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.
Camel provides the following Idempotent Consumer implementations:
The Idempotent Consumer has the following options:
Using the Fluent Builders
The following example will use the header myMessageId to filter out duplicatesThe above example will use an in-memory based MessageIdRepository which can easily run out of memory and doesn't work in a clustered environment. So you might prefer to use the JPA based implementation which uses a database to store the message IDs which have been processed In the above example we are using the header messageId to filter out duplicates and using the collection myProcessorName to indicate the Message ID Repository to use. This name is important as you could process the same message by many different processors; so each may require its own logical Message ID Repository.
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
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.The sample example in XML DSL would be:
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:
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.
Using This Pattern
If you would like to use this EIP Pattern then please read the Getting Started, you may also find the Architecture useful particularly the description of Endpoint and URIs. Then you could try out some of the Examples first before trying this pattern out.