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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: Understand eventually consistent
Date Fri, 18 Feb 2011 15:37:45 GMT
On Fri, Feb 18, 2011 at 12:00 AM, Stu Hood <stuhood@gmail.com> wrote:
> But, the reason that it isn't safe to say that we are a strongly consistent
> store is that if 2 of your 3 replicas were to die and come back with no
> data, QUORUM might return the wrong result.

 Not so.  If you allow vaporizing arbitrary numbers of machines
without a trace then only systems that block for all replicas on each
update could be considered strongly consistent, and I don't know of
any systems in the wild that actually do that.  Certainly other
systems commonly considered "strongly consisent" like HBase do not.

> A requirement of a strongly consistent store is that replicas cannot begin
> answering queries until they are consistent

The system as a whole can be consistent even if an individual replica
is not; that is the point of CL  > ONE.

-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of DataStax, the source for professional Cassandra support
http://www.datastax.com

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