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From Patrick Hunt <>
Subject Re: Cassandra Multiple DataCenter Suitability - why?
Date Fri, 18 Jun 2010 17:42:35 GMT

On 06/18/2010 01:20 AM, wrote:
> I also read about an implemenetation of Rack Awareness employing
> Zookeeper, but I gather that wasn't released by Facebook and it was more
> geared towards single-DC rack awareness because Zookeeper is a bit heavy
> on the bandwidth.

Bandwidth is not the issue with a cross-colo ZooKeeper ensemble -- 
latency is the issue.

ZK is a quorum based service, a majority of the servers need to agree to 
every change (writes, reads are serviced locally by the server and don't 
face this issue). If the latency between servers is high then write 
operations will take longer. Generally this is "4L", so if you have 10ms 
latency btw colos it will take 40ms for a write to complete, if you have 
100ms latency btw colos it will take 400ms, etc... This is not an issue 
for "in colo" deployments since latency is typically very low. If you 
are using ZK for high level coordination then 100ms latency might not be 
bad, if you are using ZK for fine grained sharding it might be...


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