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Chapter 131. SEDA

SEDA Component

The seda: component provides asynchronous SEDA behavior, so that messages are exchanged on a BlockingQueue and consumers are invoked in a separate thread from the producer.

Note that queues are only visible within a single CamelContext. If you want to communicate across CamelContext instances (for example, communicating between Web applications), see the VM component.

This component does not implement any kind of persistence or recovery, if the VM terminates while messages are yet to be processed. If you need persistence, reliability or distributed SEDA, try using either JMS or ActiveMQ.

Synchronous

The Direct component provides synchronous invocation of any consumers when a producer sends a message exchange.

URI format

seda:queueName[?options]

Where queueName can be any string that uniquely identifies the endpoint within the current CamelContext.

You can append query options to the URI in the following format, ?option=value&option=value&...

Note

When matching consumer entpoints to producer endpoints, only the queueName is considered and any option settings are ignored. That is, the identity of a consumer endpoint depends only on the queueName. If you want to attach multiple consumers to the same queue, use the approach described in the section called “Using multipleConsumers”.

Options

Name	Default	Description
`size`	Unbounded	The maximum capacity of the SEDA queue (i.e., the number of messages it can hold). Notice: Mind if you use this option, then its the first endpoint being created with the queue name, that determines the size. To make sure all endpoints use same size, then configure the size option on all of them, or the first endpoint being created. From Camel 2.11 onwards, a validation is taken place to ensure if using mixed queue sizes for the same queue name, Camel would detect this and fail creating the endpoint.
`concurrentConsumers`	`1`	Apache Camel 1.6.1/2.0: Number of concurrent threads processing exchanges.
`waitForTaskToComplete`	`IfReplyExpected`	Option to specify whether the caller should wait for the async task to complete or not before continuing. The following three options are supported: `Always`, `Never` or `IfReplyExpected`. The first two values are self-explanatory. The last value, `IfReplyExpected`, will only wait if the message is Request Reply-based. The default option is `IfReplyExpected`. See more information about Async messaging.
`timeout`	`30000`	Apache Camel 2.0: Timeout in millis a seda producer will at most waiting for an async task to complete. See `waitForTaskToComplete` and Async for more details. In Camel 2.2 you can now disable timeout by using 0 or a negative value.
`multipleConsumers`	`false`	Camel 2.2: Specifies whether multiple consumers are allowed or not. If enabled, you can use SEDA for a publish/subscribe style of messaging. Send a message to a SEDA queue and have multiple consumers receive a copy of the message.
`limitConcurrentConsumers`	`true`	Camel 2.3: Whether to limit the concurrentConsumers to maximum 500. If its configured with a higher number an exception will be thrown. You can disable this check by turning this option off.
`blockWhenFull`	`false`	Whether a thread that sends messages to a full SEDA queue will block until the queue's capacity is no longer exhausted. By default, an exception will be thrown stating that the queue is full. By enabling this option, the calling thread will instead block and wait until the message can be accepted.
`queueSize`		Component only: The maximum default size (capacity of the number of messages it can hold) of the SEDA queue. This option is used if `size` is not in use.
`pollTimeout`	`1000`	Consumer only -- The timeout used when polling. When a timeout occurs, the consumer can check whether it is allowed to continue running. Setting a lower value allows the consumer to react more quickly upon shutdown.
`purgeWhenStopping`	`false`	Whether to purge the task queue when stopping the consumer/route. This allows to stop faster, as any pending messages on the queue is discarded.
`queue`	`null`	Define the queue instance which will be used by seda endpoint
`queueFactory`	`null`	Define the QueueFactory which could create the queue for the seda endpoint
`failIfNoConsumers`	`false`	Whether the producer should fail by throwing an exception, when sending to a SEDA queue with no active consumers.

