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From Mike Malone <m...@simplegeo.com>
Subject Re: Write everywhere, read anywhere
Date Thu, 04 Aug 2011 17:58:03 GMT
On Thu, Aug 4, 2011 at 10:25 AM, Jeremiah Jordan <
JEREMIAH.JORDAN@morningstar.com> wrote:

>  If you have RF=3 quorum won’t fail with one node down.  So R/W quorum
> will be consistent in the case of one node down.  If two nodes go down at
> the same time, then you can get inconsistent data from quorum write/read if
> the write fails with TimeOut, the nodes come back up, and then read asks the
> two nodes that were down what the value is.  And another read asks the node
> that was up, and a node that was down.  Those two reads will get different
> answers.
>

So the short answer is: yea, same thing can happen with quorum...

It's true that the failure scenarios are slightly different, but it's not
entirely true that two nodes need to fail to trigger inconsistencies with
quorum. A single node could be partitioned and produce the same result.

If a network event occurs on a single host then any writes that came in
before the event, that are processed before phi evict kicks in and marks the
rest of the cluster unavailable, will be written locally. From the rest of
the cluster's perspective only one node "failed," but from that node's
perspective the entire rest of the cluster failed. Obviously, similar things
could happen with DC_QUORUM if a datacenter went offline.

Mike

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