Introduce RabbitMQ consumer pool for parallel processing

Enhanced RabbitMQ email processing by introducing a `SendingEmailConsumerPool` to enable the competing consumers pattern. Each consumer operates on its own channel, improving scalability and thread safety.

- Added `SendingEmailConsumerPool` to manage multiple consumers.
- Updated `DependencyInjection` to register the consumer pool.
- Refactored `SendingEmailConsumer` for better logging and error handling.
- Updated `AsyncInitWorker` to initialize the consumer pool.
- Added `ConsumerConcurrency` to RabbitMQ configuration.
- Improved error handling in `LimilabsEmailService` with detailed SMTP error messages.

src/infrastructure/DigitalData.MessagingService.Infrastructure/DependencyInjection.cs

@@ -33,7 +33,7 @@ public static class DependencyInjection
         services.AddSingleton<IEncryptionService, DataProtectionEncryptionService>();
 
         // --- Email Queue (RabbitMQ) ---
-        services.AddSingleton<SendingEmailConsumer>();
+        services.AddSingleton<SendingEmailConsumerPool>();
         services.AddMessagingServicePublisher();
 
         // --- RabbitMQ Configuration ---

src/infrastructure/DigitalData.MessagingService.Infrastructure/Queue/SendingEmailConsumer.cs

@@ -4,20 +4,19 @@ using DigitalData.MessagingService.Application.Common.Interfaces;
 using DigitalData.MessagingService.Abstraction;
 using DigitalData.MessagingService.RabbitMQ;
 using Microsoft.Extensions.Logging;
-using Microsoft.Extensions.Options;
 using RabbitMQ.Client;
 using RabbitMQ.Client.Events;
 
 namespace DigitalData.MessagingService.Infrastructure.Queue;
 
 /// <summary>
-/// RabbitMQ-based email queue implementation for outgoing emails.
-/// Provides message persistence, scalability, and reliability.
-/// Uses Lazy<T> initialization pattern to avoid blocking constructor.
+/// A single RabbitMQ consumer that processes one email message at a time on its own dedicated channel.
+/// Multiple instances run in parallel via <see cref="SendingEmailConsumerPool"/> (competing consumers pattern).
+/// Each instance owns exactly one channel — channels are not thread-safe and must not be shared.
 /// </summary>
 public sealed class SendingEmailConsumer : IAsyncDisposable
 {
-    private readonly RabbitMqConfiguration _config;
+    private readonly string _queueName;
 
     private readonly Lazy<Task<IChannel>> _lazyChannel;
 
@@ -25,15 +24,27 @@ public sealed class SendingEmailConsumer : IAsyncDisposable
 
     private readonly ILogger<SendingEmailConsumer>? _logger;
 
-    public SendingEmailConsumer(IOptions<RabbitMqConfiguration> config, IEmailService EmailService, RabbitMqConnectionFactory CnnFactory, ILogger<SendingEmailConsumer>? logger = null)
+    /// <summary>
+    /// Transient identifier assigned to this consumer instance at runtime.
+    /// A new value is generated each time the application starts or a new consumer is created.
+    /// Use this to correlate log entries belonging to the same consumer session across competing instances.
+    /// </summary>
+    public Guid RuntimeId { get; } = Guid.NewGuid();
+
+    public SendingEmailConsumer(string queueName, IEmailService emailService, RabbitMqConnectionFactory cnnFactory, ILogger<SendingEmailConsumer>? logger = null)
     {
         _logger = logger;
-        _config = config.Value;
+        _queueName = queueName;
 
-        _lazyChannel = new(CnnFactory.CreateChannelAsync);
-        _lazyInit = new(async () => {
+        _lazyChannel = new(cnnFactory.CreateChannelAsync);
+        _lazyInit = new(async () =>
+        {
             var channel = await _lazyChannel.Value;
 
+            // prefetchCount=1 ensures this consumer processes one message at a time before acking.
+            // Parallelism comes from running multiple consumer instances, not from within a single channel.
+            await channel.BasicQosAsync(prefetchSize: 0, prefetchCount: 1, global: false);
+
             var consumer = new AsyncEventingBasicConsumer(channel);
 
             consumer.ReceivedAsync += async (sender, args) =>
@@ -47,10 +58,14 @@ public sealed class SendingEmailConsumer : IAsyncDisposable
                     if (oMailEvent is not null)
                     {
                         // Send email via SMTP (SMTP config is injected in IEmailService via IOptions)
-                        await EmailService.SendEmailAsync(oMailEvent.Mail);
+                        await emailService.SendEmailAsync(oMailEvent.Mail, args.CancellationToken);
 
