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Date: Fri, 24 Aug 2012 21:55:09 -0700
From: Philip O'Toole <philip@loggly.com>
To: user@cassandra.apache.org
Subject: QUORUM writes, QUORUM reads -- and eventual consistency
Message-ID: <20120825045509.GA2237@loggly.com>
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Hello -- perhaps someone could provide me some clarification about this.

From:

    http://www.datastax.com/docs/1.1/dml/data_consistency#data-consistency

"If consistency is top priority, you can ensure that a read will always reflect the most recent write by using the following formula:

(nodes_written + nodes_read) > replication_factor"

But consider this. Say I have a replication factor of 3. I request a QUORUM write, and it fails because the write only reaches 1 node. Perhaps there is a temporary partition in my cluster. Now, asynchronously, a different reader performs a QUORUM read of the same cluster and just before it issues the read, the partition is resolved. The quorum read is satisfied by the two nodes that have *not* received the latest write (yet). Doesn't this mean that the read does not "reflect the most recent write"? I realise this is very unlikely to happen in practise, but I want to be sure I understand all this.

Perhaps the documentation would be more correct if the statement read as "...reflect the most recent SUCCESSFUL write..."?

Thanks,

Philip

-- 
Philip O'Toole
Senior Developer
Loggly, Inc.
San Francisco, CA