus.cs

@@ -2,6 +2,7 @@
 using System;
 using System.Threading.Tasks;
 using MQTTnet.Client.Disconnecting;
+using MQTTnet.Server.Internal;
 
 namespace MQTTnet.Server.Status
 {
@@ -24,9 +25,11 @@ namespace MQTTnet.Server.Status
 
         public MqttProtocolVersion ProtocolVersion { get; set; }
 
-        public DateTime LastPacketReceivedTimestamp { get; set; }
-
         public DateTime ConnectedTimestamp { get; set; }
+        
+        public DateTime LastPacketReceivedTimestamp { get; set; }
+        
+        public DateTime LastPacketSentTimestamp { get; set; }
 
         public DateTime LastNonKeepAlivePacketReceivedTimestamp { get; set; }
 
@@ -43,7 +46,7 @@ namespace MQTTnet.Server.Status
         public long BytesSent { get; set; }
 
         public long BytesReceived { get; set; }
-
+        
         public Task DisconnectAsync()
         {
             return _connection.StopAsync(MqttClientDisconnectReason.NormalDisconnection);

Source/MQTTnet/Server/Status/MqttSessionStatus.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Threading.Tasks;
+using MQTTnet.Server.Internal;
 
 namespace MQTTnet.Server.Status
 {
@@ -25,8 +26,11 @@ namespace MQTTnet.Server.Status
 
         public DateTime CreatedTimestamp { get; set; }
 
+        /// <summary>
+        /// This items can be used by the library user in order to store custom information.
+        /// </summary>
         public IDictionary<object, object> Items { get; set; }
-
+        
         public Task DeleteAsync()
         {
             return _sessionsManager.DeleteSessionAsync(ClientId);

Tests/MQTTnet.Benchmarks/TopicFilterComparerBenchmark.cs

@@ -2,6 +2,7 @@
 using BenchmarkDotNet.Jobs;
 using MQTTnet.Server;
 using System;
+using MQTTnet.Server.Internal;
 
 namespace MQTTnet.Benchmarks
 {

Tests/MQTTnet.Core.Tests/BaseTestClass.cs

@@ -0,0 +1,22 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Tests.Mockups;
+
+namespace MQTTnet.Tests
+{
+    public abstract class BaseTestClass
+    {
+        public TestContext TestContext { get; set; }
+        
+        protected TestEnvironment CreateTestEnvironment()
+        {
+            return new TestEnvironment(TestContext);
+        }
+
+        protected Task LongDelay()
+        {
+            return Task.Delay(TimeSpan.FromSeconds(1));
+        }
+    }
+}

Tests/MQTTnet.Core.Tests/MQTTv5/Client_Tests.cs

@@ -16,7 +16,7 @@ using MQTTnet.Tests.Mockups;
 namespace MQTTnet.Tests.MQTTv5
 {
     [TestClass]
-    public class Client_Tests
+    public sealed class Client_Tests
     {
         public TestContext TestContext { get; set; }
 
@@ -60,6 +60,7 @@ namespace MQTTnet.Tests.MQTTv5
                 Assert.AreEqual(2, receivedMessage.UserProperties.Count);
             }
         }
+        
         [TestMethod]
         public async Task Connect_With_AssignedClientId()
         {
@@ -116,7 +117,6 @@ namespace MQTTnet.Tests.MQTTv5
                 Assert.AreEqual("test123", serverConnectedClientId);
                 Assert.AreEqual("test123", serverDisconnectedClientId);
                 Assert.AreEqual("test123", clientAssignedClientId);
-
             }
         }
 
@@ -165,7 +165,6 @@ namespace MQTTnet.Tests.MQTTv5
 
                 Assert.AreEqual(1, result.Items.Count);
                 Assert.AreEqual(MqttClientSubscribeResultCode.GrantedQoS1, result.Items[0].ResultCode);
-
             }
         }
 

Tests/MQTTnet.Core.Tests/Mockups/TestEnvironment.cs

@@ -172,21 +172,25 @@ namespace MQTTnet.Tests.Mockups
             return StartServerAsync(new MqttServerOptionsBuilder());
         }
 
-        public async Task<IMqttServer> StartServerAsync(MqttServerOptionsBuilder options)
+        public IMqttServer CreateServer()
         {
-            if (options == null) throw new ArgumentNullException(nameof(options));
-
             if (Server != null)
             {
                 throw new InvalidOperationException("Server already started.");
             }
 
             Server = new TestServerWrapper(_mqttFactory.CreateMqttServer(ServerLogger), TestContext, this);
+            return Server;
+        }
 
+        public async Task<IMqttServer> StartServerAsync(MqttServerOptionsBuilder options)
+        {
+            CreateServer();
+            
             options.WithDefaultEndpointPort(ServerPort);
             options.WithMaxPendingMessagesPerClient(int.MaxValue);
-
+            
-            await Server.StartAsync(options.Build()).ConfigureAwait(false);
+            await Server.StartAsync(options.Build());
 
             return Server;
         }

Tests/MQTTnet.Core.Tests/MqttClient_Tests.cs

@@ -50,7 +50,7 @@ namespace MQTTnet.Tests
                     Topic = "Test"
                 }, CancellationToken.None);
                 
-                await Task.Delay(1000);
+                await Task.Delay(500);
                 
                 // This will simulate a device which closes the connection directly
                 // after sending the data so do delay is added between send and dispose!

