the processing of the write transaction is described in the zookeeper
internals presentation on
http://wiki.apache.org/hadoop/ZooKeeper/ZooKeeperPresentations i think
other presentations may also touch on it. we also have it in the
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
-> 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
From: Flavio Junqueira [mailto:f...@yahoo-inc.com]
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
Not really. I am just echoing comments on observers from them that
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?
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).
I don't think it is even in trunk yet. Look on Jira or at the
The ideas for federating ZK or allowing observers would likely do
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
3.2 in full, or only partially?
of this mailing list.
It is not on trunk yet.