Idempotent Consumer 3

The Idempotent Consumer![][5] from the EIP patterns is used to filter out duplicate messages. 



This pattern is implemented using the IdempotentConsumer![][5] 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 MessageIdRepository![][5] 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.<p> 



### Options



The Idempotent Consumer has the following options:



Option  Default  Description 



eager 

true 

**Camel 2.0:** Eager controls whether Camel adds the message to the repository before or after the exchange has been processed. If enabled before then Camel will be able to detect duplicate messages even when messages are currently in progress. By disabling Camel will only detect duplicates when a message has successfully been processed. 



**Using the Fluent Builders**



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

    

    RouteBuilder builder = new RouteBuilder() {

        public void configure() {

            errorHandler(deadLetterChannel("mock:error"));

    

            from("seda:a").idempotentConsumer(header("myMessageId"), MemoryIdempotentRepository.memoryIdempotentRepository(200))

                .to("seda:b");

        }

    };



The above example![][5] will use an in-memory based MessageIdRepository![][5] 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

    

    from("direct:start").idempotentConsumer(

            header("messageId"),

            jpaMessageIdRepository(lookup(JpaTemplate.class), PROCESSOR_NAME)

    ).to("mock:result");



In the above example![][5] 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![][5]



#### 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.



![][21]