Tests/MQTTnet.Core.Tests/MqttSubscriptionsManager_Tests.cs

@@ -5,6 +5,7 @@ using MQTTnet.Server;
 using MQTTnet.Tests.Mockups;
 using System.Collections.Concurrent;
 using System.Threading.Tasks;
+using MQTTnet.Server.Internal;
 
 namespace MQTTnet.Tests
 {

Tests/MQTTnet.Core.Tests/Server_Events_Tests.cs

@@ -0,0 +1,186 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Client;
+using MQTTnet.Client.Receiving;
+using MQTTnet.Formatter;
+using MQTTnet.Protocol;
+using MQTTnet.Server;
+
+namespace MQTTnet.Tests
+{
+    [TestClass]
+    public sealed class Server_Events_Tests : BaseTestClass
+    {
+        [TestMethod]
+        public async Task Fire_Client_Connected_Event()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServerAsync();
+
+                MqttServerClientConnectedEventArgs eventArgs = null;
+                server.ClientConnectedHandler = new MqttServerClientConnectedHandlerDelegate(e =>
+                {
+                    eventArgs = e;
+                });
+
+                await testEnvironment.ConnectClientAsync(o => o.WithCredentials("TheUser"));
+                
+                await LongDelay();
+                
+                Assert.IsNotNull(eventArgs);
+                
+                Assert.IsTrue(eventArgs.ClientId.StartsWith(nameof(Fire_Client_Connected_Event)));
+                Assert.IsTrue(eventArgs.Endpoint.Contains("::1"));
+                Assert.AreEqual(MqttProtocolVersion.V311, eventArgs.ProtocolVersion);
+                Assert.AreEqual("TheUser", eventArgs.UserName);
+            }
+        }
+
+        [TestMethod]
+        public async Task Fire_Client_Disconnected_Event()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServerAsync();
+
+                MqttServerClientDisconnectedEventArgs eventArgs = null;
+                server.ClientDisconnectedHandler = new MqttServerClientDisconnectedHandlerDelegate(e =>
+                {
+                    eventArgs = e;
+                });
+
+                var client = await testEnvironment.ConnectClientAsync(o => o.WithCredentials("TheUser"));
+                await client.DisconnectAsync();
+                
+                await LongDelay();
+                
+                Assert.IsNotNull(eventArgs);
+                
+                Assert.IsTrue(eventArgs.ClientId.StartsWith(nameof(Fire_Client_Disconnected_Event)));
+                Assert.IsTrue(eventArgs.Endpoint.Contains("::1"));
+                Assert.AreEqual(MqttClientDisconnectType.Clean, eventArgs.DisconnectType);
+            }
+        }
+        
+        [TestMethod]
+        public async Task Fire_Client_Subscribed_Event()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServerAsync();
+
+                MqttServerClientSubscribedTopicEventArgs eventArgs = null;
+                server.ClientSubscribedTopicHandler = new MqttServerClientSubscribedTopicHandlerDelegate(e =>
+                {
+                    eventArgs = e;
+                });
+
+                var client = await testEnvironment.ConnectClientAsync();
+                await client.SubscribeAsync("The/Topic", MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                await LongDelay();
+                
+                Assert.IsNotNull(eventArgs);
+                
+                Assert.IsTrue(eventArgs.ClientId.StartsWith(nameof(Fire_Client_Subscribed_Event)));
+                Assert.AreEqual("The/Topic", eventArgs.TopicFilter.Topic);
+                Assert.AreEqual(MqttQualityOfServiceLevel.AtLeastOnce, eventArgs.TopicFilter.QualityOfServiceLevel);
+            }
+        }
+        
+        [TestMethod]
+        public async Task Fire_Client_Unsubscribed_Event()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServerAsync();
+        
+                MqttServerClientUnsubscribedTopicEventArgs eventArgs = null;
+                server.ClientUnsubscribedTopicHandler = new MqttServerClientUnsubscribedTopicHandlerDelegate(e =>
+                {
+                    eventArgs = e;
+                });
+        
+                var client = await testEnvironment.ConnectClientAsync();
+                await client.UnsubscribeAsync("The/Topic");
+                
+                await LongDelay();
+                
+                Assert.IsNotNull(eventArgs);
+                
+                Assert.IsTrue(eventArgs.ClientId.StartsWith(nameof(Fire_Client_Unsubscribed_Event)));
+                Assert.AreEqual("The/Topic", eventArgs.TopicFilter);
+            }
+        }
+        
+        [TestMethod]
+        public async Task Fire_Application_Message_Received_Event()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServerAsync();
+        
+                MqttApplicationMessageReceivedEventArgs eventArgs = null;
+                server.ApplicationMessageReceivedHandler = new MqttApplicationMessageReceivedHandlerDelegate(e =>
+                {
+                    eventArgs = e;
+                });
+        
+                var client = await testEnvironment.ConnectClientAsync();
+                await client.PublishAsync("The_Topic", "The_Payload");
+                
+                await LongDelay();
+                
+                Assert.IsNotNull(eventArgs);
+                
+                Assert.IsTrue(eventArgs.ClientId.StartsWith(nameof(Fire_Application_Message_Received_Event)));
+                Assert.AreEqual("The_Topic", eventArgs.ApplicationMessage.Topic);
+                Assert.AreEqual("The_Payload", eventArgs.ApplicationMessage.ConvertPayloadToString());
+            }
+        }
+        
+        [TestMethod]
+        public async Task Fire_Started_Event()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = testEnvironment.CreateServer();
+        
+                EventArgs eventArgs = null;
+                server.StartedHandler = new MqttServerStartedHandlerDelegate(e =>
+                {
+                    eventArgs = e;
+                });
+
+                await server.StartAsync(new MqttServerOptionsBuilder().Build());
+        
+                await LongDelay();
+                
+                Assert.IsNotNull(eventArgs);
+            }
+        }
+        
+        [TestMethod]
+        public async Task Fire_Stopped_Event()
+        {
+            using (var testEnvironment = CreateTestEnvironment())
+            {
+                var server = await testEnvironment.StartServerAsync();
+        
+                EventArgs eventArgs = null;
+                server.StoppedHandler = new MqttServerStoppedHandlerDelegate(e =>
+                {
+                    eventArgs = e;
+                });
+
+                await server.StopAsync();
+        
+                await LongDelay();
+                
+                Assert.IsNotNull(eventArgs);
+            }
+        }
+    }
+}

