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RE: How to read messages from a queue in parallel


Hi Peter,

Given you have a header already identifying the message categories, you could try to look into the Aggregator EIP:

http://camel.apache.org/aggregator2.html

It will allow you to group all the messages by the header into 'batches' for each of the groups. Each batch can then be processed by a single consumer and it would allow you to have multiple consumers taking care of the parallel processing of different groups. This is very suitable for periodic batch processing of messages, but requires finite batch sizes and/or finite time limits.

If you have a fixed set of group headers but not a deterministic size of the messages per group or a non-deterministic time period then a Message Router might be suitable:

http://camel.apache.org/message-router.html

You can directly check for the header in the choice expression and then route the different group messages to their respective group queues where you can have single consumers processing them in sequence in a continuous way. The separate queues would allow to fulfil the parallel processing per group requirement.

There is also a Dynamic Router option, but I personally wouldn't have any experience with it, but if none of the above suit your needs it might be something worth looking into:

http://camel.apache.org/dynamic-router.html


Thanks,
Valdis

-----Original Message-----
From: Peter Billen [mailto:peter.billen@xxxxxxxxx] 
Sent: 17 December 2018 11:08
To: users@xxxxxxxxxxxxxxxx
Subject: How to read messages from a queue in parallel

Hi all,

I am reading from a RabbitMQ queue as following:

from("rabbitmq://localhost/?queue=camel&autoAck=false&concurrentConsumers=1&
threadPoolSize=1&prefetchEnabled=true&prefetchCount=50")

Some remarks about the configuration parameters:

- I set `autoAck` to false to be able to acknowledge manually in my implementation.
- I set `concurrentConsumers` and `threadPoolSize` to 1 to guarantee that I consume the messages in the same order as they were added to the queue.
- I set `prefetchCount` to 50 to have at most 50 inflight messages in memory.

Now, I want to process these 50 messages in an asynchronous fashion and manually acknowledge when done. Each message has a `group identifier` header set. Messages from the same group will be processed sequentially, while messages from other groups will be processed concurrently.

I tried to start with the following:

from("rabbitmq://...")
.process(new AsyncProcessor() {
    @Override
    public boolean process(final Exchange exchange, final AsyncCallback
callback) {
        System.out.println("ASYNC");
        // TODO: (1) read group identifier (2) submit task to executor responsible for that particular group (3) call callback.done() in the task once done
        return false;
    }

    @Override
    public void process(final Exchange exchange) {
        throw new UnsupportedOperationException();
    }
})

The problem is here that only the first message is given to `process(exchange, callback)`. Is there a way to also receive the other inflight messages?

Note that I do *not* want to increase the number of RabbitMQ consumers, as this would skew with the message order. It is important that messages from the same group will be executed sequentially, hence the necessity to have one single RabbitMQ consumer.

Thanks!

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