[rabbitmq-discuss] [Q] best way to add a sequencer to the broker

[Preston Marshall]

It sounds like you want something like EventMachine for Ruby, which I  
believe is based on Java's reactor pattern.  They have a Java  
implementation, but it is meant for JRuby so I don't know what your  
mileage will be on it.  It is here: http://rubyeventmachine.com/browser/trunk/java
On Dec 29, 2008, at 8:04 AM, Chuck Remes wrote:

> In my application I would like to stamp each message on the bus with a
> sequence number. Doing so allows me to replay events in order (amongst
> other benefits). Some services will be publishing latency-sensitive
> information.
>
> Each service connecting to the broker will publish to a topic
> exchange. One to N other services may subscribe to the published data.
> I foresee a topic hierarchy like so:
>
> out.topic1 (passive, durable)
> out.topic2 (passive, non-durable)
> out.topic3.subtopic1 (passive, non-durable)
> out.topic3.subtopic2 (passive, durable)
> etc.
>
> These exchanges are named with dot notation for simplicity of parsing
> for regular expressions. I realize the exchange name doesn't need the
> dot notation whereas that is a requirement for routing keys.
>
> All of my data is serialized as a JSON object prior to publishing. The
> object (hash) contains a key called :sequence which is set to 0 by the
> publisher. The sequencer service will subscribe to all out.# topics
> (with '#' as the routing key so it gets everything), read the content
> body as a Map, and set the map :sequence value to i++ where 'i' is a
> 32-bit (or 64-bit) integer. It will then republish the JSON object to
> a new topic of the form "in.topicX" and pass through the original
> routing key. It's routing between, for example, out.topic1 and
> in.topic1 and adding a sequence number as a side effect.
>
> I played around with the examples SimpleTopicPublisher and
> SimpleTopicConsumer as a basis for a new class. I was able to add a
> JSONReader and JSONWriter to the new java class I call Sequencer. It
> successfully performs the work as described in the prior paragraph.
> RIght now I statically declare the in & out exchanges, but I see how I
> could declare the "in" exchange based on the contents of the "out"
> exchange name received in the envelope.
>
> I have a few outstanding questions that I'm hoping some more
> experienced folks can help answer.
>
> 1. This code will likely start before other services, so it will
> declare the exchanges and block on them while waiting for traffic. The
> topic Consumer/Producer examples all illustrate how to do this with a
> single exchange. How do I accomplish this task with multiple
> exchanges? Do I need a separate thread and channel for each exchange?
>
> 2. Is there a better way to accomplish this task? I've read some notes
> about future releases of rabbitmq allowing for "internal" clients that
> run inside the rabbitmq memory space. I think this is an ideal use of
> that capability. Am I right or barking up the wrong tree? And does
> that internal client need to be written in erlang or can it be java/c/
> whatever?
>
> 3. Is there a way to make this more dynamic so I do not have to
> declare all the exchanges up-front in this sequencer code? Ideally
> this service could detect that new exchanges were declared by other
> services and automatically subscribe to them to do the sequence
> stamping and routing.
>
> Thanks for any feedback.
>
> cr
>
>
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