In Chapter 5 Michael describes the "Decoupling Middleware" pattern. He suggests using a messaging broker to decouple remote service invocation. This allows you to leverage the features of the messaging broker, like durability and delayed re-delivery, to improve the resiliency of communication with a remote service.
The following demonstrates how to use Mule and JMS to decouple interaction with Twitter's API. "Tweet Service"
transactionally accepts messages from a JMS queue and submits them to Twitter's REST API:
<model name="Twitter Services">
<service name="Tweet Service">
<inbound>
<jms:inbound-endpoint queue="tweets">
<jms:transaction action="BEGIN_OR_JOIN"/>
</jms:inbound-endpoint>
</inbound>
<http:rest-service-component
httpMethod="POST"
serviceUrl="http://user:password@twitter.com/statuses/update.json">
<http:requiredParameter key="status" value="#[payload:]"/>
</http:rest-service-component>
</service>
</model>
Let's consider some of the benefits of this indirection:
- The service can be taken down / brought up without fear of losing messages - they will queue on the broker and be delivered when the service is brought back up.
- The service can be scaled horizontally simply by adding additional instances (competing consumption off the queue)
- Messages can be re-ordered based on priority and sent to the remote service
- Requests can be re-submitted in the event of a failure.
The last point is particularly important. Its common to encounter transient errors when integrating with remote services, particularly web-based ones. These errors are usually recoverable after a certain amount of time. If your JMS broker supports it, you can use delayed redelivery to periodically re-attempt the request. HornetQ supports this by configuring address-settings on the queue. The following address-settings for the "tweets" queue specifies 12 redelivery attempts with a 5 minute interval between each request:
<address-setting match="jms.queue.tweets"> <max-delivery-attempts>12</max-delivery-attempts> <redelivery-delay>300000</redelivery-delay> </address-setting>
Its incidentally easy to employ a "circuit-breaker", another one of Michael's patterns, into the above using an exception-strategy. I'll demonstrate that in another post.