Tests/MQTTnet.Core.Tests/Server_Status_Tests.cs

@@ -10,10 +10,8 @@ using MQTTnet.Server;
 namespace MQTTnet.Tests
 {
     [TestClass]
-    public class Server_Status_Tests
+    public sealed class Server_Status_Tests : BaseTestClass
     {
-        public TestContext TestContext { get; set; }
-
         [TestMethod]
         public async Task Show_Client_And_Session_Statistics()
         {
@@ -143,7 +141,7 @@ namespace MQTTnet.Tests
                     // At most once will send one packet to the client and the server will reply
                     // with an additional ACK packet.
                     await c1.PublishAsync("a", string.Empty, MqttQualityOfServiceLevel.AtLeastOnce);
-                    await Task.Delay(50);
+                    await Task.Delay(250);
 
                     var clientStatus = await server.GetClientStatusAsync();
 

Tests/MQTTnet.Core.Tests/Server_Tests.cs

@@ -147,7 +147,7 @@ namespace MQTTnet.Tests
         }
 
         [TestMethod]
-        public async Task Will_Message_Do_Not_Send()
+        public async Task Will_Message_Do_Not_Send_On_Clean_Disconnect()
         {
             using (var testEnvironment = new TestEnvironment(TestContext))
             {
@@ -155,7 +155,7 @@ namespace MQTTnet.Tests
 
                 await testEnvironment.StartServerAsync();
 
-                var willMessage = new MqttApplicationMessageBuilder().WithTopic("My/last/will").WithAtMostOnceQoS().Build();
+                var willMessage = new MqttApplicationMessageBuilder().WithTopic("My/last/will").Build();
 
                 var clientOptions = new MqttClientOptionsBuilder().WithWillMessage(willMessage);
 
