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[infinispan-dev] Triangle and ISPN-3918

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[infinispan-dev] Triangle and ISPN-3918

Radim Vansa
Hi,

I was thinking about ISPN-3918 [1] and I've realized that while this
happens in current implementation only rarely during state transfer,
with Triangle v4 this could happen more often.

Conditional command is always executed on primary owner, and so far
during the execution of conditional command (incl. replication to
backup-owners) the other commands to the same key were blocking in the
locking layer. Triangle v4 removes this blocking, and if in thread T1
you do:

T1: replace(key, A, B)

and in second thread T2

T2: replace(key, A, C)
T2: get(key)

the T2.replace can now fail before the T1.replace (successful) is
replicated to backup owner. When T2 is, by chance, the backup owner, the
T2.replace completes with false, the T2.get will be served locally and
it will still returns A.

We should decide if this is an issue, and either close ISPN-3918 (not a
bug) or think about triangle routing of unsuccessful commands.

Radim

[1] https://issues.jboss.org/browse/ISPN-3918

--
Radim Vansa <[hidden email]>
JBoss Performance Team

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Re: [infinispan-dev] Triangle and ISPN-3918

Pedro Ruivo-2


On 14-11-2016 14:26, Radim Vansa wrote:

> Hi,
>
> I was thinking about ISPN-3918 [1] and I've realized that while this
> happens in current implementation only rarely during state transfer,
> with Triangle v4 this could happen more often.
>
> Conditional command is always executed on primary owner, and so far
> during the execution of conditional command (incl. replication to
> backup-owners) the other commands to the same key were blocking in the
> locking layer. Triangle v4 removes this blocking, and if in thread T1
> you do:
>
> T1: replace(key, A, B)
>
> and in second thread T2
>
> T2: replace(key, A, C)
> T2: get(key)
>
> the T2.replace can now fail before the T1.replace (successful) is
> replicated to backup owner. When T2 is, by chance, the backup owner, the
> T2.replace completes with false, the T2.get will be served locally and
> it will still returns A.
>
> We should decide if this is an issue, and either close ISPN-3918 (not a
> bug) or think about triangle routing of unsuccessful commands.

well... I think we could send the unsuccessful ack in FIFO order(*1). In
this way, it would force the backup owner to process the T1 operation
before processing the ack. get() will then return the correct value.

*1 or send only in FIFO when the backup owner is the originator and the
command is unsuccessful.
*1 or merge the ack command + backup-write command and send them in FIFO

>
> Radim
>
> [1] https://issues.jboss.org/browse/ISPN-3918
>
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Re: [infinispan-dev] Triangle and ISPN-3918

Dan Berindei
On Mon, Nov 14, 2016 at 4:49 PM, Pedro Ruivo <[hidden email]> wrote:

>
>
> On 14-11-2016 14:26, Radim Vansa wrote:
>> Hi,
>>
>> I was thinking about ISPN-3918 [1] and I've realized that while this
>> happens in current implementation only rarely during state transfer,
>> with Triangle v4 this could happen more often.
>>
>> Conditional command is always executed on primary owner, and so far
>> during the execution of conditional command (incl. replication to
>> backup-owners) the other commands to the same key were blocking in the
>> locking layer. Triangle v4 removes this blocking, and if in thread T1
>> you do:
>>
>> T1: replace(key, A, B)
>>
>> and in second thread T2
>>
>> T2: replace(key, A, C)
>> T2: get(key)
>>
>> the T2.replace can now fail before the T1.replace (successful) is
>> replicated to backup owner. When T2 is, by chance, the backup owner, the
>> T2.replace completes with false, the T2.get will be served locally and
>> it will still returns A.
>>
>> We should decide if this is an issue, and either close ISPN-3918 (not a
>> bug) or think about triangle routing of unsuccessful commands.
>
> well... I think we could send the unsuccessful ack in FIFO order(*1). In
> this way, it would force the backup owner to process the T1 operation
> before processing the ack. get() will then return the correct value.
>
> *1 or send only in FIFO when the backup owner is the originator and the
> command is unsuccessful.
> *1 or merge the ack command + backup-write command and send them in FIFO
>

Merging sounds like it would send too much extra stuff to the
originator. Sending only the ack command to the originator when it's
also a backup owner (and making it FIFO) sounds much better :)

OTOH, having T2 run on the backup owner guarantees that get() will
look up the key locally, but there is a chance of that happening when
T2 runs on any non-owner. So I don't think making the ack command FIFO
would really solve the problem.

The more I think about it, the more it seems this bug is just another
example of distributed caches not having session consistency [1]. The
fact is that distributed caches allow read operations to return the
values of concurrent writes in a any order, and having one of those
reads be also a write muddies the water a bit, but doesn't really
change anything. (Except, of course, an implementation that makes it
preserve the order most of the time in master.)

I vote to close ISPN-3918, but I would like to open another issue (or
reuse this one) to add a "force consistent read"
operation/flag/configuration that would force the cache to read the
value from the primary owner. We've been talking about this a lot, and
at the very least we need to have the option so we know whether users
actually choose it.

[1]: https://github.com/infinispan/infinispan/wiki/Consistency-guarantees-in-Infinispan#41-non-transactional
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