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[Benjamin Cordes]

To answer the question about cloning more precisely. You have a counter
with the publication. Then the client will know it is behind and will
request the delta. This example covers adding data to a hashmap. What you
will have to do is expand it a bit to apply adds and deletes (if you have
deletes). http://zguide.zeromq.org/py:clonesrv2


On Fri, May 30, 2014 at 8:50 PM, Thomas Rodgers <rodgert at twrodgers.com>
wrote:

> 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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>>>
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>>
>>
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