[zeromq-dev] zmq_close() semantics and handling outstanding messages

[Brian Granger]

Martin,

On Tue, Jul 6, 2010 at 10:24 AM, Martin Lucina <mato at kotelna.sk> wrote:
> Hi all,
>
> while implementing a 0MQ architecture which needs to dynamically create and
> destroy sockets during operation I ran into the current behaviour of
> zmq_close() being semantically different from the standard close() system
> call.

I have run into this issue as well.

> Consider a scenario where we wish to send a bunch of messages, then close
> the socket:
>
>  zmq_send (s, ...)
>  zmq_send (s, ...)
>  zmq_send (s, ...)
>  zmq_close (s)
>
> The current behaviour is that zmq_close() will discard any messages which
> have been queued ("sent") with zmq_send() but have not yet been pushed out
> to the network. Contrast this with the behaviour of the close() system call
> on a standard socket where the call means "please make this socket go away,
> but finish sending any outstanding data on it asynchronously if you
> can"[1].

The other issue with the current API is that it is non-deterministic.
Depending on the timing of when zmq_close is called, the messages
may or may not get sent.

> In my opinion the proper solution is to use the same semantics as the
> close() system call, in other words, zmq_close() shall invalidate the
> socket from the caller's point of view so no further operations may be
> performed on it, but 0MQ shall send any outstanding messages in the
> background *as long as a endpoint for those messages still exists* before
> destroying the socket "for real".

+1

> This would mean a second change to the API which would make zmq_term() a
> blocking call, since it would need to wait until all outstanding messages
> are sent. The analogous functionality for the close() system call is
> handled by the OS kernel -- obviously if the OS shuts down then data will
> be lost.

+1

> The downside is that zmq_term() could freeze for an arbitrary amount of
> time if the remote end is "stuck". For applications where this is
> undesirable it would mean adding a "KILL" flag or separate zmq_term_kill()
> function which means "we don't care, really go away now".
>
> Please let me know your opinions on this change; ultimately I think it's
> the right way to go especially if OS integration of 0MQ sockets is (a long
> way) down the road.

I think this would be a great change in the API.

Cheers,

Brian

> -mato
>
> [1] This behaviour can be changed using the SO_LINGER option, we'd probably
> want to implement a similar option for 0MQ sockets.
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-- 
Brian E. Granger, Ph.D.
Assistant Professor of Physics
Cal Poly State University, San Luis Obispo
bgranger at calpoly.edu
ellisonbg at gmail.com