[infinispan-dev] Dangers of getCacheEntry (ISPN-4424)

Tue Jun 24 14:04:18 EDT 2014

I must admit this seems a bit heavy handed to have to enable
transactions, when are using them solely for the purpose of having
implicit transactions.

Can we not instead just tweak the NonTransactionalLockingInterceptor
to obey the FORCE_WRITE_LOCK, so it would guarantee a consistent value
in the face of a concurrent write when doing getCacheEntry?

Although thinking again on the locking, I don't think it alone is
sufficient either.  As the cache entry is serialized after releasing
the lock, which means there is still a window when only the value may
be changed on an owner node.  We really need immutable CacheEntries
returned from getCacheEntry even with locking to work properly.

 - Will

On Tue, Jun 24, 2014 at 1:51 PM, Pedro Ruivo <pedro at infinispan.org> wrote:
>
>
> On 06/24/2014 05:11 PM, Mircea Markus wrote:
>>
>> On Jun 24, 2014, at 16:50, Galder Zamarreño <galder at redhat.com> wrote:
>>
>>>
>>> On 24 Jun 2014, at 16:51, Mircea Markus <mmarkus at redhat.com> wrote:
>>>
>>>>
>>>> On Jun 24, 2014, at 15:27, Galder Zamarreño <galder at redhat.com> wrote:
>>>>
>>>>> To fix this, I’ve been working with Dan on some solutions and we’ve taken inspiration of the new requirements appearing as a result of ISPN-2956. To be able to deal with partial application of conditional operations properly, transactional caches are needed. So, the solution that can be seen in [4] takes that, and creates a transaction around the replaceIfUmodified and forces the getCacheEntry() call to acquire lock via FORCE_WRITE_LOCK flag.
>>>>
>>>> so the entire underlaying cache needs to be transactional then?
>>>
>>> Yeah, it needs to be transactional, but the code I’ve written also deals with the fact that the cache might not be transactional. I’ll probably add a WARN message when it’s not transactional. This goes in hand with the recommendations for ISPN-2956, whereby failover for conditional operations relying on return values require transactional caches to properly deal with failover situations.
>>>
>>> To sum up, if using conditional operations or CRUD methods with Flag.FORCE_RETURN_VALUE, caches should be transactional. Moreover, to achieve concurrency guarantees of counter tests such as the one tested for 4424, locking needs to be pessimistic too. If not using conditional operations or CRUD methods without the flag, the cache could be non-transactional.
>>
>> Thanks for the analysis. I think we should go with your patch for ISPN 7.0 and consider the proper solution for the future, as you suggest below.
>> +1 for the warning, users should be made aware for the limitation.
>
> +1 for the patch.
>
> Another suggestion: in *InternalEntryFactory.update()*, you can

I don't know if that would cover all the places since we also set the
value in the various WriteCommands themselves.

> synchronize in the cache entry, and create a new method
> *InternalCacheEntry copy(InternalCacheEntry)* that makes a copy while it
> also synchronizes in the existing cache entry. I think in this way, you
> don't need the cache to be transactional neither to force the lock on
> reads. Also, I would suggest the copy() to be invoked in your case (or
> in the conditional commands accesses to DataContainer?).

Yeah I was thinking of this approach as well, we would either have to
make the copy or synchronize in the serialization.  I personally think
making a copy would be better, but the entire copy operation would
have to be synchronized then as you mentioned.

>
>>>
>>>>
>>>>> This solves the issue explained above, but of course it has an impact of the performance. The test now runs about 1.5 or 2 times slower.
>>>>>
>>>>> This is probably the best that we can do in the 7.0 time frame, but there’s several things that could improve this:
>>>>>
>>>>> 1. True immutable entries in the cache. If the entries in the cache were truly immutable, there would be no risk of sending back a partially correct entry back to the client.
>>>>>
>>>>> 2. A cache replace method that does not compare objects based on equality (the current replace()), but a replace method that takes a function. The function could compare that the old entry’s version and the cached entry’s version match. The function would only be executed inside the inner container, with all the locks held properly. I already hinted something similar in [5].
>>
>>
>> Shall we raise a new JIRA for the permanent solution then?
>
> +1
>
> I think the immutable entries would be better, but it will destroy
> ReadCommitted isolation. The ReadCommitted is based on the mutability of
> the entry (i.e. it will not invoke the data container if the entry
> exists in transaction context). When (and if) is removed, then we can
> make the entries immutable.

If we only make getCacheEntry immutable, it wouldn't affect the normal
get operations.  I was thinking we could probably make the copy only
in the GetKeyValueCommand.

>
>>
>>>>>
>>>>> Finally, this was not a problem when the value stored in Hot Rod was a ByteArrayValue wrapping the byte array and the version, because the value was treated atomically, and in hindsight, maybe adding getCacheEntry might have been premature, but this method has proven useful for other use cases too (rolling upgrades…etc).
>>>>>
>>>>> Cheers,
>>>>>
>>>>> [1] https://issues.jboss.org/browse/ISPN-4424
>>>>> [2] https://issues.jboss.org/browse/ISPN-2956
>>>>> [3] https://github.com/infinispan/infinispan/blob/master/server/core/src/main/scala/org/infinispan/server/core/AbstractProtocolDecoder.scala#L302
>>>>> [4] https://github.com/galderz/infinispan/tree/t_4424
>>>>> [5] https://github.com/infinispan/infinispan/blob/master/core/src/main/java/org/infinispan/AdvancedCache.java#L374
>>>>> --
>>>>> 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
>>>>
>>>> Cheers,
>>>> --
>>>> Mircea Markus
>>>> Infinispan lead (www.infinispan.org)
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> infinispan-dev mailing list
>>>> infinispan-dev at lists.jboss.org
>>>> https://lists.jboss.org/mailman/listinfo/infinispan-dev
>>>
>>>
>>> --
>>> 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
>>
>> Cheers,
>>
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