MQTTnet/Samples/Server/Server_Retained_Messages_Samples.cs

using System.Text.Json;

namespace MQTTnet.Samples.Server;

public static class Server_Retained_Messages_Samples
{
    public static async Task Persist_Retained_Messages()
    {
        /*
         * This sample starts a simple MQTT server which will store all retained messages in a file.
         */

        var storePath = Path.Combine(Path.GetTempPath(), "RetainedMessages.json");

        var mqttFactory = new MqttFactory();

        // Due to security reasons the "default" endpoint (which is unencrypted) is not enabled by default!
        var mqttServerOptions = mqttFactory.CreateServerOptionsBuilder().WithDefaultEndpoint().Build();

        using (var server = mqttFactory.CreateMqttServer(mqttServerOptions))
        {
            // Make sure that the server will load the retained messages.
            server.LoadingRetainedMessageAsync += async eventArgs =>
            {
                try
                {
                    eventArgs.LoadedRetainedMessages = await JsonSerializer.DeserializeAsync<List<MqttApplicationMessage>>(File.OpenRead(storePath));
                    Console.WriteLine("Retained messages loaded.");
                }
                catch (FileNotFoundException)
                {
                    // Ignore because nothing is stored yet.
                    Console.WriteLine("No retained messages stored yet.");
                }
                catch (Exception exception)
                {
                    Console.WriteLine(exception);
                }
            };

            // Make sure to persist the changed retained messages.
            server.RetainedMessageChangedAsync += async eventArgs =>
            {
                try
                {
                    // This sample uses the property _StoredRetainedMessages_ which will contain all(!) retained messages.
                    // The event args also contain the affected retained message (property ChangedRetainedMessage). This can be
                    // used to write all retained messages to dedicated files etc. Then all files must be loaded and a full list
                    // of retained messages must be provided in the loaded event.

                    var buffer = JsonSerializer.SerializeToUtf8Bytes(eventArgs.StoredRetainedMessages);
                    await File.WriteAllBytesAsync(storePath, buffer);
                    Console.WriteLine("Retained messages saved.");
                }
                catch (Exception exception)
                {
                    Console.WriteLine(exception);
                }
            };

            // Make sure to clear the retained messages when they are all deleted via API.
            server.RetainedMessagesClearedAsync += _ =>
            {
                File.Delete(storePath);
                return Task.CompletedTask;
            };

            await server.StartAsync();

            Console.WriteLine("Press Enter to exit.");
            Console.ReadLine();
        }
    }
}