[zeromq-dev] Load distribution vs fail over

[gonzalo diethelm]

Hi Martin,

> Yes. You are right. The feature you are describing is absolutely
needed.
> It's on the roadmap, but as always, the share amount of stuff on the
> roadmap... Anyway, here are my comments:

It is great to see one is not alone and thinking of weird, useless
ideas... Thanks for validating my musings!

> Failover as described in your scenario (but needed in other scenarios
as
> well - just think of multihoming) should be available for both
> UPSTREAM/DOWNSTEAM and REQ/REP sockets.
> 
> It should kick in if primary service is disconnected, dead or busy.
> 
> Once the primary service is ready to process messages the system
should
> fail back automatically.
> 
> The user should be able to combine failover with load balancing. I.e.
> load balance among instances R1..R10. When none is available, fail
over
> to R11.
> 
> There should be multiple failover levels: If R1 is not available fail
> over to R2. If even R2 is not available fail over to R3.

You captured all my requirements, and then some. I was starting small,
but everything you mention is, of course, desirable.

> The current idea is to introduce ZMQ_PRIORITY socket option:
> 
> socket_t s (ctx, ZMQ_DOWNSTREAM);
> s.setsockopt (ZMQ_PRIORITY, 1);
> s.bind ("tcp://eth0:5555");
> s.setsockopt (ZMQ_PRIORITY, 2);
> s.bind ("tcp://eth0:5556");
> 
> This way load balancing would be done among all the peers connected to
> port 5555. If none of them is available, the system will fail over to
> peers connected to port 5556 and load balance among them.
> 
> One of the important applications of such mechanism is using local
> instances (say those located in the same datacenter) if possible and
> failing over to remote instances (say those located in a distant
> datacenter connected by slow satellite link) in case no local services
> are available.

This would be perfect.

> As for the condition for fail-over, queue limits (ZMQ_HWM) should be
> used. These still weren't ported from 0MQ/1.0, but as I am said the

code
> is ready and will be commited shortly.

Ok, knowing this is on the pipeline is great. Thank you very much.

-- 
Gonzalo Diethelm



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