Monday, February 28, 2011

Akka Producer Actors: New Features and Best Practices

In a previous post I wrote about new features and best practices for Akka consumer actors. In this post, I'll cover Akka producer actors. For the following examples to compile and run, you'll need the current Akka 1.1-SNAPSHOT.

Again, I assume that you already have a basic familiarity with Akka, Apache Camel and the akka-camel integration module. If you are new to it, you may want to read the Akka and Camel chapter (free pdf) of the Camel in Action book or the Introduction section of the official akka-camel documentation first.

Basic usage

Akka producer actors can send messages to any Camel endpoint, provided that the corresponding Camel component is on the classpath. This allows Akka actors to interact with external systems or other components over a large number of protocols and APIs.

Let's start with a simple producer actor that sends all messages it receives to an external HTTP service and returns the response to the initial sender. For sending messages over HTTP we can use the Camel jetty component which features an asynchronous HTTP client.

Concrete producer actors inherit a default implementation of Actor.receive from the Producer trait. For simple use cases, only an endpoint URI must be defined. Producer actors also require a started CamelContextManager for working properly. A CamelContextManager is started when an application starts a CamelService e.g. via CamelServiceManager.startCamelService or when starting the CamelContextManager directly via

The latter approach is recommended when an application uses only producer actors but no consumer actors. This slightly reduces the overhead when starting actors. After starting the producer actor, clients can interact with the HTTP service via the actor API.

Here, !! is used for sending the message and waiting for a response. Alternatively, one can also use ! together with an implicit sender reference.

In this case the sender will receive an asynchronous reply from the producer actor. Before, the producer actor itself receives an asynchronous reply from the jetty endpoint. The asynchronous jetty endpoint doesn't block a thread waiting for a response and the producer actor doesn't do that either. This is important from a scalability perspective, especially for longer-running request-response cycles.

By default, a producer actor initiates an in-out message exchange with its Camel endpoint i.e. it expects a response from it. If a producer actor wants to initiate an in-only message exchange then it must override the oneway method to return true. The following example shows a producer actor that initiates an in-only message exchange with a JMS endpoint.

This actor adds any message it receives to the test JMS queue. By default, producer actors that are configured with oneway = true don't reply. This behavior is defined in the Producer.receiveAfterProduce method which is implemented as follows.

The receiveAfterProduce method has the same signature as Actor.receive and is called with the result of the message exchange with the endpoint (please note that in-only message exchanges with Camel endpoints have a result as well). The result type for successful message exchanges is Message, for failed message exchanges it is Failure (see below).

Concrete producer actors can override this method. For example, the following producer actor overrides onReceiveAfterProduce to reply with a constant "done" message.

The result of the message exchange with the JMS endpoint is ignored (case _).


Messages exchanges with a Camel endpoint can fail. In this case, onReceiveAfterProduce is called with a Failure message containing the cause of the failure (a Throwable). Let's extend the HttpProducer usage example to deal with failure responses.

In addition to a failure cause, a Failure message can also contain endpoint-specific headers with failure details such as the HTTP response code, for example. When using ! instead of !!, together with an implicit sender reference (as shown in the previous section), that sender will then receive the Failure message asynchronously. The JmsReplyingProducer example can also be extended to return more meaningful responses: a "done" message only on success and an error message on failure.

Failed message exchanges never cause the producer actor to throw an exception during execution of receive. Should Producer implementations want to throw an exception on failure (for whatever reason) they can do so in onReceiveAfterProduce.

In this case failure handling should be done in combination with a supervisor (see below).

Let's look at another example. What if we want

to throw an exception on failure (instead of returning a Failure message) but to respond with a normal Message on success? In this case, we need to use self.senderFuture inside onReceiveAfterProduce and complete it with an exception.

Forwarding results

Another option to deal with message exchange results inside onReceiveAfterProduce is to forward them to another actor. Forwarding a message also forwards the initial sender reference. This allows the receiving actor to reply to the initial sender.

With producer actors that forward message exchange results to other actors (incl. other producer actors) one can build actor-based message processing pipelines that integrate external systems. In combination with consumer actors, this could be extended towards a scalable and distributed enterprise service bus (ESB) based on Akka actors ... but this is a topic for another blog post.

