[rabbitmq-discuss] rabbitmq-c API improvements for multiple channels

Fri Jan 13 21:19:50 GMT 2012

I haven't really had a huge amount of time to work on it since I sent the
first message message, with the holidays and starting up a new project at
work.

Also I was hoping to get some input from the rabbitmq guys so that any
changes I make would have a better chance of being accepted.

Taking a quick look at what I wrote a month ago, today my plan would be to
implement having a amqp_pool_t associated with each amqp_channel_t and see
how far I could get with that.

-Alan

On Thu, Jan 12, 2012 at 11:39 AM, Alexis Richardson <alexis at rabbitmq.com>wrote:

> Alan
>
> It appears that you stunned everyone into silence ;-)
>
> How are you getting on with this?
>
> alexis
>
>
> On Wed, Dec 14, 2011 at 8:15 PM, Alan Antonuk <alan.antonuk at gmail.com>
> wrote:
> > I've been working on a c++ wrapper around rabbitmq-c that presents a
> > "humane" API to the programmer heavily inspired by the Puka project.
> > (see: https://github.com/alanxz/SimpleAmqpClient).
> >
> > In developing this library I've run across several limitations of the
> > rabbitmq-c when working with multiple channels, the biggest issue being:
> >
> > - There is no way to wait for a list of methods on a channel.
> > There is amqp_simple_wait_method() - however this suffers from some
> serious
> > drawbacks:
> > + you can only specify one method to listen for
> > + it calls abort() if a different method, or a method on a different
> channel
> > is received
> > A use case for this might be: doing a basic.publish, and you want to
> want to
> > wait for a basic.ack or a basic.return on a channel with confirms enabled
> >
> > The way I got around this in SimpleAmqpClient was to only use
> > amqp_simple_wait_frame() and maintain queues of amqp_frame_t for each
> > channel that I have open.
> >
> > However, this comes with one serious drawback: memory management. Each
> > decoded frame is allocated in a connection-wide amqp_pool_t.  Because of
> > this - it is impossible to recycle the pool and release memory unless you
> > have dealt with all of your pending amqp_frame_t's.  This becomes a
> problem
> > in pathological cases where you have two consumers sending
> simultaneously,
> > you can get in the situation that even though the client program
> eventually
> > deals with every consumed message, memory never gets released, because
> there
> > is always at least one frame queued up.
> >
> > The above demonstrates the second biggest issue with the rabbitmq-c API:
> > memory management when dealing with multiple channels.  There is no way
> to
> > separate out memory allocation on, for example, a per-channel basis (with
> > the library client keeping track of the memory pools used for example).
> >
> > Before I go on I'd like to mention one feature that I find useful with
> the
> > current API: it is possible to use something like select() before calling
> > amqp_simple_wait_frame() to setup a timeout while waiting for a consumer,
> > which is useful when developing single-threaded RPC apps.
> >
> >
> > So now the interesting part: how could the API be improved?
> >
> > Some thoughts I've had dealing with the memory management:
> > 1. Create a amqp_simple_wait_frame() variant that as one of the
> parameters
> > provides a callback to allocate memory, either something like a malloc()
> > call, which the client would then be responsible for freeing, or perhaps
> > something like get_amqp_pool(amqp_frame_t) which allows the client to
> return
> > a memory pool which rabbitmq-c would use to allocate memory for that
> frame.
> >  The amqp_frame_t would have to have some minimal amount of information
> > filled in - such as frame type and channel to be useful.
> >
> > 2. amqp_channel_t becomes a struct containing both the the channel_id
> and a
> > amqp_pool_t.  The amqp_pool would be used by the library to allocate
> frames
> > received on that channel.  The client would then be responsible for
> calling
> > amqp_pool_recycle at an appropriate point.
> >
> > Some thoughts on improving the API to deal with multiple channels:
> > 1. Add the following to the API:
> > amqp_simple_wait_frame_on_channel - wait for a frame on a specified
> channel
> > amqp_simple_wait_methods - wait for multiple methods on a specified
> channel.
> > Don't abort() if the wrong method, or channel is received, instead queue
> up
> > the frame as is done in amqp_simple_rpc
> > amqp_frames_enqueued_for_channel to add feature parity with
> > amqp_frames_enqueued
> > I started to code this up at one point but abandoned it as it didn't
> support
> > the interesting property of being able to use select() to specify a
> timeout
> > when calling amqp_simple_wait_frame_on_channel.  At least not without
> adding
> > timeout to the rabbitmq-c api, which didn't fit with how the current
> > rabbitmq-c api. Here's the implementation I came up with before I
> abandoned
> > it. I can't guarantee is completely free of
> > bugs. https://github.com/alanxz/rabbitmq-c/commits/simple_wait_frame/
> >
> > 2. Continue to duck the issue and allow clients to write their own code
> to
> > deal with multiple channels (fixing the memory management issue using
> one of
> > the above techniques)
> >
> > 3. Something I haven't thought of yet.
> >
> > So anyone have any thoughts on all of this?
> >
> > -Alan
> >
> > _______________________________________________
> > rabbitmq-discuss mailing list
> > rabbitmq-discuss at lists.rabbitmq.com
> > https://lists.rabbitmq.com/cgi-bin/mailman/listinfo/rabbitmq-discuss
> >
>
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