[zeromq-dev] Not receiving unsubscription messages in XPUB socket with ZMQ_XPUB_VERBOSE when using a proxy

[Ricardo Catalinas Jiménez]

On Mon, Jul 20, 2015 at 08:43:55PM +0200, Pieter Hintjens wrote:
> Hi Ricardo,
> 
> Thanks for this analysis and proposal. In general we welcome patches
> (pull requests).

I'd gladly prepare a PR to fix the issue but given that the solution is
non-trivial without breaking the current API, I was asking for some
advice on the matter of how to proceed.

> On Mon, Jul 20, 2015 at 2:31 PM, Ricardo Catalinas Jiménez
> <jimenezrick at gmail.com> wrote:
> > (a) Stop filtering in XSUB socket at all and rely on this in XPUB
> >     sockets.  So that we modify the trie_t to not to have a refcnt field
> >     and xsub_t forwards all the (un)subscription messages it received.
> >
> >     This is my preferred solution although it could break applications
> >     that subscribe multiple times to the same topic and expect to stop
> >     receiving messages only when they unsubscribe the same number of
> >     times.  Although I'm not aware that this behavior is documented
> >     which could mean it isn't really a problem.

I checked the docs again and no, solution (a) isn't possible as it
breaks the contract described in the current API:

ZMQ_UNSUBSCRIBE:

"If the socket has several instances of the same filter attached the
ZMQ_UNSUBSCRIBE option shall remove only one instance, leaving the rest
in place and functional."

That leave us in a position where we need to keep the reference counting
in the XSUB side.

The only solution I can think about includes this two modifications:

1. Don't filter repeated unsubscribe messages in XPUB but keep the
   reference counting to not to break the API: we are already not
   filtering subscribe messages, so the other half of the logic should
   be symmetrical.

2. Add another flag to zmq_setsockopt named ZMQ_XPUB_UNSUBSCRIBE_VERBOSE
   so that this should be enabled in the XPUB sockets used in proxies to
   not to filter repeated unsubscribe messages to make sure the
   reference counting works properly propagating always these upstream
   up to the producer application which won't use this flag in its XPUB
   socket.

I don't like adding more flags to the API, but I cannot think of any
other way to solve this right now. Suggestions are welcome.


Best,
/Ricardo