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Thomas Rodgers rodgert at twrodgers.com
Fri May 30 20:50:17 CEST 2014


zmq_socket_monitor?

http://api.zeromq.org/4-0:zmq-socket-monitor


On Fri, May 30, 2014 at 9:21 AM, Jeremy Richemont <jrichemont at gmail.com>
wrote:

> 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?
>
> Jeremy
>
>
> 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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