[zeromq-dev] Implementing failover in client side

Pieter Hintjens ph at imatix.com
Tue Oct 5 06:15:17 CEST 2010


Hi Brian,

I've not yet covered reliable request-reply / failover in the user
guide yet.  0MQ doesn't do it directly.

I'd probably look at using XREQ rather than REQ, so you can keep the
socket open, and possibly even XREP so you can explicitly address the
different servers.  Look at the custom load balancing and LRU examples
in Ch3 of the guide, they may help.

Most probably you'll want a queue device in between your applications
and your servers that does LRU workload distribution and retries if a
server dies / does not respond.

-Pieter

On Tue, Oct 5, 2010 at 1:28 AM, Iñaki Baz Castillo <ibc at aliax.net> wrote:
> Hi, I'm trying to understand the proper way to implement failover in a
> ZMQ requester (not just load balancing). Let me expose the scenario I
> want to achieve:
>
>
> 1) An application processes SIP requests (VoIP) in a single process.
>
> 2) The application has a ZMQ REQ socket connected to N servers (ZMQ
> REP), so there is load balanding.
>
> 3) When a SIP request arrives the application sends a message using
> ZMQ REQ (so it expects a response) to a server (which runs a ZMQ REP
> socket). Let's say it chooses server-1.
>
> 4) The client waits (blocking) the reply for some ms (using zmq_poll).
>
> 4.1) If it arrives then it does something with it, and continues
> processing the SIP request.
>
> 4.2) If the response doesn't arrive (perhaps server-1 is down) the
> client should try with server-2 (I expect ZMQ provides this failover
> capability in someway but couldn't find it).
>
> 4.3) In case all the servers are down then the client would know it
> because no response was received in the REQ socket. Then it would
> terminate the SIP request processing in any other way, or continue
> with it and log an error, or whatever.
> Then when a new SIP request arrives the client must re-open the ZMQ
> REQ socket (if not it can not send a new request because no response
> was received). Also, I don't want the previous lost response to be
> received now (I don't want the queue feature), this is, if the
> response didn't arrive within N ms due to network issues, I don't want
> the response to arrive later. The ZMQ response message only makes
> sense when processing the SIP request for which the correspnding ZMQ
> request was sent. The only way to get it I can imagine is closing and
> re-opening the ZMQ REQ socket after detecting error running zm_send().
>
>
> Unfortunatelly I cannot find in the documentation how to achieve this
> scenario, it seems that ZMQ doesn't provide failover in client side at
> all (but just load balancing when using REQ-REP). So I've thinking in
> how to get this behavior with ZMQ and the resulting code would change
> a lot the "expected" use case of ZMQ, becoming very ugly so it seems
> I'm not using ZMQ. Or perhaps I miss something :)
>
>
> Any recommendation about it? Thanks a lot.
>
> --
> Iñaki Baz Castillo
> <ibc at aliax.net>
> _______________________________________________
> zeromq-dev mailing list
> zeromq-dev at lists.zeromq.org
> http://lists.zeromq.org/mailman/listinfo/zeromq-dev
>



-- 
-
Pieter Hintjens
iMatix - www.imatix.com



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