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

[Alexis Richardson]

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
>
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