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From Ignasi Rius <irius...@gmail.com>
Subject N+1 redundancy with Curator
Date Wed, 10 Jul 2013 10:18:18 GMT
Hi all,

I am a newbie to Zookeeper and Curator. After a couple of weeks
reading, testing and playing with it, I am quite convinced that it is
a great solution for our cluster management needs.

I would like to ask you some advice on how could I implement the
following behaviour based on Curator recipes:

We provide a service which is split/distributed among N nodes, and we
would like to implement a N+1 redundancy strategy. That is N active
nodes plus 1 spare node which can "behave" as any of the others.

Indeed, when each node starts, it should take the following role:
 - a) Behave like a Spare Node: iif there are already "N" nodes working.
 - b) Behave like node (M+1): if  (M < N) (assuming M is the number of
current workers).

Then, in case that any of the N working nodes dies, the Spare one
should behave like the one that died.


Summarizing, as an exemple with  3 hosts (1,2 and 3) this is a
possible sequence of events:

- Host 1 starts and acts as Node 1.
- Host 2 starts and acts as Node 2.
- Host 3 starts and acts as a Spare Node.
- Host 2 dies.
- Host 3 takes over and acts as Node 2.


My question is about what do you guys think would be the best
"practice" for implementing such scenario?

I thought that the simplest thing to do was using an
InterProcessSemaphoreV2 (N), so if a node can acquire the semaphore,
it means that it is not the "Spare" one. However, I don't see a way in
Curator to find out "which node I am" in case I can acquire de
semaphore.

On the contrary, LeaderLatch seems to handle participant IDs, etc...
but there is only "one leader" at a time (obviusly).

Thanks for your time. Any ideas are welcome.

Best,
Ignasi

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