[rabbitmq-discuss] java client library rpc client

Fri Apr 23 09:21:01 BST 2010

Hi Matthias,

The .net client has a overload call() defined which invokes cast()(Sends
an asynchronous/one-way message) and then Retrieves the reply from the
Subscription object which is consuming from the reply queue of the
consumer.
So in this way you get the first reply back immediately and then you can
poll the subscription for any more replies.

I have extended the java rpcclient to work in a similar way by using
QueingConsumer instead of Default consumer.

Thanks
Rajesh

-----Original Message-----
From: Matthias Radestock [mailto:matthias at rabbitmq.com] 
Sent: 23 April 2010 09:09
To: Reddy, Rajesh (GPT EMEA)
Cc: rabbitmq-discuss at lists.rabbitmq.com
Subject: Re: [rabbitmq-discuss] java client library rpc client

Rajesh,

Reddy, Rajesh (GPT EMEA) wrote:
> Basically my process makes an rpccall and gets reply from one or more
> other process. This works if the rpcclient receives one reply for its
> request, but as soon as it gets multiple replies it fails. I get the
> following exception which doesn't say much about the root cause.

The rpc abstractions that ship with the RabbitMQ clients are not 
designed to handle multiple replies.

> This is possible in .NET library of rabbitmq and working fine.

The difference between the Java and .net version is that the former 
processes replies eagerly, i.e. any message received from the rpc 
client's reply queue is processed immediately, whereas the .net version 
processes replies lazily, i.e. only when an rpc caller is waiting for a 
result. If you tried re-using the same .net SimpleRpcClient instance for

another call things would break in much the same way as for the Java 
client since the code would find a reply that is not correlated to the 
current request.

To implement the "wait for first reply and discard all others" pattern 
properly I suggest you amend the rpc client code with an "unexpected 
reply" handler/callback that by default throws an exception (thus 
mimicking the current behaviour). In your application code you could 
then set that handler to ignore unexpected replies instead.

Patches are welcome.

Regards,

Matthias.

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