[zeromq-dev] Notification of peer disconnection for ZMQ_STREAM sockets.

[André Caron]

Hi Pieter,

Created issue #48 (https://github.com/zeromq/zeromq4-x/issues/48) and sent
pull request #49 (https://github.com/zeromq/zeromq4-x/pull/49) to zeromq4-x.

As for the null message, I don't think it's a problem.  You can probably
signal all new connections using a null message provided appropriate
documentation.  In the meantime, I'll ignore null messages sent from
unknown peers (those from whom I haven't heard of yet).

Cheers,

André


On Mon, Jan 13, 2014 at 6:48 AM, Pieter Hintjens <ph at imatix.com> wrote:

> If you want a patch to the 4.0 fork you can make a test case and an
> issue. I can then backport it. Otherwise it'll come to libzmq master
> (= 4.1).
>
> One thing about receiving a null message; we'd thought at some stage
> to use this to (also) signal a new connection ready, for outgoing
> ZMQ_STREAM connections. This isn't really contradictory with what
> you're doing, and worth keeping in mind.
>
> On Mon, Jan 13, 2014 at 4:36 AM, André Caron <andre.l.caron at gmail.com>
> wrote:
> > Thanks Pieter,
> >
> > I've spent some time fiddling around with the code in order to submit a
> > patch.
> >
> > At first glance, it looks like I need to make a change in
> > "stream_engine.cpp" in order to push an empty message into the session
> (iff
> > "options.raw_sock" is true) in the "stream_engine_t::error()" method
> which
> > is called on disconnections of all kinds.  Everything else should take
> care
> > of itself provided that empty messages don't get caught up somewhere in
> the
> > routing back to the application.
> >
> > I wrote a small test program and it works nicely for ZMQ_STREAM sockets
> over
> > the TCP transport on Windows (so far, so good :-)  I'll run the test
> suite
> > on my Linux box tomorrow to see if I've broken anything for other types
> of
> > sockets.
> >
> > When I'm confident about my patch, I assume I should send a pull request
> to
> > the main development fork?  Would it also be possible to submit a pull
> > request to the 4.x fork (would probably mean a faster deployment to PyZMQ
> > which is my primary binding)?
> >
> > Cheers,
> >
> > André
> >
> >
> > On Sat, Jan 11, 2014 at 4:51 AM, Pieter Hintjens <ph at imatix.com> wrote:
> >>
> >> Hi André,
> >>
> >> Yes, this would be a good solution. You'll have to ask someone to help
> >> make the patch, or learn enough to make it yourself.
> >>
> >> -Pieter
> >>
> >> On Thu, Jan 9, 2014 at 6:05 AM, André Caron <andre.l.caron at gmail.com>
> >> wrote:
> >> > Hi there!
> >> >
> >> > First and foremost, kudos for all your awesome work on this excellent
> >> > library :-)
> >> >
> >> > I'm experimenting with ZMQ_STREAM sockets and I'm not sure how to
> handle
> >> > disconnection of peers.  The man page is pretty clear on how to
> forcibly
> >> > disconnect a peer (send 0-length message), but there is no information
> >> > about
> >> > handling disconnections.  Local tests using a simple Telnet server
> >> > implementation (pet project to accept control commands via Telnet in
> >> > ZMQ-based nodes) show me that the program never gets notified if the
> >> > peer
> >> > disconnects (at least, zmq_poll never marks the socket as readable).
> >> >
> >> > This piece is quite critical because an application that receives
> >> > non-framed
> >> > messages (such as an HTTP server) maintains per-connection state: in
> >> > particular, you need to buffer request data until a full request has
> >> > arrived.  This state must be dropped if the peer disconnects or the
> >> > connection is lost.
> >> >
> >> > Without some means to detect that a connection is no longer usable,
> ZMQ
> >> > programs must hold a per-connection timeout, and zmq_poll() using the
> >> > min of
> >> > all these timeouts and forcibly drop the state for whichever
> connection
> >> > has
> >> > expired, and then tell ZMQ to drop that connection.  Apart from
> >> > introducing
> >> > an unnecessary delay in cleanup, this is quite a bit of work to be
> >> > repeated
> >> > in each place where we use ZMQ_STREAM sockets... (I'm not hoping to do
> >> > this
> >> > routinely, it would still be a pain to maintain).
> >> >
> >> > It seems to me that ZMQ_STREAM is so close to looking like a real ZMQ
> >> > socket.  The last thing it needs IMO to be close enough would be to
> have
> >> > the
> >> > following behavior on peer-initiated disconnection:
> >> > - zmq_poll() shows disconnected ZMQ_STREAM sockets as readable; and
> >> > - zmq_msg_recv() return as zero-length message.
> >> > This would make for such a smoother experience of bridging ZMQ with
> >> > existing
> >> > protocols!
> >> >
> >> > Any thoughts on this?
> >> >
> >> > Cheers,
> >> >
> >> > André
> >> >
> >> > _______________________________________________
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> >> > zeromq-dev at lists.zeromq.org
> >> > http://lists.zeromq.org/mailman/listinfo/zeromq-dev
> >> >
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> >
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