@@ -171,6 +171,38 @@ namespace MQTTnet.Tests
                 Assert.AreEqual(0, receivedMessagesCount);
             }
         }
+        
+        [TestMethod]
+        public async Task Will_Message_Do_Not_Send_On_Takeover()
+        {
+            using (var testEnvironment = new TestEnvironment(TestContext))
+            {
+                var receivedMessagesCount = 0;
+
+                await testEnvironment.StartServerAsync();
+                
+                // C1 will receive the last will!
+                var c1 = await testEnvironment.ConnectClientAsync();
+                c1.ApplicationMessageReceivedHandler = new MqttApplicationMessageReceivedHandlerDelegate(c => Interlocked.Increment(ref receivedMessagesCount));
+                await c1.SubscribeAsync(new MqttTopicFilterBuilder().WithTopic("#").Build());
+
+                // C2 has the last will defined.
+                var willMessage = new MqttApplicationMessageBuilder().WithTopic("My/last/will").Build();
+                
+                var clientOptions = new MqttClientOptionsBuilder()
+                    .WithWillMessage(willMessage)
+                    .WithClientId("WillOwner");
+                
+                var c2 = await testEnvironment.ConnectClientAsync(clientOptions);
+                
+                // C3 will do the connection takeover.
+                var c3 = await testEnvironment.ConnectClientAsync(clientOptions);
+                
+                await Task.Delay(1000);
+
+                Assert.AreEqual(0, receivedMessagesCount);
+            }
+        }
 
         [TestMethod]
         public async Task Will_Message_Send()
@@ -571,7 +603,12 @@ namespace MQTTnet.Tests
             using (var testEnvironment = new TestEnvironment(TestContext))
             {
                 var server = await testEnvironment.StartServerAsync();
-                server.ClientConnectedHandler = new MqttServerClientConnectedHandlerDelegate(e => server.SubscribeAsync(e.ClientId, "topic1"));
+                
+                server.ClientConnectedHandler = new MqttServerClientConnectedHandlerDelegate(async e =>
+                {
+                    // Every client will automatically subscribe to this topic.
+                    await server.SubscribeAsync(e.ClientId, "topic1");
+                });
 
                 var client = await testEnvironment.ConnectClientAsync();
                 var receivedMessages = new List<MqttApplicationMessage>();
@@ -636,11 +673,11 @@ namespace MQTTnet.Tests
                 Assert.AreEqual(1, clientConnectedCalled);
                 Assert.AreEqual(0, clientDisconnectedCalled);
 
-                await Task.Delay(500);
+                await Task.Delay(1000);
 
                 await c1.DisconnectAsync();
 
-                await Task.Delay(500);
+                await Task.Delay(1000);
 
                 Assert.AreEqual(1, clientConnectedCalled);
                 Assert.AreEqual(1, clientDisconnectedCalled);

Tests/MQTTnet.Core.Tests/TopicFilterComparer_Tests.cs

@@ -1,5 +1,6 @@
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.Server;
+using MQTTnet.Server.Internal;
 
 namespace MQTTnet.Tests
 {

Tests/MQTTnet.TestApp.NetCore/Program.cs

@@ -31,6 +31,7 @@ namespace MQTTnet.TestApp.NetCore
             Console.WriteLine("a = Start QoS 2 benchmark");
             Console.WriteLine("b = Start QoS 1 benchmark");
             Console.WriteLine("c = Start QoS 0 benchmark");
+            Console.WriteLine("d = Start server with logging");
 
             var pressedKey = Console.ReadKey(true);
             if (pressedKey.KeyChar == '1')
@@ -83,6 +84,10 @@ namespace MQTTnet.TestApp.NetCore
             {
                 Task.Run(PerformanceTest.RunQoS0Test);
             }
+            else if (pressedKey.KeyChar == 'd')
+            {
+                Task.Run(ServerTest.RunEmptyServerWithLogging);
+            }
 
             Thread.Sleep(Timeout.Infinite);
         }

Tests/MQTTnet.TestApp.NetCore/ServerTest.cs

@@ -2,8 +2,10 @@
 using System.Text;
 using System.Threading.Tasks;
 using MQTTnet.Client.Receiving;
+using MQTTnet.Diagnostics;
 using MQTTnet.Protocol;
 using MQTTnet.Server;
+using MQTTnet.Server.Internal;
 
 namespace MQTTnet.TestApp.NetCore
 {
@@ -13,6 +15,19 @@ namespace MQTTnet.TestApp.NetCore
         {
             var mqttServer = new MqttFactory().CreateMqttServer();
             mqttServer.StartAsync(new MqttServerOptions()).GetAwaiter().GetResult();
+            
+            Console.WriteLine("Press any key to exit.");
+            Console.ReadLine();
+        }
+
+        public static void RunEmptyServerWithLogging()
+        {
+            var logger = new MqttNetLogger();
+            MqttNetConsoleLogger.ForwardToConsole(logger);
+           
+            var mqttFactory = new MqttFactory(logger);
+            var mqttServer = mqttFactory.CreateMqttServer();
+            mqttServer.StartAsync(new MqttServerOptions()).GetAwaiter().GetResult();
 
             Console.WriteLine("Press any key to exit.");
             Console.ReadLine();