[zeromq-dev] Forwarding ROUTER to PUB

[Lindley French]

That will work, of course....I'm just curious what the resistance is to
letting the HWM policy be settable.


On Thu, Jan 16, 2014 at 3:27 PM, Goswin von Brederlow <goswin-v-b at web.de>wrote:

> On Thu, Jan 16, 2014 at 02:45:31PM -0500, Lindley French wrote:
> > > This is a common issue. If you can?t recover from dropped messages,
> > PUB/SUB is not the correct pattern.
> >
> > In many cases, this is correct. I do not believe inproc is one of those
> > cases, however. With inproc, you should have full control of who is
> > subscribing and when they come up relative to the publisher. If your
> > subscribers aren't running when you expect them to be running, that's a
> > bug. If they aren't running fast enough, dropping messages *might* be a
> > solution, or it might not. I don't feel that's a decision that can be
> made
> > in the general case.
> >
> > Let me put it this way: If I need one-to-many semantics with backpressure
> > and filtering, what should I use? PUB is the only one-to-many socket
> type.
> > I can write my own filtering code, keep a vector of push sockets, etc.
> but
> > that seems to defeat the point of ZMQ patterns. PUB is exactly what I
> want
> > in every way *except* the HWM behavior.
> >
> >
> > On Thu, Jan 16, 2014 at 2:11 PM, Charles Remes <lists at chuckremes.com>
> wrote:
> >
> > > This is a common issue. If you can?t recover from dropped messages,
> > > PUB/SUB is not the correct pattern.
> > >
> > > This of PUB as a radio antenna. It broadcasts regardless of whether or
> not
> > > anyone is listening. If there are no listeners, every packet gets
> dropped.
> > > If a listener is slow, then packets will get dropped. If you need
> further
> > > guarantees about delivery, then you need to build some kind of protocol
> > > (ack, nak, ack window, etc) on top of DEALER/ROUTER.
> > >
> > > Also, as of libzmq 3.3, I believe the default HWM is 1000 (to prevent
> > > memory exhaustion in the default configuration). If you want ?infinite?
> > > then setsockopt to -1.
> > >
> > > As for the dropped messages on inproc, you need to be careful to
> confirm
> > > that a listener (SUB) is actually up, running and *connected* before
> you
> > > start PUB?ing otherwise the PUB socket will drop messages.
> Synchronization
> > > for this is discussed in the guide. Alternately, just have your PUB
> ?sleep?
> > > for a second after the SUB bind/connects and you should be okay.
> > >
> > >
> > > On Jan 16, 2014, at 1:03 PM, Lindley French <lindleyf at gmail.com>
> wrote:
> > >
> > > Well, aside from the router issue I do like the arrangement for easily
> > > handling different messages in different places. However, there may be
> a
> > > fatal flaw at the moment: PUB's desire to drop messages at the HWM.
> While
> > > making "drop" a default behavior for PUB is fine, I really don't like
> that
> > > it's the *only* behavior possible.
> > >
> > > Then again, that may or may not be the issue here. I haven't touched
> the
> > > HWM, so it should still be 0 which is theoretically infinite.
> Nonetheless,
> > > a bunch of my messages in a row vanished into the ether somewhere
> between
> > > PUB and SUB inproc sockets.
> > >
> > >
> > > On Thu, Jan 16, 2014 at 1:02 PM, Lindley French <lindleyf at gmail.com
> >wrote:
> > >
> > >> I tend to stuff in as many different features as I can when I'm first
> > >> learning something new, it helps me get a feel for it.
> > >>
> > >> You should have seen my first major python program.....
> > >>
> > >>
> > >> On Thu, Jan 16, 2014 at 12:46 PM, Charles Remes <lists at chuckremes.com
> >wrote:
> > >>
> > >>> Create a socket for each worker thread and have your main thread
> resend
> > >>> the message down the appropriate socket. Sometimes it isn?t a good
> idea to
> > >>> try and shoe-horn every zeromq socket pattern into your app. :)
> > >>>
> > >>> On Jan 16, 2014, at 9:54 AM, Lindley French <lindleyf at gmail.com>
> wrote:
> > >>>
> > >>> > A problem I was wrestling with was, how do I deal with a TCP
> > >>> connection where messages of different types may arrive, and may
> need to be
> > >>> dealt with in different threads? The TCP socket can't be touched
> directly
> > >>> by multiple threads, of course. The obvious solution was to
> immediately
> > >>> forward messages arriving on the TCP socket to an inproc socket.
> > >>> >
> > >>> > I then took it one step further: why not make that inproc socket a
> PUB
> > >>> socket and make the first part of each message be a topic
> identifier, so
> > >>> that whichever thread knows how to deal with a particular message
> can just
> > >>> subscribe to it and ignore the rest?
> > >>> >
> > >>> > That's a great design, right up until I try to do it with the TCP
> > >>> socket being a ROUTER. Now, no matter what the first part of the sent
> > >>> message is, the identity will end up being the first part on the
> receiving
> > >>> end. The PUB/SUB won't work without some tweaking.
> > >>> >
> > >>> > I don't want to just drop the identity; that's useful information.
> I
> > >>> could swap the first two parts; that will work, but it's unintuitive
> and
> > >>> could cause confusion down the road.
> > >>> >
> > >>> > Any other ideas?
>
> Add a splitter that simply checks the first frame, looks up the right
> target socket and sends the remainder on the message onwards.
> Then you can use a simple PUSH/PULL pattern.
>
> MfG
>         Goswin
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