Added support for persisted retained messages.

Source/MQTTnet.Server/Mqtt/MqttServerStorage.cs

@@ -1,27 +1,131 @@
 using System;
 using System.Collections.Generic;
+using System.IO;
+using System.Text;
+using System.Threading;
 using System.Threading.Tasks;
+using Microsoft.Extensions.Logging;
 using MQTTnet.Server.Configuration;
+using Newtonsoft.Json;
 
 namespace MQTTnet.Server.Mqtt
 {
     public class MqttServerStorage : IMqttServerStorage
     {
-        private readonly SettingsModel _settings;
+        private readonly List<MqttApplicationMessage> _messages = new List<MqttApplicationMessage>();
 
-        public MqttServerStorage(SettingsModel settings)
+        private readonly SettingsModel _settings;
+        private readonly ILogger<MqttServerStorage> _logger;
+
+        private string _filename;
+        private bool _messagesHaveChanged;
+
+        public MqttServerStorage(SettingsModel settings, ILogger<MqttServerStorage> logger)
         {
             _settings = settings ?? throw new ArgumentNullException(nameof(settings));
+            _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+        }
+
+        public void Configure()
+        {
+            if (_settings.RetainedApplicationMessages?.Persist != true)
+            {
+                _logger.LogInformation("Persisting of retained application messages is disabled.");
+                return;
+            }
+
+            _filename = ExpandFilename();
+
+            // The retained application messages are stored in a separate thread.
+            // This is mandatory because writing them to a slow storage (like RaspberryPi SD card) 
+            // will slow down the whole message processing speed.
+            Task.Run(SaveRetainedMessagesInternalAsync, CancellationToken.None);
         }
 
         public Task SaveRetainedMessagesAsync(IList<MqttApplicationMessage> messages)
         {
+            lock (_messages)
+            {
+                _messages.Clear();
+                _messages.AddRange(messages);
+
+                _messagesHaveChanged = true;
+            }
+
             return Task.CompletedTask;
         }
 
-        public Task<IList<MqttApplicationMessage>> LoadRetainedMessagesAsync()
+        private async Task SaveRetainedMessagesInternalAsync()
         {
-            return Task.FromResult<IList<MqttApplicationMessage>>(null);
+            while (true)
+            {
+                try
+                {
+                    await Task.Delay(TimeSpan.FromSeconds(_settings.RetainedApplicationMessages.WriteInterval)).ConfigureAwait(false);
+
+                    List<MqttApplicationMessage> messages;
+                    lock (_messages)
+                    {
+                        if (!_messagesHaveChanged)
+                        {
+                            continue;
+                        }
+
+                        messages = new List<MqttApplicationMessage>(_messages);
+                        _messagesHaveChanged = false;
+                    }
+
+                    var json = JsonConvert.SerializeObject(messages);
+                    await File.WriteAllTextAsync(_filename, json, Encoding.UTF8).ConfigureAwait(false);
+                    
+                    _logger.LogInformation($"{messages.Count} retained MQTT messages written.");
+                }
+                catch (Exception exception)
+                {
+                    _logger.LogError(exception, "Error while writing retained MQTT messages.");
+                }
+            }
+        }
+
+        public async Task<IList<MqttApplicationMessage>> LoadRetainedMessagesAsync()
+        {
+            if (_settings.RetainedApplicationMessages?.Persist != true)
+            {
+                return null;
+            }
+
+            if (!File.Exists(_filename))
+            {
+                return null;
+            }
+
+            try
+            {
+                var json = await File.ReadAllTextAsync(_filename).ConfigureAwait(false);
+                var applicationMessages = JsonConvert.DeserializeObject<List<MqttApplicationMessage>>(json);
+
+                _logger.LogInformation($"{applicationMessages.Count} retained MQTT messages loaded.");
+
+                return applicationMessages;
+            }
+            catch (Exception exception)
+            {
+                _logger.LogWarning(exception, "Error while loading persisted retained application messages.");
+                return null;
+            }
+        }
+
+        private string ExpandFilename()
+        {
+            var filename = _settings.RetainedApplicationMessages.Filename;
+
+            var uri = new Uri(filename, UriKind.RelativeOrAbsolute);
+            if (!uri.IsAbsoluteUri)
+            {
+                filename = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, filename);
+            }
+
+            return filename;
         }
     }
 }