                         // Acknowledge message after successful processing
                         await channel.BasicAckAsync(args.DeliveryTag, false, args.CancellationToken);
+
+                        logger?.LogDebug(
+                            "Email successfully sent and acknowledged. RuntimeId={RuntimeId}, Queue={QueueName}, DeliveryTag={DeliveryTag}, To={Recipients}, Subject={Subject}, EventId={EventId}",
+                            RuntimeId, _queueName, args.DeliveryTag, oMailEvent.Mail.Recipients, oMailEvent.Mail.Subject, oMailEvent.Id);
                     }
                     else
                     {
@@ -89,19 +104,18 @@ public sealed class SendingEmailConsumer : IAsyncDisposable
 
             // Start consuming messages (event-driven, non-blocking)
             await channel.BasicConsumeAsync(
-                queue: _config.QueueName,
+                queue: _queueName,
                 autoAck: false,
                 consumer: consumer,
-                cancellationToken: CnnFactory.CancellationToken);
+                cancellationToken: cnnFactory.CancellationToken);
 
-            logger?.LogInformation("RabbitMQ consumer started for queue: {QueueName}", _config.QueueName);
+            logger?.LogInformation("RabbitMQ consumer started for queue: {QueueName}", _queueName);
         });
     }
 
     /// <summary>
-    /// Initialize RabbitMQ connection, channel, exchanges, and queues asynchronously.
-    /// Start event-driven consumer that processes messages as they arrive
-    /// Called lazily on first use via EnsureInitializedAsync.
+    /// Starts the consumer: opens a channel, sets QoS, and registers the event handler.
+    /// Called by <see cref="SendingEmailConsumerPool.InitAsync"/>.
     /// </summary>
     public async Task InitAsync()
     {

src/infrastructure/DigitalData.MessagingService.Infrastructure/Queue/SendingEmailConsumerPool.cs

@@ -0,0 +1,51 @@
+using DigitalData.MessagingService.Application.Common.Interfaces;
+using DigitalData.MessagingService.RabbitMQ;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+
+namespace DigitalData.MessagingService.Infrastructure.Queue;
+
+/// <summary>
+/// Manages a pool of <see cref="SendingEmailConsumer"/> instances that compete for messages
+/// on the same RabbitMQ queue (competing consumers pattern).
+/// Each consumer owns a dedicated channel, so they process messages fully in parallel
+/// without any shared locking or synchronization primitives.
+/// </summary>
+public sealed class SendingEmailConsumerPool : IAsyncDisposable
+{
+    private readonly List<SendingEmailConsumer> _consumers;
+    private readonly ILogger<SendingEmailConsumerPool>? _logger;
+    private readonly int _concurrency;
+
+    public SendingEmailConsumerPool(
+        IOptions<RabbitMqConfiguration> config,
+        IEmailService emailService,
+        RabbitMqConnectionFactory cnnFactory,
+        ILogger<SendingEmailConsumerPool>? logger = null,
+        ILogger<SendingEmailConsumer>? consumerLogger = null)
+    {
+        _logger = logger;
+        _concurrency = config.Value.ConsumerConcurrency;
+
+        _consumers = [.. Enumerable
+            .Range(0, _concurrency)
+            .Select(_ => new SendingEmailConsumer(config.Value.QueueName, emailService, cnnFactory, consumerLogger))];
+    }
+
+    /// <summary>
+    /// Starts all consumers in parallel. Each consumer opens its own channel and begins listening.
+    /// </summary>
+    public async Task InitAsync()
+    {
+        _logger?.LogInformation("Starting {Count} competing email consumers.", _concurrency);
+
+        await Task.WhenAll(_consumers.Select(c => c.InitAsync()));
+
+        _logger?.LogInformation("All {Count} email consumers started.", _concurrency);
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await Task.WhenAll(_consumers.Select(async c => await c.DisposeAsync().AsTask()));
+    }
+}

src/infrastructure/DigitalData.MessagingService.Infrastructure/Services/Background/AsyncInitWorker.cs

