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From AJ>
Subject Re: Is LOCAL_QUORUM as strong as QUORUM?
Date Thu, 23 Jun 2011 02:53:27 GMT
On 6/22/2011 8:20 PM, mcasandra wrote:
> Well it depends on the requirements. If you use any combination of CL with
> EACH_QUORUM it means you are accepting the fact that you are ok if one of
> the DC is down. And in your scenario you care more about DCs being
> consistent even if writes were to fail. Also you are ok with network
> latency.
> I think there is a broader design question here and you might be able to
> solve it with LOCAL_QUORUM if you handled it at application or load
> balancing layer. Is this active/active data center? What's your actual
> requirements? Are these external clients that can go to any data center?
> --
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I require 3 (or more) geographically diverse dc's serving local users.  
The next arbitrary closest dc will serve as a 1-replica fail-over for 
the previous dc in case it becomes unavail altogether.  So, each dc is 
active for it's locale and a failover for one of the others; like a 
daisy chain configuration.  I was imagining a series of events where the 
primary dc gets updated at local_quorum, followed by that dc losing all 
network connectivity before the backup gets the change.  Then, the same 
user gets redirected to the backup dc and does a read at local_quorum 
and gets stale data.

But, I realize now if I substituted each_quorum for local_quorum for 
writes, then, in the case of fail-over, the writes would fail.  That's 
fine for consistency's sake, but is a high price to pay.  I have to 
think on this more and what I want.  Thanks for the help.

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