Flavio, Ted, Henry, Scott, this would perfectly well for my use case

        GROUP A : ZK Servers w/ read/write AND Leader Elections
        GROUP B : ZK Servers w/ read/write W/O Leader Elections

So, we can craft this via Observers and Hiererarchial Quorum groups?
Great. Problem solved.

When will this be production ready? :o)


Scott brought up a multi-feature that is very interesting for me.

1. Offline ZK servers that sync & merge on reconnect

The offline servers seems conceptually simple, it's kind of like a
messaging system. However, the merge and resolve step when two servers
reconnect might be challenging. Cool idea though.

2. Partial memory graph subscriptions

The second idea is partial memory graph subscriptions. This would enable
virtual ensembles to interract on the same physical ensemble. For my use
case, this would prevent unnecessary cross talk between nodes on a WAN,
allowing me to define the subsets of the memory graph that need to be
replicated, and to whom. This would be a huge scalability win for WAN
use cases.


-----Original Message-----
From: Scott Carey [mailto:sc...@richrelevance.com] 
Sent: Monday, July 20, 2009 11:00 AM
To: zookeeper-user@hadoop.apache.org
Subject: Re: Leader Elections

Observers would be awesome especially with a couple enhancements /

An option for the observers to enter a special state if the WAN link
goes down to the "master" cluster.  A read-only option would be great.
However, allowing certain types of writes to continue on a limited basis
would be highly valuable as well.  An observer could "own" a special
node and its subnodes.  Only these subnodes would be writable by the
observer when there was a session break to the master cluster, and the
master cluster would take all the changes when the link is
reestablished.  Essentially, it is a portion of the hierarchy that is
writable only by a specitfic observer, and read-only for others.
The purpose of this would be for when the WAN link goes down to the
"master" ZKs for certain types of use cases - status updates or other
changes local to the observer that are strictly read-only outside the
Observer's 'realm'.

On 7/19/09 12:16 PM, "Henry Robinson" <he...@cloudera.com> wrote:

You can. See ZOOKEEPER-368 - at first glance it sounds like observers
be a good fit for your requirements.

Do bear in mind that the patch on the jira is only for discussion
I would not consider it currently fit for production use. I hope to put
up a
much better patch this week.


On Sat, Jul 18, 2009 at 7:38 PM, Ted Dunning <ted.dunn...@gmail.com>

> Can you submit updates via an observer?
> On Sat, Jul 18, 2009 at 6:38 AM, Flavio Junqueira <f...@yahoo-inc.com>
> wrote:
> > 2- Observers: you could have one computing center containing an
> > and observers around the edge just learning committed values.
> --
> Ted Dunning, CTO
> DeepDyve

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