[infinispan-dev] Reliability of return values

[Sanne Grinovero]

I don't think we are in a position to decide what is a reasonable
compromise; we can do better.
For example - as Radim suggested - it might seem reasonable to have
the older value around for a little while. We'll need a little bit of
history of values and tombstones anyway for many other reasons.


Sanne

On 12 May 2014 09:37, Dan Berindei <dan.berindei at gmail.com> wrote:
> Radim, I would contend that the first and foremost guarantee that put()
> makes is to leave the cache in a consistent state. So we can't just throw an
> exception and give up, leaving k=v on one owner and k=null on another.
>
> Secondly, put(k, v) being atomic means that it either succeeds, it writes
> k=v in the cache, and it returns the previous value, or it doesn't succeed,
> and it doesn't write k=v in the cache. Returning the wrong previous value is
> bad, but leaving k=v in the cache is just as bad, even if the all the owners
> have the same value.
>
> And last, we can't have one node seeing k=null, then k=v, then k=null again,
> when the only write we did on the cache was a put(k, v). So trying to undo
> the write would not help.
>
> In the end, we have to make a compromise, and I think returning the wrong
> value in some of the cases is a reasonable compromise. Of course, we should
> document that :)
>
> I also believe ISPN-2956 could be fixed so that HotRod behaves just like
> embedded mode after the ISPN-3422 fix, by adding a RETRY flag to the HotRod
> protocol and to the cache itself.
>
> Incidentally, transactional caches have a similar problem when the
> originator leaves the cluster: ISPN-3421 [1]
> And we can't handle transactional caches any better than non-transactional
> caches until we expose transactions to the HotRod client.
>
> [1] https://issues.jboss.org/browse/ISPN-2956
>
> Cheers
> Dan
>
>
>
>
> On Mon, May 12, 2014 at 10:21 AM, Radim Vansa <rvansa at redhat.com> wrote:
>>
>> Hi,
>>
>> recently I've stumbled upon one already expected behaviour (one instance
>> is [1]), but which did not got much attention.
>>
>> In non-tx cache, when the primary owner fails after the request has been
>> replicated to backup owner, the request is retried in the new topology.
>> Then, the operation is executed on the new primary (the previous
>> backup). The outcome has been already fixed in [2], but the return value
>> may be wrong. For example, when we do a put, the return value for the
>> second attempt will be the currently inserted value (although the entry
>> was just created). Same situation may happen for other operations.
>>
>> Currently, it's not possible to return the correct value (because it has
>> already been overwritten and we don't keep a history of values), but
>> shouldn't we rather throw an exception if we were not able to fulfil the
>> API contract?
>>
>> Radim
>>
>> [1] https://issues.jboss.org/browse/ISPN-2956
>> [2] https://issues.jboss.org/browse/ISPN-3422
>>
>> --
>> Radim Vansa <rvansa at redhat.com>
>> JBoss DataGrid QA
>>
>> _______________________________________________
>> infinispan-dev mailing list
>> infinispan-dev at lists.jboss.org
>> https://lists.jboss.org/mailman/listinfo/infinispan-dev
>
>
>
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