Henry's observation sounds right. I'd like to point out, though, that for BCP it might be interesting in some cases to allow multiple groups to contain prospective leaders. To tolerate any group of a set of groups failing completely, you would need at least 3 groups, so it is probably not applicable to your case of two groups.

On Jul 20, 2009, at 3:04 PM, Todd Greenwood wrote:

Henry, cool. When youre patch is ready for testing, I'll devote some
time to take a test pass on it.

-----Original Message-----
From: Henry Robinson [mailto:he...@cloudera.com]
Sent: Monday, July 20, 2009 2:54 PM
To: zookeeper-user@hadoop.apache.org
Subject: Re: Leader Elections

On Mon, Jul 20, 2009 at 7:50 PM, Todd Greenwood

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)

Looks to me like you don't even need hierarchical quorums for this -
everyone in group B an Observer and you're done.

I've been working on this feature. Recently we've been discussing a
proof-of-concept patch on the JIRA. I have nearly finished a less rough
patch which I will submit for discussion and potentially commit this
At that point it would be extremely helpful if you could help test the
patch, and you can start considering it for production. To get into
trunk I
will have to write a comprehensive test suite and update the
and then making sure all the boxes are ticked and no regressions are
up can take a little while.



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
virtual ensembles to interract on the same physical ensemble. For my
case, this would prevent unnecessary cross talk between nodes on a
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
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
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

2- Observers: you could have one computing center containing an
and observers around the edge just learning committed values.

Ted Dunning, CTO

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