Async

Available as of Camel 2.0

The asynchronous API in Camel have been rewritten for Camel 2.0, and the information on this page applies for Camel 2.0 and later.

The Async API in Camel is primarily divided in two areas
1. Initiating an Async messaging from the client
2. Turning a route into Async using the threads DSL

Before we look at these two areas we start with a bit of background information and looks at the concept from at a higher level using diagrams.
Then we check out the first area how a client can initiate an Async message exchange and we also throw in the synchronous message exchange in the mix as well so we can compare and distill the difference.
And finally we turn our attention towards the last area the new threads DSL and what it can be used for.

Background

The new Async API in Camel 2.0 leverages in much greater detail the Java Concurrency API and its support for executing tasks asynchronous.
Therefore the Camel Async API should be familiar for users with knowledge of the Java Concurrency API.

A few concepts to master

When doing messaging there are a few aspects to keep in mind.

First of all a caller can initiate a message exchange as either:

Request only is when the caller sends a message but do not expect any reply. This is also known as fire and forget or event message.

The Request Reply is when the caller sends a message and then waits for a reply. This is like the HTTP protocol that we use every day when we surf the web.
We send a request to fetch a web page and wait until the reply message comes with the web content.

In Camel a message is labeled with a Message Exchange Pattern that labels whether its a request only or request reply message. Camel uses the JBI term for this an uses InOnly for the request only, and InOut for the request reply.

For all message exchange they can be executed either:

  • synchronous
  • asynchronous

Synchronous Request Reply

A synchronous exchange is defined as the caller sends a message and waits until its complete before continuing. This is illustrated in the diagram below:

1. The client sends a sync Request Reply message over HTTP to Camel. The client application will wait for the response that Camel routes and processes.
2. The message invokes an external TCP service using synchronous Request Reply. The client application still waits for the response.
3. The response is send back to the client.

Asynchronous Request Reply

On the other hand the asynchronous version is where the caller sends a message to an Endpoint and then returns immediately back to the caller. The message however is processed in another thread, the asynchronous thread. Then the caller can continue doing other work and at the same time the asynchronous thread is processing the message. This is illustrated in the diagram below:

1. The client sends an Async Request Reply message over HTTP to Camel. The control is immediately returned to the client application, that can continue and do other work while Camel routes the message.
2. Camel invokes an external TCP service using synchronous Request Reply. The client application can do other work simultaneously.
3. The client wants to get the reply so it uses the Future handle it got as response from step 1. With this handle it retrieves the reply, wait if nessasary if the reply is not ready.

Synchronous Request Only

You can also do synchronous Request only with Camel. The client sends a message to Camel in which a reply is not expected. However the client still waits until the message is processed completely. This is illustrated in the diagram below:

1. The client sends a Request only and we can still use HTTP despite http being Request Reply by nature.
2. Camel invokes an external TCP service using synchronous Request Reply. The client application is still waiting.
3. The message is processed completely and the control is returned to the client.

So why do you want to use synchronous Request Only? Well if you want to know whether the message was processed successfully or not before continuing. With synchronous it allows you to wait while the message is being processed. In case the processing was succesful the control is returned to the client with no notion of error. In case of failure the client can detect this as an exception is thrown. (and exchange.isFailed() returns true).

Asynchronous Request Only

As opposed to the synchronous Request Only the Async counter part will not wait for the processing of the message to complete. In this case the client can immediately continue doing other work while the message is being routed and processed in Camel. This is illustrated in the diagram below:

1. The client sends a Request only and we can still use HTTP despite http being Request Reply by nature. The control is immediately returned to the client application, that can continue and do other work while Camel routes the message.
2. Camel invokes an external TCP service using synchronous Request Reply. The client application can do other work simultaneously.
3. The message completes but no result is returned to the client.

Notice: As Camel always returns a Future handle for Async messaging to the client. The client can use this handler to get hold of the status of the processing whether the task is complete or an Exception occured during processing. Note that the client is not required to do so, its perfect valid to just ignore the Future handle.

Icon

In case you want to know whether the Async Request Only failed, then you can use the Future handle and invoke get() and if it throws a ExecutionException then the processing failed. The caused exception is wrapped. You can invoke isDone() first to test whether the task is done or still in progress. Otherwise invoking get() will wait until the task is done.

With these diagrams in mind lets turn out attention to the Async API and how to use it with Camel.

