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From Ted Dunning <ted.dunn...@gmail.com>
Subject Re: Using ZK for real-time group membership notification
Date Fri, 18 Mar 2011 22:51:13 GMT
For a very close to correct answer, take a look at KeptCollections.

On Fri, Mar 18, 2011 at 2:02 PM, Otis Gospodnetic <
otis_gospodnetic@yahoo.com> wrote:

> Hi,
>
> Short version:
> How can ZK be used to make sure that all group members know *the exact
> number of
> members at all times*?
>
> I have an app that can be run on 1 or more servers.  New instances of the
> app
> come and go, may die, etc. -- the number of the app instances is completely
> dynamic.  At any one time, as these apps come and go, each live instance of
> the
> app needs to know how many instances are there total.  If a new instance of
> the
> app is started, all instances need to know the new total number of
> instances.
> If an app is stopped or if it dies, the remaining apps need to know the new
> number of app instances.
>
> Also, and this is critical, they need to know about these
> additions/removals of
> apps right away and they all need to find out them at the same time.
> Basically,
> all members of some group need to know *the exact number of members at all
> times*.
>
> This sounds almost like we need to watch a "parent group znode" and monitor
> the
> number of its ephemeral children, which represent each app instance that is
> watching the "parent groups znode".  Is that right?  If so, then all I'd
> need to
> know is the answer to "How many watchers are watching this znode?" of "How
> many
> kids does this znode have?". And I'd need ZK to notify all watchers
> whenever the
> answer to this question changes.  Ideally it would send/push the answer
> (the
> number of watchers) to all watchers, but if not, I assume any watcher that
> is
> notified about the change would go poll ZK to get the number of ephemeral
> kids.
>
> I think the above is essentially what's described on
>
> http://eng.wealthfront.com/2010/01/actually-implementing-group-management.html,
> but doesn't answer the part that's critical for me (the very first Q up
> above).
>
> Thanks,
> Otis
> ----
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>
>

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