Distributed ActiveMQ with Camel

Question

I am in the process of learning ActiveMQ and Camel, with the goal to create a little prototype system that works something like this:

(big)

When an order is placed in the Orders system, a message is sent out to any subscribers (a pub/sub system), and they can play their part in processing the order. The Orders, Shipping and Invoicing applications have their own ActiveMQ installations, so that if any of the three systems are offline, the others can continue to function. Something takes care of moving messages between the ActiveMQ installs.

Getting Apache Camel to move messages from one queue to another via routes is quite easy, if they are on the same ActiveMQ instance. So this works for managing the subscription queues.

The next challenge is pushing messages from one ActiveMQ instance to another, and it's the bit where I am not sure what to look at next.

1. Can Camel route between different ActiveMQ installations? (I can't figure out what the JMI endpoint URI would be if they are on different machines).
2. I understand ActiveMQ has store and forward capabilities. Is this what I would use to move messages between Orders and Shipping/Invoicing?
3. Or is this what Apache ServiceMix is meant to solve?

Answer 1

Hmmmm, I've only dabbled at best, and not for a fair while, but I'll try and offer something.

ActiveMQ can route between different installations and just uses standard URIs to my knowledge so I'm not sure what the problem is here. I would think that using TCP you'd be fine. Using ServiceMix (you mention it later) you'd just specify a connectionFactory & then provide the URI in that. This link shows some examples http://servicemix.apache.org/servicemix-jms-new-endpoints.html.

Camel has support for Durable Subscriber if that's what you were after (http://camel.apache.org/durable-subscriber.html)? This pattern will ensure that if the subscriber is offline when the message is ready, it will be held until the subscriber is back online. This is also supported by ServiceMix (see link given above and look for 'subscriptionDurable'.

Answer 2

This is a pretty straightforward asynchronous, event-driven application that is well-suited for ActiveMQ and Camel.

Actually you do not move messages explicitly from one ActiveMQ instance to another. The way it works is using what's known as a network of brokers. In your case, you'd have three brokers: ActiveMQ-purple, ActiveMQ-green and ActiveMQ-blue. ActiveMQ-purple creates a uni-directional broker network with ActiveMQ-green and ActiveMQ-blue. This allows ActiveMQ-purple to store-and-forward messages to ActiveMQ-green and ActiveMQ-blue based on consumer demand.

The Orders app accepts orders on the orders queue on ActiveMQ-purple. The Orders app uses Camel to consume and process a message to determine if it is an invoicing message or a shipping message. Camel routes the messages to either the invoicing queue or the shipping queue on ActiveMQ-purple.

Consumer demand comes from the Invoicing app and the Shipping app. The Invoicing uses Camel to consume messages from the invoicing queue on ActiveMQ-green. The Shipping app uses Camel to consume messages from the shipping queue on ActiveMQ-blue. Because of the broker network and because of the consumer demand on the ActiveMQ-green.invoicing queue and the ActiveMQ-blue.shipping queue, messages will be forwarded from ActiveMQ-purple to the appropriate broker and queue. There is no need to explicitly route messages to specific broker.

I hope this answers your questions. Let me know if you have anymore.

Bruce