Camel Component: SEDA - 6.3

Talend ESB Mediation Developer Guide

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6.3
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Talend Data Fabric
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Design and Development
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Talend ESB

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 Camel Component: 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 Camel Component: JMS or Camel Component: ActiveMQ.

Note

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

URI format and options

seda:someName[?options]

Where someName 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: the same queue name must be used for both producer and consumer.

An exactly identical SEDA queue name must be used for both the producer endpoint and the consumer endpoint. Otherwise Camel will create a second Camel Component: SEDA endpoint, even though the someName portion of the queue is identical. For example:

from("direct:foo").to("seda:bar?concurrentConsumers=5");
from("seda:bar?concurrentConsumers=5").to("file://output");

And where URI option can be:

Table 18. 

Name

Default

Description

size

 

The maximum capacity of the SEDA queue (i.e., the number of messages it can hold). The default value in Camel 2.2 or older is 1000. From Camel 2.3 onwards, the size is unbounded by default. 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.

concurrent-Consumers

1

Number of concurrent threads processing exchanges.

waitForTaskTo-Complete

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

Timeout in milliseconds a seda producer will at most waiting for an async task to complete. See waitForTaskToComplete and Async for more details. You can disable timeout by using 0 or a negative value.

multipleConsumers

false

Specifies whether multiple consumers are allowed. If enabled, you can use Camel Component: SEDA for Publish-Subscribe messaging. That is, you can send a message to the SEDA queue and have each consumer receive a copy of the message. When enabled, this option should be specified on every consumer endpoint.

limitConcurrent-Consumers

true

Whether to limit the concurrentConsumers to maximum 500. By default, an exception will be thrown if a SEDA endpoint is configured with a greater number. You can disable that check by turning this option off.

blockWhenFullfalse

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.
pollTimeout1000Consumer only. The timeout used when polling. When a timeout occurs then the consumer can check whether its allowed to continue to run. Setting a lower value allows the consumer to react faster upon shutting down.
purgeWhenStoppingfalse

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.

queuenull

Define the queue instance which will be used by seda endpoint

queueFactorynull

Define the QueueFactory which could create the queue for the seda endpoint

failIfNoConsumersfalse

Whether the producer should fail by throwing an exception, when sending to a SEDA queue with no active consumers.


See the Camel Website for the most up-to-date examples of this component in use.

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 Camel Component: 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.

Using Request Reply over Camel Component: SEDA or Camel Component: VM, you can 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(...)

As for the difference between the two, note a thread pool can increase/shrink dynamically at runtime depending on load, whereas the number of concurrent consumers is 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 wish to configure a Camel Component: 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.