How to do error handling with EasyNetQ / RabbitMQ

I've implemented exactly what you describe. Here are some tips based on my experience and related to each of your questions.

Q1 (how to retry X times):

For this, you can use IMessage.Body.BasicProperties.Headers. When you consume a message off an error queue, just add a header with a name that you choose. Look for this header on each message that comes into the error queue and increment it. This will give you a running retry count.

It's very important that you have a strategy for what to do when a message exceeds the retry limit of X. You don't want to lose that message. In my case, I write the message to disk at that point. It gives you lots of helpful debugging information to come back to later, because EasyNetQ automatically wraps your originating message with error info. It also has the original message so that you can, if you like, manually (or maybe automated, through some batch re-processing code) requeue the message later in some controlled way.

You can look at the code in the Hosepipe utility to see a good way of doing this. In fact, if you follow the pattern you see there then you can even use Hosepipe later to requeue the messages if you need to.

Q2 (how to create an error queue per originating queue):

You can use the EasyNetQ Advanced Bus to do this cleanly. Use IBus.Advanced.Container.Resolve<IConventions> to get at the conventions interface. Then you can set the conventions for the error queue naming with conventions.ErrorExchangeNamingConvention and conventions.ErrorQueueNamingConvention. In my case I set the convention to be based on the name of the originating queue so that I get a queue/queue_error pair of queues every time I create a queue.

Q3 (how to process messages in the error queues):

You can declare a consumer for the error queue the same way you do any other queue. Again, the AdvancedBus lets you do this cleanly by specifying that the type coming off of the queue is EasyNetQ.SystemMessage.Error. So, IAdvancedBus.Consume<EasyNetQ.SystemMessage.Error>() will get you there. Retrying simply means republishing to the original exchange (paying attention to the retry count you put in the header (see my answer to Q1, above), and information in the Error message that you consumed off the error queue can help you find the target for republishing.

I know this is an old post but - just in case it helps someone else - here is my self-answered question (I needed to ask it because existing help was not enough) that explains how I implemented retrying failed messages on their original queues. The following should answer your question #1 and #3. For #2, you may have to use the Advanced API, which I haven't used (and I think it defeats the purpose of EasyNetQ; one might as well use RabbitMQ client directly). Also consider implementing IConsumerErrorStrategy, though.

1) Since there can be multiple consumers of a message and all may not need to retry a msg, I have a Dictionary<consumerId, RetryInfo> in the body of the message, as EasyNetQ does not (out of the box) support complex types in message headers.

public interface IMessageType
{
    int MsgTypeId { get; }

    Dictionary<string, TryInfo> MsgTryInfo {get; set;}

}

2) I have implemented a class RetryEnabledErrorMessageSerializer : IErrorMessageSerializer that just updates the TryCount and other information every time it is called by the framework. I attach this custom serializer to the framework on a per-consumer basis via the IoC support provided by EasyNetQ.

 public class RetryEnabledErrorMessageSerializer<T> : IErrorMessageSerializer where T : class, IMessageType
 {
        public string Serialize(byte[] messageBody)
        {
             string stringifiedMsgBody = Encoding.UTF8.GetString(messageBody);
             var objectifiedMsgBody = JObject.Parse(stringifiedMsgBody);

             // Add/update RetryInformation into objectifiedMsgBody here
             // I have a dictionary that saves <key:consumerId, val: TryInfoObj>

             return JsonConvert.SerializeObject(objectifiedMsgBody);
        }
  }

And in my EasyNetQ wrapper class:

    public void SetupMessageBroker(string givenSubscriptionId, bool enableRetry = false)
    {
        if (enableRetry)
        {
            _defaultBus = RabbitHutch.CreateBus(currentConnString,
                                                        serviceRegister => serviceRegister.Register<IErrorMessageSerializer>(serviceProvider => new RetryEnabledErrorMessageSerializer<IMessageType>(givenSubscriptionId))
                                                );
        }
        else // EasyNetQ's DefaultErrorMessageSerializer will wrap error messages
        {
            _defaultBus = RabbitHutch.CreateBus(currentConnString);
        }
    }

    public bool SubscribeAsync<T>(Func<T, Task> eventHandler, string subscriptionId)
    {
        IMsgHandler<T> currMsgHandler = new MsgHandler<T>(eventHandler, subscriptionId);
        // Using the msgHandler allows to add a mediator between EasyNetQ and the actual callback function
        // The mediator can transmit the retried msg or choose to ignore it
        return _defaultBus.SubscribeAsync<T>(subscriptionId, currMsgHandler.InvokeMsgCallbackFunc).Queue != null;
    }

