[infinispan-dev] PutForExternalRead consistency

[William Burns]

I wonder if we are over analyzing this.  It seems the main issue is
that the replication is done asynchronously.  Infinispan has many ways
to be make something asynchronous.  In my opinion we just chose the
wrong way.  Wouldn't it be easier to just change the PFER to instead
of passing along the FORCE_ASYNCHRONOUS flag we instead just have the
operation performed asynchronous using putIfAbsentAsync ?  This way
the lock is held during the duration of the replication and should be
consistent with other operations.  Also the user can regain control
back faster as it doesn't even have to process the local interceptor
chain.  We could also change the putForExternalRead method declaration
to also return a NotifiableFuture<Void> or something so they know when
the operation is completed (if they want).

 - Will

On Thu, Nov 21, 2013 at 9:54 AM, Dan Berindei <dan.berindei at gmail.com> wrote:
>
>
>
> On Thu, Nov 21, 2013 at 12:35 PM, Galder Zamarreño <galder at redhat.com>
> wrote:
>>
>>
>> On Nov 18, 2013, at 12:42 PM, Dan Berindei <dan.berindei at gmail.com> wrote:
>>
>> >
>> >
>> >
>> > On Mon, Nov 18, 2013 at 9:43 AM, Galder Zamarreño <galder at redhat.com>
>> > wrote:
>> >
>> > On Nov 14, 2013, at 1:20 PM, Pedro Ruivo <pedro at infinispan.org> wrote:
>> >
>> > > Hi,
>> > >
>> > > Simple question: shouldn't PFER ensure some consistency?
>> > >
>> > > I know that PFER is asynchronous but (IMO) it can create
>> > > inconsistencies
>> > > in the data. the primary owner replicates the PFER follow by a PUT
>> > > (PFER
>> > > is sent async log the lock is released immediately) for the same key,
>> > > we
>> > > have no way to be sure if the PFER is delivered after or before in all
>> > > the backup owners.
>> > >
>> > > comments?
>> >
>> > Assuming that PFER and PUT happen in the same thread, we're normally
>> > relying on the JGroups sequence of events to send the first, wait no
>> > response, and then send the second put. That should guarantee order in which
>> > puts are received in the other nodes, but after that yeah, there's a risk
>> > that it could happen. PFER and PUT for a given key normally happen in the
>> > same thread in cache heavy use cases such as Hibernate 2LC, but there's no
>> > guarantee.
>> >
>> > I don't think that's correct. If the cache is synchronous, the PUT will
>> > be sent as an OOB message, and as such it can be delivered on the target
>> > before the previous PFER command. That's regardless of whether the PFER
>> > command was sent as a regular or as an OOB message.
>>
>> ^ Hmmmm, that's definitely risky. I think we should make PFER local only.
>>
>> The fact that PFER is asynchronous is nice to have. IOW, if you read a
>> value from a database and you want to store it in the cache for later read,
>> the fact that it's replicated asynchronously is just so that other nodes can
>> take advantage of the value being in the cache. Since it's asynchronous some
>> nodes could fail to apply, but that's fine since you can go to the database
>> and re-retrieve it from there. So, making PFER local only would be the
>> degenerate case, where all nodes fail to apply except the local node, which
>> is fine. This is better than having the reordering above.
>>
>> In a chat I had with Dan, he pointed out that having PFER local only would
>> be problematic for DIST mode w/ L1 enabled, since the local write would not
>> invalidate other nodes, but this is fine because PFER only really makes
>> sense for situations where the Infinispan is used as a cache. So, if the
>> data is in the DB, you might as well go there (1 network trip), as opposed
>> to askign the other nodes for data and the database in the worst case (2
>> network trips).
>>
>> PFER is really designed for replication or invalidation use cases, which
>> are precisely the ones configured for Hibernate 2LC.
>>
>> Thoughts?
>>
>
> +1 to make PFER local-only in replicated caches, but I now think we should
> go all the way and disallow PFER completely in dist caches.
>
> I still think having L1 enabled would be a problem, because a regular put()
> won't invalidate the entry on all the nodes that did a PFER for that key
> (there are no requestors, and even if we assume that we do a remote get
> before the PFER we'd still have race conditions).
>
> With L1 disabled, we have the problem that you mentioned: we're trying to
> read the value from the proper owners, but we never write it to the proper
> owners, so the hit ratio will be pretty bad. Using the SKIP_REMOTE_LOOKUP
> flag on reads, we'll avoid the extra RPC in Infinispan, but that will make
> the hit ratio even worse. E.g. in a 4-nodes cluster with numOwners=2, the
> hit ratio will never go above 50%.
>
> I don't think anyone would use a cache knowing that its hit ratio can never
> get above 50%, so we should just save ourselves some effort and stop
> supporting PFER in DIST mode.
>
> Cheers
> Dan
>
>
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