Apache Camel Meets Redis

The term idempotent is used in mathematics to describe a function that produces the same result if it is applied to itself. In Messaging this concepts translates into the a message that has the same effect whether it is received once or multiple times. In Camel this pattern is implemented using the IdempotentConsumer class which 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. RedisIdempotentRepository is using a set structure to store and check for existing Ids.

<bean id="idempotentRepository" class="org.apache.camel.component.redis.processor.idempotent.RedisIdempotentRepository">
  <constructor-arg value="test-repo"/>
</bean>

<route>
  <from uri="direct:start"/>
    <idempotentConsumer messageIdRepositoryRef="idempotentRepository">
      <simple>${in.body.id}</simple>
      <to uri="mock:result"/>
    </idempotentConsumer>
</route>

One of the main uses of Redis is as LRU cache. It can store data inmemory as Memcached or can be tuned to be durable flushing data to a log file that can be replayed if the node restarts.The various policies when maxmemory is reached allows creating caches for specific needs:

• volatile-lru remove a key among the ones with an expire set, trying to remove keys not recently used.
• volatile-ttl remove a key among the ones with an expire set, trying to remove keys with short remaining time to live.
• volatile-random remove a random key among the ones with an expire set.
• allkeys-lru like volatile-lru, but will remove every kind of key, both normal keys or keys with an expire set.
• allkeys-random like volatile-random, but will remove every kind of keys, both normal keys and keys with an expire set.

Once your Redis server is configured with the right policies and running, the operation you need to do are SET and GET:

<?xml version="1.0" encoding="UTF-8"?>
<route>
    <from uri="direct:start"/>
        <setHeader headerName="CamelRedis.Command">
            <constant>SET</constant>
        </setHeader>
        <setHeader headerName="CamelRedis.Key">
            <constant>keyOne</constant>
        </setHeader>
        <setHeader headerName="CamelRedis.Value">
            <constant>valueOne</constant>
        </setHeader>
    <to uri="redis://localhost:6379"/>
</route>

Camel has various components for interacting between routes:
direct: provides direct, synchronous invocation in the same camel context.
seda: asynchronous behavior, where messages are exchanged on a BlockingQueue, again in the same camel context.
vm: asynchronous behavior like seda, but also supports communication across CamelContext as long as they are in the same JVM.
Complex applications usually consist of more than one standalone Camel instances running on separate machines. For this kind of scenarios, Camel provides jms, activemq, combination of AWS SNS with SQS, for messaging between instances.
Redis has a simpler solution for the Publish/Subscribe messaging paradigm. Subscribers subscribes to one or more channels, by specifying the channel names or using pattern matching for receiving messages from multiple channels. Then the publisher publishes the messages to a channel, and Redis makes sure it reaches all the matching subscribers.

<?xml version="1.0" encoding="UTF-8"?>
<camelContext id="camel" xmlns="http://camel.apache.org/schema/spring">
    <route startupOrder="1">
        <from uri="redis://localhost:6379?command=SUBSCRIBE&channels=testChannel"/>
        <to uri="mock:result"/>
    </route>

    <route startupOrder="2">
        <from uri="direct:start"/>
        <setHeader headerName="CamelRedis.Command">
            <constant>PUBLISH</constant>
        </setHeader>
        <setHeader headerName="CamelRedis.CHANNEL">
            <constant>testChannel</constant>
        </setHeader>
        <setHeader headerName="CamelRedis.MESSAGE">
            <constant>Test Message</constant>
        </setHeader>
        <to uri="redis://localhost:6379"/>
    </route>
</camelContext>