the processing of the write transaction is described in the zookeeper internals presentation on i think other presentations may also touch on it. we also have it in the ZooKeeper documentation:


Todd Greenwood wrote:
Flavio & Ted, thank you for your comments.

So it sounds like the only way to currently deploy to the WAN is to
deploy ZK Servers to the central DC and open up client connections to
these ZK servers from the edge nodes. True?

In the future, once the Observers feature is implemented, then we should
be able to deploy zk servers to both the DC and to the pods...with all
the goodness that Flavio mentions below.

Flavio - do you have a doc that describes exactly what happens in the
transaction of a write operation? For instance, I'd like to know at
exactly what stage a write has been commited to the ensemble, and not
just the zk server the client is connected to. I figure it must be
something like:

clientA.write(path, value)
-> serverA writes to memory
-> serverA writes to transacted disk every n/seconds or m/bytes
-> serverA sends write to Leader
-> Leader stamps with transaction id
-> Leader responds to ensemble with update + transaction id


-----Original Message-----
From: Flavio Junqueira [] Sent: Friday, July 24, 2009 4:50 PM
Subject: Re: Zookeeper WAN Configuration

Just a few quick observations:

On Jul 24, 2009, at 4:40 PM, Ted Dunning wrote:

On Fri, Jul 24, 2009 at 4:23 PM, Todd Greenwood

Could you explain the idea behind the Observers feature, what this
concept is supposed to address, and how it applies to the WAN
configuration problem in particular?

Not really. I am just echoing comments on observers from them that know.

Without observers, increasing the number of servers in an ensemble enables higher read throughput, but causes write throughput to drop because the number of votes to order each write operation increases. Essentially, observers are zookeeper servers that don't vote when ordering updates to the zookeeper state. Adding observers enables higher read throughput affecting minimally write throughput (leader still has to send commits to everyone, at least in the version we have been working on).

The ideas for federating ZK or allowing observers would likely do what
want.  I can imagine that an observer would only care that it can see
local peers and one of the observers would be elected to get updates
thus would care about the central service).
This certainly sounds like exactly what I want...Was this introduced in
3.2 in full, or only partially?

I don't think it is even in trunk yet. Look on Jira or at the recent logs
of this mailing list.

It is not on trunk yet.


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