[zeromq-dev] disabling receiving messages from queue

[Martin Sustrik]

Hi Pavel,

> Can I disable receiving messages from spcecified queue by 
> api_thread_t::receive()?
> Real use case: my application has one api_thread_t, it receives 
> requests, process it and sends responses. While processing request, it 
> sends/receives messages to another applications. I want to implement 
> limit for number of currently processing messages. Easiest 
> implementation for me looks like this: when number of currently 
> processing request goes above limit, i block receiving messages from 
> requests queue. In this case only messages that used in processing will 
> be received, no new request messages.
> 
> Now i'm thinking about:
> 1) more complex realization with two api_thread_t, non-blocking 
> receive() and sleep() functions.
> 2) more complex realization with blocking receive() but with two 
> application threads with syncronization between them.
> 
> May be threre is more elegant solution for my situation?

No, there isn't :(

What you speak about actually boils down to the need for polling on 
arbitrary subset of queues.

Rather than implementing 1. or 2. you may consider submitting a patch to 
0MQ that would allow for receiving only from a subset of queues. Say:

api->receive (&msg, false, 1 << queue_id1 | 1 << queue_id2);

Have a look at the implementation of zmq::api_thread_t::receive 
function. It's actually a pretty dumb loop that round robins through the 
list of queues and fetches a message from one of them.

Making it ignore subset of queues seems to be a pretty trivial change.

Thoughts?
Martin