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From Ignasi Rius <irius...@gmail.com>
Subject Re: N+1 redundancy with Curator
Date Thu, 11 Jul 2013 15:26:23 GMT
Hi Jordan, Eric, thanks for your advices.

InterProcessSemaphoreV2.getParticipantNodes() is fine, but as Jordan
says, I would like that each participant knows who it is ordinally.
The reason is that I would like to use this number as an identifier
for the "role" I want that node to fulfill.

Eric's suggestion will do the job and looks  pretty easy to implement
and mantain. I will store a "list of roles" somewhere, and once a node
acquires de semaphore, it will try to create each "role node" until it
succeeds. That's it.

Thanks!

2013/7/10 Eric Tschetter <echeddar@gmail.com>:
> The other option is to have a specific znode for each "role" that you want
> fulfilled.  On startup, you have each process race to claim one of those
> znodes by creating an ephemeral node.  If it manages to create its ephemeral
> node, then it acts as that role.  If it gets a NodeExistsException, then it
> tries another role.
>
> This is similar to electing a leader for each role.  It does cause some
> stampeding when a node goes down, though.  There also is no recipe for this
> currently.
>
> --Eric
>
>
> On Wednesday, July 10, 2013, Jordan Zimmerman wrote:
>>
>> I would use InterProcessSemaphoreV2. Does each node need to know which
>> participant it is ordinally? If so, I'm curious why. In any event, you can
>> use setNodeData() to set an identifier for each participant. Just like
>> LeaderLatch InterProcessSemaphoreV2 has a getParticipantNodes() method.
>>
>> -Jordan
>>
>> On Jul 10, 2013, at 3:18 AM, Ignasi Rius <iriusvsn@gmail.com> wrote:
>>
>> > 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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