How to resend from Dead Letter Queue using Redis MQ?

Question

Just spent my first few hours looking at Redis and Redis MQ.

Slowly getting the hang of Redis and was wondering how you could resend a message that is in a dead letter queue?

Also, where are the configuration options which determine how many times a message is retried before it goes into the dead letter queue?

Answer 1

Currently, there's no way to automatically resend messages in the dead letter queue in ServiceStack. However, you can do this manually relatively easily:

To reload messages from the dead letter queue by using:

  public class AppHost {

      public override Configure(){

           // create the hostMq ...
           var hostMq = new RedisMqHost( clients, retryCount = 2 );

           // with retryCount = 2, 3 total attempts are made. 1st + 2 retries

           // before starting hostMq
           RecoverDLQMessages<TheMessage>(hostMq);

           // add handlers
           hostMq.RegisterHandler<TheMessage>( m => 
                     this.ServiceController.ExecuteMessage( m ) );

           // start hostMq
           hostMq.Start();
      }
  }

Which ultimately uses the following to recover (requeue) messages:

    private void RecoverDLQMessages<T>( RedisMqHost hostMq )
    {
        var client = hostMq.CreateMessageQueueClient();

        var errorQueue = QueueNames<T>.Dlq;
        log.Info( "Recovering Dead Messages from: {0}", errorQueue );
        var recovered = 0;

        byte[] msgBytes;
        while( (msgBytes = client.Get( errorQueue, TimeSpan.FromSeconds(1) )) != null )
        {
            var msg = msgBytes.ToMessage<T>();
            msg.RetryAttempts = 0;
            client.Publish( msg );
            recovered++;
        }

        log.Info( "Recovered {0} from {1}", recovered, errorQueue );
    }

Note

At the time of this writing, there's a possibility of ServiceStack losing messages. Please See Issue 229 Here, so, don't kill the process while it's moving messages from the DLQ (dead letter queue) back to the input queue. Under the hood, ServiceStack is POPing messages from Redis.