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Jeremy Richemont jrichemont at gmail.com
Fri May 30 16:21:21 CEST 2014

Thanks, Charles. I did, in fact, find that pattern. The problem is it does
not match what I am trying to do. That pattern for when you have state +
deltas. What I have is a continuous message stream which, once started to
client x must be preserved even if client x dies for a bit (not forever of
course, I put an SLA of 1 million messages/client) and then reconnects,
every message it missed is replayed, in order, then the live stream resumes.

It needs to handle n clients, any of which may drop and reconnect so each
one will need an independent message cache. PUB/SUB will not do for this
because I may need to send messages 10 - 100 to client x on reconnect but
50 - 200 to client y.

Asking for state is a good idea - ask for missing updates in my case - but
the question remains; how does the server know the client is no longer
available and it must therefore start backing up messages from a PUB
socket? The client can't tell it over OOB because it died already.

If I could just query PUB and get a list of clients plus a notification
when one drops that'd solve the problem I think. But how to do that?


On 30 May 2014 15:00, Charles Remes <lists at chuckremes.com> wrote:

> Take a look at the Clone pattern in the zguide.
> http://zguide.zeromq.org/page:all#Reliable-Pub-Sub-Clone-Pattern
> This might be what you need.
> cr
> On May 29, 2014, at 11:20 AM, Jeremy Richemont <jrichemont at gmail.com>
> wrote:
> >
> > Hi. I am struggling to work out how to use zmq to implement the
> architecture I need. I have a classic publish/subscribe situation except
> that once client x has subscribed to a topic I need the topic data to be
> sent to it to be cached if the client dies and resent on reconnect. The
> data order is important and I can't miss messages should the client be
> offline for a while.
> >
> > The PUB/SUB pattern doesn't seem to know about individual clients and
> will just stop sending to client x if it dies. Plus I can't find out this
> has happened and cache the messages, or know when it reconnects.
> >
> > To try to get around this I used the REQ/REP pattern so the clients can
> announce themselves and have some persistence but this is not ideal for a
> couple of reasons:
> >
> > 1) The clients must constantly ask "got any data for me?" which offends
> my sensibilities
> >
> > 2) What happens if there's no data to send to client x but there is to
> client y? Without zmq I'd have had a thread per client and simply block the
> one with no data but I can't block client x without also blocking client y
> in a single thread.
> >
> > Am I trying to shove a round peg in a square hole, here? Is there some
> way I can get feedback from PUB saying 'failed to send to client x'? so I
> can cache the messages instead? Or is there some other pattern I should be
> using?
> >
> > Otherwise it's back to low level tcp for me...
> >
> > Many thanks;
> >
> > Jeremy
> >
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