3) Once the message is added to the default error queue, you can have a simple console app/windows service that periodically republishes existing error messages on their original queues. Something like:

var client = new ManagementClient(AppConfig.BaseAddress, AppConfig.RabbitUsername, AppConfig.RabbitPassword);
var vhost = client.GetVhostAsync("/").Result;
var aliveRes = client.IsAliveAsync(vhost).Result;
var errQueue = client.GetQueueAsync(Constants.EasyNetQErrorQueueName, vhost).Result;
var crit = new GetMessagesCriteria(long.MaxValue, Ackmodes.ack_requeue_false);
var errMsgs = client.GetMessagesFromQueueAsync(errQueue, crit).Result;
foreach (var errMsg in errMsgs)
{
    var innerMsg = JsonConvert.DeserializeObject<Error>(errMsg.Payload);
    var pubInfo = new PublishInfo(innerMsg.RoutingKey, innerMsg.Message);
    pubInfo.Properties.Add("type", innerMsg.BasicProperties.Type);
    pubInfo.Properties.Add("correlation_id", innerMsg.BasicProperties.CorrelationId);
    pubInfo.Properties.Add("delivery_mode", innerMsg.BasicProperties.DeliveryMode);
    var pubRes = client.PublishAsync(client.GetExchangeAsync(innerMsg.Exchange, vhost).Result, pubInfo).Result;
}

4) I have a MessageHandler class that contains a callback func. Whenever a message is delivered to the consumer, it goes to the MessageHandler, which decides if the message try is valid and calls the actual callback if so. If try is not valid (maxRetriesExceeded/the consumer does not need to retry anyway), I ignore the message. You can choose to Dead Letter the message in this case.

public interface IMsgHandler<T> where T: class, IMessageType
{
    Task InvokeMsgCallbackFunc(T msg);
    Func<T, Task> MsgCallbackFunc { get; set; }
    bool IsTryValid(T msg, string refSubscriptionId); // Calls callback only 
                                                      // if Retry is valid
}

Here is the mediator function in MsgHandler that invokes the callback:

    public async Task InvokeMsgCallbackFunc(T msg)
    {
        if (IsTryValid(msg, CurrSubscriptionId))
        {
            await this.MsgCallbackFunc(msg);
        }
        else
        {
            // Do whatever you want
        }
    }

The problem you are running into with EasyNetQ/RabbitMQ is that it's much more "raw" when compared to other messaging services like SQS or Azure Service Bus/Queues, but I'll do my best to point you in the right direction.

Question 1.

This will be on you to do. The simplest way is that you can No-Ack a message in RabbitMQ/EasyNetQ, and it will be placed at the head of the queue for you to retry. This is not really advisable because it will be retried almost immediately (With no time delay), and will also block other messages from being processed (If you have a single subscriber with a prefetch count of 1).

I've seen other implementations of using a "MessageEnvelope". So a wrapper class that when a message fails, you increment a retry variable on the MessageEnvelope and redeliver the message back onto the queue. YOU would have to do this and write the wrapping code around your message handlers, it would not be a function of EasyNetQ.

Using the above, I've also seen people use envelopes, but allow the message to be dead lettered. Once it's on the dead letter queue, there is another application/worker reading items from the dead letter queue.

All of these approaches above have a small issue in that there isn't really any nice way to have a logarithmic/exponential/any sort of increasing delay in processing the message. You can "hold" the message in code for some time before returning it to the queue, but it's not a nice way around.

Out of all of these options, your own custom application reading the dead letter queue and deciding whether to reroute the message based on an envelope that contains the retry count is probably the best way.

Question 2.

You can specify a dead letter exchange per queue using the advanced API. (https://github.com/EasyNetQ/EasyNetQ/wiki/The-Advanced-API#declaring-queues). However this means you will have to use the advanced API pretty much everywhere as using the simple IBus implementation of subscribe/publish looks for queues that are named based on both the message type and subscriber name. Using a custom declare of queue means you are going to be handling the naming of your queues yourself, which means when you subscribe, you will need to know the name of what you want etc. No more auto subscribing for you!

Question 3

An Error Queue/Dead Letter Queue is just another queue. You can listen to this queue and do what you need to do with it. But there is not really any out of the box solution that sounds like it would fit your needs.