1) The Async Client API

Camel provides the Async Client API in the ProducerTemplate where we have added about 10 new methods to Camel 2.0. We have listed the most important in the table below:

Method

Returns

Description

setExecutorService

void

Is used to set the Java ExecutorService. Camel will by default provide a ScheduledExecutorService with 5 thread in the pool.

asyncSend

Future<Exchange>

Is used to send an async exchange to a Camel Endpoint. Camel will imeddiately return control to the caller thread after the task has been submitted to the executor service. This allows you to do other work while Camel processes the exchange in the other async thread.

asyncSendBody

Future<Object>

As above but for sending body only. This is a request only messaging style so no reply is expected. Uses the InOnly exchange pattern.

asyncRequestBody

Future<Object>

As above but for sending body only. This is a Request Reply messaging style so a reply is expected. Uses the InOut exchange pattern.

extractFutureBody

T

Is used to get the result from the asynchronous thread using the Java Concurrency Future handle.

The asyncSend and asyncRequest methods return a Future handle. This handle is what the caller must use later to retrieve the asynchronous response. You can do this by using the extractFutureBody method, or just use plain Java but invoke get() on the Future handle.

The Async Client API with callbacks

In addition to the Client API from above Camel provides a variation that uses callbacks when the message Exchange is done.

Method

Returns

Description

asyncCallback

Future<Exchange>

In addition a callback is passed in as a parameter using the org.apache.camel.spi.Synchronization Callback. The callback is invoked when the message exchange is done.

asyncCallbackSendBody

Future<Object>

As above but for sending body only. This is a request only messaging style so no reply is expected. Uses the InOnly exchange pattern.

asyncCallbackRequestBody

Future<Object>

As above but for sending body only. This is a Request Reply messaging style so a reply is expected. Uses the InOut exchange pattern.

These methods also returns the Future handle in case you need them. The difference is that they invokes the callback as well when the Exchange is done being routed.

The Future API

The java.util.concurrent.Future API have among others the following methods:

Method

Returns

Description

isDone

boolean

Returns a boolean whether the task is done or not. Will even return true if the tasks failed due to an exception thrown.

get()

Object

Gets the response of the task. In case of an exception was thrown the java.util.concurrent.ExecutionException is thrown with the caused exception.

Example: Asynchronous Request Reply

Suppose we want to call a HTTP service but it is usually slow and thus we do not want to block and wait for the response, as we can do other important computation. So we can initiate an Async exchange to the HTTP endpoint and then do other stuff while the slow HTTP service is processing our request. And then a bit later we can use the Future handle to get the response from the HTTP service. Yeah nice so lets do it:

First we define some routes in Camel. One for the HTTP service where we simulate a slow server as it takes at least 1 second to reply. And then other route that we want to invoke while the HTTP service is on route. This allows you to be able to process the two routes simultaneously:

And then we have the client API where we call the two routes and we can get the responses from both of them. As the code is based on unit test there is a bit of mock in there as well:

All together it should give you the basic idea how to use this Async API and what it can do.

Example: Synchronous Request Reply

This example is just to a pure synchronous version of the example from above that was Async based.

The route is the same, so its just how the client initiate and send the messages that differs:

Using the Async API with callbacks

Suppose we want to call a HTTP service but it is usually slow and thus we do not want to block and wait for the response, but instead let a callback gather the response. This allows us to send multiple requests without waiting for the replies before we can send the next request.

First we define a route in Camel for the HTTP service where we simulate a slow server as it takes at least 1 second to reply.

Then we define our callback where we gather the responses. As this is based on an unit test it just gathers the responses in a list. This is a shared callback we use for every request we send in, but you can use your own individual or use an anonymous callback. The callback supports different methods, but we use onDone that is invoked regardless if the Exchange was processed successfully or failed. The org.apache.camel.spi.Synchronization API provides fine grained methods for onCompletion and onFailure for the two situations.

And then we have the client API where we call the HTTP service using asyncCallback 3 times with different input. As the invocation is Async the client will send 3 requests right after each other, so we have 3 concurrent exchanges in progress. The response is gathered by our callback so we do not have to care how to get the response.

Using the Async API with the Camel classic API

When using the Camel API to create a producer and send an Exchange we do it like this:

But to do the same with Async we need a little help from a helper class, so the code is:

2) Using the Threads DSL

In Camel 2.0 the threads DSL replaces the old thread DSL.

Camel 2.0 to 2.3 behavior

The threads DSL leverages the JDK concurrency framework for multi threading. It can be used to turn a synchronous route into Async. What happens is that from the point forwards from threads the messages is routed asynchronous in a new thread. The caller will either wait for a reply if a reply is expected, such as when we use Request Reply messaging. Or the caller will complete as well if no reply was expected such as Request Only messaging.