Choosing BlockingQueue implementation

Available as of Camel 2.12

By default, the SEDA component always intantiates LinkedBlockingQueue, but you can use different implementation, you can reference your own BlockingQueue implementation, in this case the size option is not used

<bean id="arrayQueue" class="java.util.ArrayBlockingQueue">
<constructor-arg index="0" value="10" ><!-- size -->
<constructor-arg index="1" value="true" ><!-- fairness -->
</bean>
<!-- ... and later -->
<from>seda:array?queue=#arrayQueue</from>

Or you can reference a BlockingQueueFactory implementation, 3 implementations are provided LinkedBlockingQueueFactory, ArrayBlockingQueueFactory and PriorityBlockingQueueFactory:

<bean id="priorityQueueFactory" class="org.apache.camel.component.seda.PriorityBlockingQueueFactory">
<property name="comparator">
<bean class="org.apache.camel.demo.MyExchangeComparator" />
</property>
</bean>
<!-- ... and later -->
<from>seda:priority?queueFactory=#priorityQueueFactory&size=100</from>

Use of Request Reply

The SEDA component supports using Request Reply, where the caller will wait for the Async route to complete. For instance:

from("mina:tcp://0.0.0.0:9876?textline=true&sync=true").to("seda:input");

from("seda:input").to("bean:processInput").to("bean:createResponse");

In the route above, we have a TCP listener on port 9876 that accepts incoming requests. The request is routed to the seda:input queue. As it is a Request Reply message, we wait for the response. When the consumer on the seda:input queue is complete, it copies the response to the original message response.

until 2.2: Works only with 2 endpoints

Using Request Reply over SEDA or VM only works with 2 endpoints. You cannot chain endpoints by sending to A -> B -> C etc. Only between A -> B. The reason is the implementation logic is fairly simple. To support 3+ endpoints makes the logic much more complex to handle ordering and notification between the waiting threads properly.

This has been improved in Camel 2.3 onwards, which allows you to chain as many endpoints as you like.

Concurrent consumers

By default, the SEDA endpoint uses a single consumer thread, but you can configure it to use concurrent consumer threads. So instead of thread pools you can use:

from("seda:stageName?concurrentConsumers=5").process(...)

Difference between thread pools and concurrent consumers

The thread pool is a pool that can increase/shrink dynamically at runtime depending on load, whereas the concurrent consumers are always fixed.

Thread pools

Be aware that adding a thread pool to a SEDA endpoint by doing something like:

from("seda:stageName").thread(5).process(...)

Can wind up with two BlockQueues: one from the SEDA endpoint, and one from the workqueue of the thread pool, which may not be what you want. Instead, you might want to consider configuring a Direct endpoint with a thread pool, which can process messages both synchronously and asynchronously. For example:

from("direct:stageName").thread(5).process(...)

You can also directly configure number of threads that process messages on a SEDA endpoint using the concurrentConsumers option.

Sample

In the route below we use the SEDA queue to send the request to this async queue to be able to send a fire-and-forget message for further processing in another thread, and return a constant reply in this thread to the original caller.

public void configure() throws Exception {
    from("direct:start")
        // send it to the seda queue that is async
        .to("seda:next")
        // return a constant response
        .transform(constant("OK"));

    from("seda:next").to("mock:result");
}

Here we send a Hello World message and expect the reply to be OK.

Object out = template.requestBody("direct:start", "Hello World");
assertEquals("OK", out);

The "Hello World" message will be consumed from the SEDA queue from another thread for further processing. Since this is from a unit test, it will be sent to a mock endpoint where we can do assertions in the unit test.

Using multipleConsumers

Available as of Camel 2.2

In this example we have defined two consumers and registered them as spring beans.

<!-- define the consumers as spring beans -->
<bean id="consumer1" class="org.apache.camel.spring.example.FooEventConsumer"/>

<bean id="consumer2" class="org.apache.camel.spring.example.AnotherFooEventConsumer"/>

<camelContext xmlns="http://camel.apache.org/schema/spring">
    <!-- define a shared endpoint which the consumers can refer to instead of using url -->
    <endpoint id="foo" uri="seda:foo?multipleConsumers=true"/>
</camelContext>

Since we have specified multipleConsumers=true on the seda foo endpoint we can have those two consumers receive their own copy of the message as a kind of pub-sub style messaging.

As the beans are part of an unit test they simply send the message to a mock endpoint, but notice how we can use @Consume to consume from the seda queue.

public class FooEventConsumer {

    @EndpointInject(uri = "mock:result")
    private ProducerTemplate destination;

    @Consume(ref = "foo")
    public void doSomething(String body) {
        destination.sendBody("foo" + body);
    }

}

Extracting queue information.

If you need it, you can also get information like queue size etc without using JMX like this:

SedaEndpoint seda = context.getEndpoint("seda:xxxx");
int size = seda.getExchanges().size()