Re: [infinispan-dev] ISPN-263 and handling partitions
On Wed, Apr 17, 2013 at 5:53 PM, Manik Surtani <msurtani(a)redhat.com> wrote:
On 17 Apr 2013, at 08:23, Dan Berindei <dan.berindei(a)gmail.com> wrote:
I like the idea of always clearing the state in members of the minority
partition(s), but one problem with that is that there may be some keys that
only had owners in the minority partition(s). If we wiped the state of the
minority partition members, those keys would be lost.
Right, this is my concern with such a wipe as well.
Of course, you could argue that the cluster already lost those keys when
we allowed the majority partition to continue working without having those
keys... We could also rely on the topology information, and say that we
only support partitioning when numOwners >= numSites (or numRacks, if there
is only one site, or numMachines, if there is a single rack).
This is only true for an embedded app. For an app communicating with the
cluster over Hot Rod, this isn't the case as it could directly read from
the minority partition.
For that to happen, the client would have to be able to keep two (or more)
active consistent hashes at the same time. I think, at least in the first
phase, the servers in a minority partition should send a "look somewhere
else" response to any request from the client, so that it installs the
topology update of the majority partition and not the topology of one of
the minority partitions.
One other option is to perform a more complicated post-merge state
transfer, in which each partition sends all the data it has to all
the
other partitions, and on the receiving end each node has a "conflict
resolution" component that can merge two values. That is definitely more
complicated than just going with a primary partition, though.
That sounds massively expensive. I think the right solution at this point
is entry versioning using vector clocks and the vector clocks are exchanged
and compared during a merge. Not the entire dataset.
True, it would be very expensive. I think for many applications just
selecting a winner should be fine, though, so it might be worth
implementing this algorithm with the versioning support we already have as
a POC.
One final point... when a node comes back online and it has a local cache
store, it is very much as if we had a merge view. The current
approach is
to join as if the node didn't have any data, then delete everything from
the cache store that is not mapped to the node in the consistent hash.
Obviously that can lead to consistency problems, just like our current
merge algorithm. It would be nice if we could handle both these cases the
same way.
+1
I've been thinking about this some more... the problem with the local cache
store is that nodes don't necessary start in the same order in which they
were shut down. So you might have enough nodes for a cluster to consider
itself "available", but only a slight overlap with the cluster as it looked
the last time it was "available" - so you would have stale data.
We might be able to save the topology on shutdown and block startup until
the same nodes that were in the last "available" partition are all up, but
it all sounds a bit fragile.
Cheers
Dan
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