Sage Weil wrote:
On Wed, 14 May 2008, Jamie Lokier wrote:
In that model, neighbour sensing is used to find the largest coherency
domains fitting a set of parameters (such as "replicate datum X to N
nodes with maximum comms latency T"). If the parameters are able to
be met, quorum gives you the desired robustness in the event of
node/network failures. During any time while the coherency parameters
cannot be met, the robustness reduces to the best it can do
temporarily, and recovers when possible later. As a bonus, you have
some timing guarantees if they are more important.
Anything that silently relaxes consistency like that scares me. Does
anybody really do that in practice?
Well, there's Amazon Dynamo, a distributed system that places most
importance on writes succeeding, if inconsistent. They choose to relax
consistency up front, and on the backend absorb the cost of merging
multiple versions of objects:
http://www.allthingsdistributed.com/2007/10/amazons_dynamo.html
(full paper)
Jeff
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