I've created a small project that shows how to use JBoss Infinispan with Apache Camel and the Idempotent Consumer Pattern to guarantee a message will not be processed twice in a clustered environment.

Imagine you have an application that has to scale out easily by deploying it on multiple containers. But the application has to process each unique request only once across the cluster.
The solution is simple: use Idempotent Consumer Pattern in Camel with a repository that can scale out easily. This is where Infinispan comes into play. Infinispan is extremely scalable, highly available key/value store and data grid. If you use InfinispanIdempotentRepository with an idempotent consumer, it will create an in-memory cache to store the requests, and the moment you start another instance of the application, the cache instances will sync and the idempotent consumers in all applications will not process existing requests any longer.

With this project (idempotent consumer demo5) you can start as many containers as you want, each container will start a rest endpoint on a new port starting from 8080 (http://localhost:8080/idempotent/KEY), and if you perform a GET request with a key, the subsequent requests with the same key to any other container will be rejected. Behind the scene Infinispan is replicating all the processed keys across the cluster of Camel applications and ensuring consistency.

The core of the application is the following route definition that finds a new free port number for each instance of the application:

public class IdempotentRoute extends RouteBuilder {
    private static final transient Logger LOGGER = LoggerFactory.getLogger(IdempotentRoute.class);

    private InfinispanIdempotentRepository infinispanRepo;
    private int port;

    @Override
    public void configure() throws Exception {
        from("restlet:http://localhost:" + port + "/idempotent/{key}?restletMethods=GET")

                .idempotentConsumer(header("key"), infinispanRepo)
                    .setBody(simple("UNIQUE REQUEST ACCEPTED: ${header.key}"))
                    .stop()
                 .end()

                .setBody(simple("REQUEST REJECTED: ${header.key}"));
    }

    public InfinispanIdempotentRepository getInfinispanRepo() {
        return infinispanRepo;
    }

    public void setInfinispanRepo(InfinispanIdempotentRepository infinispanRepo) {
        this.infinispanRepo = infinispanRepo;
    }

    public void start() {
         port = AvailablePortFinder.getNextAvailable(8080);
         LOGGER.info("Using port: " + port);
    }
}

Simple, isn't it.