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From Ben Bangert <...@groovie.org>
Subject Re: Use zookeeper with Cassandra
Date Fri, 19 Jul 2013 15:29:09 GMT
On Jul 19, 2013, at 5:59 AM, Brian Tarbox <briantarbox@gmail.com> wrote:

> Can you say more about this?  We use Zookeeper to manage a large set of
> "work units" which are selected, locked, processed by EC2 worker
> computers...which then do work on and with Cassandra data.

Sure, there's two parts to it. First, holding a lock external to Cassandra itself has its
own issue. This is because while you might be able to guarantee that only one person is doing
something with Cassandra, short of using CL.ALL, there's no way to fully ensure that two people
don't stomp on updates. If you use CL.QUORUM, its still possible that a node could be holding
hints, client releases ZK lock, another client gets the lock, does a read and if timed poorly
will not see the prior change.

Cassandra's CAS operation properly deals with this since its handled inside Cassandra itself.

Jonathan Ellis went over why client locks don't work for Cassandra at the Cassandra Summit,
slides from his talk here:
http://www.slideshare.net/jbellis/cassandra-summit-2013-keynote

He goes over CAS on slide 52-60.

Cheers,
Ben
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