@@ -1,19 +1,18 @@
-using DigitalData.MessagingService.Application.Common.Interfaces;
 using DigitalData.MessagingService.Infrastructure.Queue;
 using Microsoft.Extensions.Hosting;
 
 namespace DigitalData.MessagingService.Infrastructure.Services.Background;
 
 /// <summary>
-/// A hosted background service responsible for initializing the outgoing email queue consumer.
+/// A hosted background service responsible for initializing the competing email consumer pool.
 /// Leverages a push-based, event-driven RabbitMQ consumer to eliminate polling overhead.
 /// Email account configuration is resolved exclusively from application settings; no database access is performed.
 /// </summary>
-public class AsyncInitWorker(SendingEmailConsumer EmailConsumer) : BackgroundService
+public class AsyncInitWorker(SendingEmailConsumerPool ConsumerPool) : BackgroundService
 {
     protected override async Task ExecuteAsync(CancellationToken stoppingToken)
     {
-        await EmailConsumer.InitAsync();
+        await ConsumerPool.InitAsync();
 
         await Task.Delay(Timeout.Infinite, stoppingToken).ConfigureAwait(ConfigureAwaitOptions.SuppressThrowing);
     }

src/infrastructure/DigitalData.MessagingService.Infrastructure/Services/LimilabsEmailService.cs

@@ -28,7 +28,7 @@ public class LimilabsEmailService(
     public async Task SendEmailAsync(EmailContext context, CancellationToken cancellationToken = default)
     {
         using var smtp = new Smtp();
-
+        ISendMessageResult? result = null;
         try
         {
             await ConnectAndAuthenticateSmtpAsync(smtp, context.Sender);
@@ -48,25 +48,24 @@ public class LimilabsEmailService(
 
             var mail = builder.Create();
 
-            var result = await smtp.SendMessageAsync(mail, cancellationToken);
+            result = await smtp.SendMessageAsync(mail, cancellationToken);
 
             if (result.Status != SendMessageStatus.Success)
             {
-                throw new InvalidOperationException($"Failed to send email. Status: {result.Status}");
+                throw new InvalidOperationException($"Failed to send email. Status: {result.Status}. {ErrorMessageBuilder(result)}");
             }
 
             await smtp.CloseAsync(cancellationToken);
-            await Task.CompletedTask; // For async consistency
         }
         catch (Limilabs.Client.ServerException ex)
         {
             await smtp.CloseSafelyAsync();
-            throw new AuthenticationFailedException("SMTP authentication failed. Check credentials or OAuth2 configuration.", ex);
+            throw new AuthenticationFailedException($"SMTP authentication failed. Check credentials or OAuth2 configuration. {ErrorMessageBuilder(result)}", ex);
         }
         catch (Exception ex)
         {
             await smtp.CloseSafelyAsync();
-            throw new InvalidOperationException("Failed to send email via SMTP server.", ex);
+            throw new InvalidOperationException($"Failed to send email via SMTP server. {ErrorMessageBuilder(result)}", ex);
         }
     }
 
@@ -92,4 +91,21 @@ public class LimilabsEmailService(
             await smtp.LoginAsync(smtpAccount.Username, password);
         }
     }
-}
+
+    private static string ErrorMessageBuilder(ISendMessageResult? result = null)
+    {
+        if(result is null || result.GeneralErrors.Count == 0)
+            return string.Empty;
+        else if(result.GeneralErrors.Count == 1)
+            return $"Error: {result.GeneralErrors.FirstOrDefault()}";
+
+        var message = new StringBuilder("Errors:\n");
+
+        foreach (var error in result.GeneralErrors)
+        {
+            message.AppendLine($"   • {error}");
+        }
+
+        return message.ToString();
+    }
+}

src/infrastructure/DigitalData.MessagingService.RabbitMQ/RabbitMqConfiguration.cs

@@ -75,5 +75,11 @@ namespace DigitalData.MessagingService.RabbitMQ
         /// Routing key used to bind <see cref="DlqQueueName"/> to <see cref="DlqExchangeName"/>.
         /// </summary>
         public string DlqRoutingKey { get; set; } = "email.outbox.dlq";
+
+        /// <summary>
+        /// Maximum number of email messages processed concurrently by the consumer.
+        /// Maps directly to RabbitMQ prefetchCount. Recommended: 3–5.
+        /// </summary>
+        public ushort ConsumerConcurrency { get; set; } = 5;
     }
 }