[infinispan-dev] Fix or document? Concurrent replaceWithVersion w/ same value might all return true - ISPN-4972

[Sanne Grinovero]

On 26 November 2014 at 14:17, Dan Berindei <dan.berindei at gmail.com> wrote:
> Sanne, it will work as long as the previous value is not the same as
> the new value.
>
> If multiple threads read value 5 with version 5, and all of them want
> to replace it with value 6, only one of them will succeed.

Ok I see I might be confusing value and versions. I hope :)

> But if multiple threads read value 5 with version 5, and want to
> replace it with value *5*, all of them might succeed.

This paragraph is confusing me more. What "value" are you referring to
at the third "5"? Is it even legal to replace an entry with a new
value but not incrementing its version?

Thanks!
Sanne

>
> Indeed, it's not atomic, but a basic counter will work. And it's all
> we can do with the actual core cache API (unless we want to go back to
> including the HotRod version in the value).
>
> Cheers
> Dan
>
>
> On Wed, Nov 26, 2014 at 2:33 PM, Sanne Grinovero <sanne at infinispan.org> wrote:
>> That's not Atomic. How can I implement a counter on this?
>>
>> Say the current version is 5, I read it, and then issue a "replace 5
>> with 6" command.
>> If I send a couple of such commands in parallel I need a guarantee
>> that only one succeeds, so that the other one can retry and get the
>> counter up to 7.
>>
>> Over Hot Rod I have no locking so I have no alternatives other than
>> atomic replacement commands, that's not unlikely to happen: that's a
>> critical showstopper for users.
>>
>> Sanne
>>
>>
>> On 20 November 2014 at 16:35, Dan Berindei <dan.berindei at gmail.com> wrote:
>>> I guess you could say this is a regression, this wouldn't have been possible
>>> when the version was part of the value :)
>>>
>>> But I agree an application is very unlikely call replaceWithVersion with the
>>> same value as before, so +1 to document it for now and implement
>>> replaceWithVersion/replaceWithPredicate in the embedded cache for 8.0.
>>>
>>> Cheers
>>> Dan
>>>
>>>
>>> On Thu, Nov 13, 2014 at 3:08 PM, Radim Vansa <rvansa at redhat.com> wrote:
>>>>
>>>> I agree with Galder, fixing it is not worth the cost.
>>>>
>>>> Actually, there are often bugs that I'd call rather 'quirks', not
>>>> honoring the ConcurrentMap contract (recently we have discussed with Dan
>>>> [1] and [2]) which are quite complex to fix. Another one that's
>>>> considered not a bug is that a read does not have transactional semantics.
>>>> Galder, where will you document that? I think that special page in
>>>> documentation should accumulate such cases, linked to JIRAs for case
>>>> that eventually we'll resolve them (with that glorious MVCC). And of
>>>> course, link from javadoc to this document (though I am not sure whether
>>>> we can correctly keep that in sync with latest release. Could we have a
>>>> redirection from http://infinispan.org/docs/latest to
>>>> http://infinispan.org/docs/7.0.x/ ?
>>>>
>>>> Radim
>>>>
>>>> [1] https://issues.jboss.org/browse/ISPN-3918
>>>> [2] https://issues.jboss.org/browse/ISPN-4286
>>>>
>>>> On 11/13/2014 01:51 PM, Galder Zamarreño wrote:
>>>> > Hi all,
>>>> >
>>>> > Re: https://issues.jboss.org/browse/ISPN-4972
>>>> >
>>>> > Embedded cache provides atomicity of a replace() call passing in the
>>>> > previous value. This limitation might be lifted when we adopt Java 8 and we
>>>> > can pass in a lambda or similar, which can be executed right when the value
>>>> > is compared now, and if it returns true it’s applied. The lambda could
>>>> > compare both value and metadata for example.
>>>> >
>>>> > Anyway, given the current status, I’m considering whether it’s worth
>>>> > fixing this particular issue. Fixing the issue would require adding some
>>>> > kind of locking in the Hot Rod server so that the version retrieval,
>>>> > comparison and replace call, can all happen atomically.
>>>> >
>>>> > This is not ideal, and on top of that, as Radim said, the chances of
>>>> > this happening in real life are limited, or more precisely it’s effects are
>>>> > minimal. In other words, if two concurrent threads call replace with the
>>>> > same value, the end result is that the new value would be stored, but as a
>>>> > result of the code, both replaces would return true which is not strictly
>>>> > right.
>>>> >
>>>> > I’d rather document this than add unnecessary locking in the Hot Rod
>>>> > server where it deals with the versioned replace call.
>>>> >
>>>> > Thoughts?
>>>> > --
>>>> > Galder Zamarreño
>>>> > galder at redhat.com
>>>> > twitter.com/galderz
>>>> >
>>>> >
>>>> > _______________________________________________
>>>> > infinispan-dev mailing list
>>>> > infinispan-dev at lists.jboss.org
>>>> > https://lists.jboss.org/mailman/listinfo/infinispan-dev
>>>>
>>>>
>>>> --
>>>> Radim Vansa <rvansa at redhat.com>
>>>> JBoss DataGrid QA
>>>>
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>>>
>>>
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