Camel 2.4 onwards behavior

The threads DSL leverages the JDK concurrency framework for multi threading. It can be used to turn a synchronous route into Async. What happens is that from the point forwards from threads the messages is routed asynchronous in a new thread. Camel leverages the asynchronous routing engine, which was re-introduced in Camel 2.4, to continue routing the Exchange asynchronously.

The threads DSL supports the following options:

Option

Description

poolSize

A number to indicate the core pool size of the underlying Java ExecutorService that is actually doing all the heavy lifting of handling Async tasks and correlate replies etc. By default a pool size of 10 is used.

maxPoolSize

A number to indicate the maximum pool size of the of the underlying Java ExecutorService

keepAliveTime

A number to indicate how long to keep inactive threads alive

timeUnit

Time unit for the keepAliveTime option

maxQueueSize

A number to indicate the maximum number of tasks to keep in the worker queue for the underlying Java ExecutorService

threadName

To use a custom thread name pattern. See Threading Model for more details.

rejectedPolicy

How to handle rejected tasks. Can be either Abort, CallerRuns, Discard, or DiscardOldest. See below for more details.

callerRunsWhenRejected

A boolean to more easily configure between the most common rejection policies. This option is default enabled. true is the same as rejectedPolicy=CallerRuns, and false is the same as rejectedPolicy=Abort.

executorService

You can provide a custom ExecutorService to use, for instance in a managed environment a J2EE container could provide this service so all thread pools is controlled by the J2EE container.

executorServiceRefYou can provide a named reference to the custom ExecutorService from the Camel registry. Keep in mind that reference to the custom executor service cannot be used together with the executor-related options (like poolSize or maxQueueSize) as referenced executor service should be configured already.

waitForTaskToComplete

@deprecated (removed in Camel 2.4): Option to specify if the caller should wait for the async task to be complete or not before continuing. The following 3 options is supported: Always, Never or IfReplyExpected. The first two options is self explained. The last will only wait if the message is Request Reply based. The default option is IfReplyExpected.

About rejected tasks

The threads DSL uses a thread pool which has a worker queue for tasks. When the worker queue gets full, the task is rejected. You can customize how to react upon this using the rejectedPolicy and callerRunsWhenRejected option. The latter is used for easily switch between the two most common and recommended settings. Either let the current caller thread execute the task (eg it will become synchronous), but also give time for the thread pool to process its current tasks, without adding more tasks - sort of self throttling. This is the default behavior. If setting callerRunsWhenRejected you use the Abort policy, which mean the task is rejected, and a RejectedExecutionException is set on the Exchange, and the Exchange will stop continue being routed, and its UnitOfWork will be regarded as failed.

The other options Discard and DiscardOldest works a bit like Abort, however they do not set any Exception on the Exchange, which mean the Exchange will not be regarded as failed, but the Exchange will be successful. When using Discard and DiscardOldest then the Exchange will not continue being routed. Notice: There is a issue with these two options in Camel 2.9 or below, that cause the UnitOfWork not to be triggered, so we discourage you from using these options in those Camel releases. This has been fixed in Camel 2.10 onwards.

Example: threads DSL

Suppose we receive orders on a JMS queue. Some of the orders expect a reply while other do not (either a JMSReplyTo exists or not). And lets imagine to process this order we need to do some heavy CPU calculation. So how do we avoid the messages that does not expect a reply to block until the entire message is processed? Well we use the threads DSL to turn the route into multi threading asynchronous routing before the heavy CPU task. Then the messages that does not expect a reply can return beforehand. And the messages that expect a reply, well yeah they have to wait anyway. So this can be accomplished like the route below:

Transactions and threads DSL

Icon

Mind that when using transactions its often required that the Exchange is processed entirely in the same thread, as the transaction manager often uses ThreadLocal to store the intermediate transaction status. For instance Spring Transaction does this. So when using threads DSL the Exchange that is processed in the async thread cannot participate in the same transaction as the caller thread.

Notice: This does not apply to the ProducerTemplate Async API as such as the client usually does not participate in a transaction. So you can still use the Camel Client Async API and do async messaging where the processing of the Exchange is still handled within transaction. Its only the client that submitted the Exchange that does not participate in the same transaction.

See Also

© 2004-2014 The Apache Software Foundation.
Apache Camel, Camel, Apache, the Apache feather logo, and the Apache Camel project logo are trademarks of The Apache Software Foundation. All other marks mentioned may be trademarks or registered trademarks of their respective owners.
Graphic Design By Hiram