Correlation identifiers

The Producer trait also supports correlation identifiers. This allows clients to correlate request messages with asynchronous response messages. A correlation identifier is a message header that can be set by clients. The following example uses the correlation identifier (or message exchange identifier) 123.

An asynchronous response (Message or Failure) from httpProducer will contain that correlation identifier as well.


A failed message exchange by default does not cause a producer actor to throw an exception. However, concrete producer actors may decide to throw an exception inside onReceiveAfterProduce, for example, or there can be a system-level Camel problem that causes a runtime exception. An application that wants to handle these exceptions should supervise its producer actors.

The following example shows how to implement a producer actor that replies to the initial sender with a Failure message when it is restarted or stopped by a supervisor.

To handle restart callbacks, producer actors must override the preRestartProducer method instead of preRestart. The preRestart method is implemented by the Producer trait and does additional resource de-allocation work after calling preRestartProducer. More information about replies within preRestart and postStop can be found in my previous blog post about consumer actors.

Thursday, February 17, 2011

Akka Consumer Actors: New Features and Best Practices

In this blog post I want to give some guidance how to implement consumer actors with the akka-camel module. Besides basic usage scenarios, I will also explain how to make consumer actors fault-tolerant, redeliver messages on failure, deal with bounded mailboxes etc. The code examples shown below require the current Akka 1.1-SNAPSHOT to compile and run.

In the following, I assume that you already have a basic familiarity with Akka, Apache Camel and the akka-camel integration module. If you are new to it, you may want to read the Akka and Camel chapter (free pdf) of the Camel in Action book or the Introduction section of the official akka-camel documentation first.

Basic usage

Akka consumer actors can receive messages from any Camel endpoint, provided that the corresponding Camel component is on the classpath. This allows clients to interact with Akka actors over a large number of protocols and APIs.

Camel endpoints either initiate in-only (one-way) message exchanges with consumer actors or in-out (two-way) message exchanges. Replies from consumer actors are mandatory for in-out message exchanges but optional for in-only message exchanges. For replying to a Camel endpoint, the consumer actor uses the very same interface as for replying to any other sender (e.g. to another actor). Examples are self.reply or self.reply_?.

Let's start by defining a simple consumer actor that accepts messages via tcp on port 6200 and replies to the tcp client (tcp support is given by Camel's mina component).

For consumer actors to work, applications need to start a CamelService which is managed by the CamelServiceManager.

When starting a consumer actor, the endpoint defined for that actor will be activated asynchronously by the CamelService. If your application wants to wait for consumer endpoints to be finally activated you can do so with the awaitEndpointActivation method (which is especially useful for testing).

For sending a test message to the consumer actor, the above code uses a Camel ProducerTemplate which can be obtained from the CamelContextManager.

If Camel endpoints, such as the file endpoint, create in-only message exchanges then consumer actors need not reply, by default. The message exchange is completed once the input message has been added to the consumer actor's mailbox.

When placing a file into the data/input directory, the Camel file endpoint will pick up that file and send its content as message to the consumer actor. Once the message is in the actor's mailbox, the file endpoint will delete the corresponding file (see delete=true in the endpoint URI).

If you want to let the consumer actor decide when the file should be deleted, then you'll need to turn auto-acknowledgements off as shown in the following example (autoack = false). In this case the consumer actor must reply with a special Ack message when message processing is done. This asynchronous reply finally causes the file endpoint to delete the consumed file.

Turning auto-acknowledgements on and off is only relevant for in-only message exchanges because, for in-out message exchanges, consumer actors need to reply in any case with an (application-specific) message. Consumer actors may also reply with a Failure message to indicate a processing failure. Failure replies can be made for both in-only and in-out message exchanges. A Failure reply can be done inside receive method but there are better ways as shown in the next sections.

Fault-tolerance and message redelivery

Message processing inside receive may throw exceptions which usually requires a failure response to Camel (i.e. to the consumer endpoint). This is done with a Failure message that contains the failure reason (an instance of Throwable). Instead of catching and handling the exception inside receive, consumer actors should be part of supervisor hierarchies and send failure responses from within restart callback methods. Here's an example of a fault-tolerant file consumer.

