[infinispan-dev] Eventual consistency

[Sanne Grinovero]

Hi Manik,
can you explain the first cause, why is it that during a rehash you're
unable to get an answer to a GET?
If a node A having installed the view T'' is receiving a GET request
from B which is still having the outdated view T', while he's not the
owner any more as the new view changed to T'' and so he just
transferred the requested value to a node C,
he definitely knows how to handle the request by forwarding it to C: B
was the owner before - otherwise it wouldn't receive the request, and
because it isn't any more he must be aware of the new hash
configuration.
He still stays in the middle of communication, and then sends the
requested value to A along with enough information about the new view
to avoid more erroneous requests.

About your proposal, what would happen if the owner crashes before he
has async-written the changes to a secondary node?

Cheers,
Sanne

2011/3/2 Manik Surtani <manik at jboss.org>:
> As consistency models go, Infinispan is primarily strongly consistent (with
> 2-phase commit between data owners), with the exception of during a rehash
> where because of eventual consistency (inability to get a valid response to
> a remote GET) forces us to wait for more responses, a quorum if you like.
>  Not dissimilar to PAXOS [1] in some ways.
> I'm wondering whether, for the sake of performance, we should also offer a
> fully eventually consistent model?  What I am thinking is that changes
> *always* occur only on the primary data owner.  Single phase, no additional
> round trips, etc.  The primary owner then asynchronously propagates changes
> to the other data owners.  This would mean things run much faster in a
> stable cluster, and durability is maintained.  However, during rehashes when
> keys are moved, the notion of the primary owner may change.  So to deal with
> this, we could use vector clocks [2] to version each entry.  Vector clocks
> allow us to "merge" state nicely in most cases, and in the case of reads,
> we'd flip back to a PAXOS style quorum during a rehash to get the most
> "correct" version.
> In terms of implementation, almost all of this would only affect the
> DistributionInterceptor and the DistributionManager, so we could easily have
> eventually consistent flavours of these two components.
> Thoughts?
> Cheers
> Manik
> [1] http://en.wikipedia.org/wiki/Paxos_algorithm
> [2] http://en.wikipedia.org/wiki/Vector_clock
> --
> Manik Surtani
> manik at jboss.org
> twitter.com/maniksurtani
> Lead, Infinispan
> http://www.infinispan.org
>
>
>
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