Threading Model
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Threading Model

Available as of Camel 2.3

The threading model in Camel is based on leveraging the JDK concurrency API which provides thread pools, named ExecutorService.

Camel leverages thread pools in the following places:

  • several EIP patterns supports using thread pools for concurrency
  • SEDA component for asynchronous connectivity
  • Threads DSL in the Camel route
  • ServicePool for pooling services
  • And some component provide thread pools by nature such as JMS, Jetty

Thread pool profiles

By default when a thread pool is to be created then its based on the default thread pool profile which is:

What that means is that for example when you use Multicast with parallelProcessing=true enabled, then it would create a thread pool based on the profile above. The rejectedPolicy has four options: Abort, CallerRuns, Discard, DiscardOldest which corresponds to the same four options provided out of the box in the JDK.

You can define as many thread pool profiles as you like. But there must only one default profile. A custom thread pool profile will inherit from the default profile. Which means that any option you do not explicit define will fallback and use the option from the default profile.

You can use -1 in maxQueueSize to indicate a unbounded queue.

In Java DSL you can configure the default thread pool profile from the ExecutorServiceStrategy/ExecutorServiceManager which you access from CamelContext.

Using thread pool profiles

Suppose you want to use a custom thread pool profile for a Multicast EIP pattern in a Camel route you can do it using the executorServiceRef attribute as shown:

What Camel will do at runtime is to lookup in the Registry for a ExecutorService with the id = fooProfile. If none found it will fallback and see if there is a ThreadPoolProfile defined with that id. And in this example there is and so the profile is used for creating a new ExecutorService which is handed back to the Multicast EIP to use in the route.

Create custom thread pool

You can also use the <threadPool/> tag in Spring XML to create a specific thread pool (eg ExecutorService). Notice that any options you do not explicit define, will have Camel to use the default thread pool profile as fallback. For example if you omit setting the maxQueueSize then Camel will fallback and use the value from the default thread pool profiles, which by default is 1000.

Management

All the thread pools that Camel creates are managed and thus you can see them in JConsole under the threadpools category.

ExecutorServiceStrategy

Available as of Camel 2.3 to 2.8.x
Camel provides a pluggable strategy to hook in your own thread pool provider, for example from a WorkManager in a J2EE server etc.
See the org.apache.camel.spi.ExecutorServiceStrategy interface which you should implement and hook into the WorkManager.

See Advanced configuration of CamelContext using Spring for how to configure it.

You can configure it on the CamelContext from Java DSL using the getter/setter.

ExecutorServiceManager

Available as of Camel 2.9

In camel 2.9.0 the ExecutorServiceManager replaces the ExecutorServiceStrategy. It is renamed to manager as is not only provides a strategy for thread pool creation but also keeps track of thread pools and thread pool profiles.

To hook in custom thread pool providers (e.g. for J2EE servers) a ThreadPoolFactory interface can be implemented. The implementation can be set in the ExecutorServiceManager. The Factory interface is much simpler then the former ExecutorServiceStrategy and makes the job of integrators much easier.

See Advanced configuration of CamelContext using Spring for how to configure it.

Customizing thread names

On the ExecutorServiceStrategy/ExecutorServiceManager you can configure the thread name pattern using the setThreadNamePattern method, which defines the thread names used when a thread pool creates a thread.

The default pattern is for:

  • Camel 2.9.x or older: Camel (${camelId}) thread #${counter} - ${name}
  • Camel 2.10 onwards: Camel (#camelId#) thread ##counter# - #name#

Notice we renamed the tokens from Camel 2.10 onwards to not clash with tokens by the Property Placeholder.

In the pattern you can use the following placeholders

  • ${camelId} Camel 2.6: is the CamelContext name
  • ${counter} is a unique incrementing counter.
  • ${name} is the regular thread name.
  • ${longName} is the long thread name which can includes endpoint parameters etc.

Notice the pattern name has changed from Camel 2.10 onwards, use #name# instead.

In Camel 2.11 onwards its easier to set the thread name pattern on the CamelContext using the threadNamePattern attribute in the XML files as shown below:

Component developers

If you develop your own Camel component and are in need of a thread pool, then its advised to use the ExecutorServiceStrategy/ExecutorServiceManager to create the thread pool you need.

Shutdown

All thread pools created by Camel will be properly shutdown when CamelContext shutdowns which ensures no leaks in the pools in case you run in a server environment with hot deployments and the likes.

The ExecutorServiceManager has APIs for shutting down thread pools graceful and aggressively. Its encourage to use this API for creating and shutting down thread pools.

From Camel 2.11 onwards Camel the graceful shutdownGraceful(executorService) method from ExecutorServiceManager will shutdown graceful at first, until a timeout value is hit. After that it shutdown aggressively, again using the timeout value to wait for the operation to complete. This means you can wait at most 2 x timeout for shutting down the thread pool.
The timeout value is by default 10000 millis. You can configure a custom value on the ExecutorServiceManager if needed. During shutdown Camel will log every 2nd second at INFO level progress of shutting down the thread pool. For example in case a shutdown takes a while, then there is activity in the logs.

The APIs on ExecutorServiceManager that is related to shutting down a thread pool is as follows:

Method

Description

shutdown

Marks the thread pool as shutdown, eg just as calling ExecutorService.shutdown() method

shutdownNow

Forces the thread pool to shutdown now, eg just as calling ExecutorService.shutdownNow() method

shutdownGraceful

Camel 2.11: Marks the thread pool as shutdown, and graceful shutdown the pool, by waiting for tasks to complete. A default timeout value of 10 sec is used, before
shutdown becomes aggressive using shutdownNow, to force threads to shutdown.

shutdownGraceful(timeout)

Camel 2.11: As above but with custom timeout value

awaitTermination

Camel 2.11: To wait graceful for the termination of a thread pool (eg to wait for its tasks to complete). Will wait until all tasks is completed or a timeout value is hit.

See Also

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