The above file consumer overrides the preRestart and postStop callback methods to send reply messages to Camel. A reply within preRestart and postStop is possible after receive has thrown an exception (new feature since Akka 1.1). When receive returns normally it is expected that any necessary reply has already been done within receive.
  • If the lifecycle of the SupervisedFileConsumer is configured to be PERMANENT, a supervisor will restart the consumer upon failure with a call to preRestart. Within preRestart a Failure reply is sent which causes the file endpoint to redeliver the content of the consumed file and the consumer actor can try to process it again. Should the processing succeed in a second attempt, an Ack is sent within receive. A reply within preRestart must be a safe reply via self.reply_? because an unsafe self.reply will throw an exception when the consumer is restarted without having failed. This can be the case in context of all-for-one restart strategies.
  • If the lifecycle of the SupervisedFileConsumer is configured to be TEMPORARY, a supervisor will shut down the consumer upon failure with a call to postStop. Within postStop an Ack is sent which causes the file endpoint to delete the file. One can, of course, choose to reply with a Failure here, so that files that couldn't be processed successfully are kept in the input directory. A reply within postStop must be a safe reply via self.reply_? because an unsafe self.reply will throw an exception when the consumer has been stopped by the application (and not by a supervisor) after successful execution of receive.

Another frequently discussed consumer actor example is a fault-tolerant JMS consumer. A JMS consumer actor should acknowledge a message receipt upon successful message processing and trigger a message redelivery on failure. This is exactly the same pattern as described for the SupervisedFileConsumer above. You just need to change the file endpoint URI to a jms or activemq endpoint URI and you're done (of course, you additionally need to configure the JMS connection with a redelivery policy and, optionally, use transacted queues. An explanation how to do this would however exceed the scope of this blog post).

Simplifications and tradeoffs with blocking=true

In all the examples so far the internally created Camel routes use the ! (bang) operator to send the input message to the consumer actor. This means that the Camel route does not block a thread waiting for a response. It's an asynchronous reply will cause the Camel route to resume processing. That's also the reason why any exception thrown by receive isn't reported back to Camel directly but must be done explicitly with a Failure response.

If you want that exceptions thrown by receive are reported back to Camel directly (i.e. without sending Failure responses) then you'll need to set blocking = true for the consumer actor. This causes the Camel route to send the input message with the !! (bangbang) operator and to wait for a response. However, this will block a thread until the consumer sends a response or throws an exception within receive. The advantage of this approach is that error handling is strongly simplified in this case but scalability will likely decrease.

Here's an example of a consumer actor that uses the simplified approach to error handling. Any exception thrown by receive will still cause the file endpoint to redeliver the message but a thread will be blocked by Camel during the execution of receive.

No supervisor is needed here. It depends on the non-functional requirements of your application whether to go for this simple but blocking approach or to use a more scalable, non-blocking approach in combination with a supervisor.

Bounded mailboxes and error handling with custom Camel routes

For consumer actors that require a significant amount of time for processing a single message, it can make sense to install a bounded mailbox. A bounded mailbox throws an exception if its capacity is reached and the Camel route tries to add additional messages to the mailbox. Here's an example of a file consumer actor that uses a bounded mailbox with a capacity of 5. Processing is artificially delayed by 1 second using a Thread.sleep.

When, let's say, 10 files are put into the data/input directory, they will be picked up by the file endpoint and added to the actor's mailbox. The capacity of the mailbox will be reached soon because the file endpoint can send messages much faster than the consumer actor can process it. Exceptions thrown by the mailbox are directly reported to the Camel route which causes the file consumer to redeliver messages until they can be added to the mailbox. The same applies to JMS and other endpoints that support redelivery.

When dealing with endpoints that do not support redelivery, one needs to customize the Camel route to the consumer actor with a special error handler that does the redelivery. This is shown for a consumer actor that consumes messages from a direct endpoint.

Here we use onRouteDefinition to define how the Camel route should be customized during its creation. In this example, an error handler is defined that attempts max. 3 redeliveries with a delay of 1000 ms. For details refer to the intercepting route construction section in the akka-camel documentation. When using a producer template to send messages to this endpoint, some of them will be added to the mailbox on first attempt, some of them after a second attempt triggered by the error handler.

The examples presented in this post cover many of the consumer-actor-related questions and topics that have been asked and discussed on the akka-user mailing list. In another post I plan to cover best practices for implementing Akka producer actors.