Idempotent Consumer

Idempotent Consumer

The Idempotent Consumer from the EIP patterns is used to filter out duplicate messages.

This pattern is implemented using the IdempotentConsumer class. This uses an Expression to calculate a unique message ID string for a given message exchange; this ID can then be looked up in the IdempotentRepository to see if it has been seen before; if it has the message is consumed; if its not then the message is processed and the ID is added to the repository.

The Idempotent Consumer essentially acts like a Message Filter to filter out duplicates.

Camel will add the message id eagerly to the repository to detect duplication also for Exchanges currently in progress.
On completion Camel will remove the message id from the repository if the Exchange failed, otherwise it stays there.

Camel provides the following Idempotent Consumer implementations:

• MemoryIdempotentRepository
• FileIdempotentRepository
• HazelcastIdempotentRepository (Available as of Camel 2.8)
• JdbcMessageIdRepository (Available as of Camel 2.7)
• JpaMessageIdRepository

• InfinispanIdempotentRepository (Available as of Camel 2.13.0)

• JCacheIdempotentRepository (Available as of Camel 2.17.0)

• SpringCacheIdempotentRepository (Available as of Camel 2.17.1)
  

• EhcacheIdempotentRepository (Available as of Camel 2.18.0)

• KafkaIdempotentRepository (Available as of Camel 2.19.0)

Options

The Idempotent Consumer has the following options:

Using the Fluent Builders

The following example will use the header myMessageId to filter out duplicates

For further examples of this pattern in use you could look at the junit test case

Spring XML example

The following example will use the header myMessageId to filter out duplicates

How to handle duplicate messages in the route

Available as of Camel 2.8

You can now set the skipDuplicate option to false which instructs the idempotent consumer to route duplicate messages as well. However the duplicate message has been marked as duplicate by having a property on the Exchange set to true. We can leverage this fact by using a Content Based Router or Message Filter to detect this and handle duplicate messages.

For example in the following example we use the Message Filter to send the message to a duplicate endpoint, and then stop continue routing that message.

How to handle duplicate message in a clustered environment with a data grid

Available as of Camel 2.8

If you have running Camel in a clustered environment, a in memory idempotent repository doesn't work (see above). You can setup either a central database or use the idempotent consumer implementation based on the Hazelcast data grid. Hazelcast finds the nodes over multicast (which is default - configure Hazelcast for tcp-ip) and creates automatically a map based repository:

  HazelcastIdempotentRepository idempotentRepo = new HazelcastIdempotentRepository("myrepo");

  from("direct:in").idempotentConsumer(header("messageId"), idempotentRepo).to("mock:out");

You have to define how long the repository should hold each message id (default is to delete it never). To avoid that you run out of memory you should create an eviction strategy based on the Hazelcast configuration. For additional information see camel-hazelcast.

See this little tutorial, how setup such an idempotent repository on two cluster nodes using Apache Karaf.

Available as of Camel 2.13.0

Another option for using Idempotent Consumer in a clustered environment is Infinispan. Infinispan is a data grid with replication and distribution clustering support. For additional information see camel-infinispan.