On Tue, Jun 24, 2014 at 9:04 PM, William Burns <mudokonman(a)gmail.com> wrote:
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?
I suggested that as well, but ISPN-2956 means we can't guarantee the
atomicity of non-tx conditional operations anyway.
And HotRod doesn't work nicely with optimistic locking (yet), so we have to
require pessimistic locking. I'm not sure about total order, though.
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.
Galder didn't mention this, but his proposal also copies the entry in
GetKeyValueCommand.perform, so the serialization happens on an
ImmutableCacheEntryView.
- Will
On Tue, Jun 24, 2014 at 1:51 PM, Pedro Ruivo <pedro(a)infinispan.org> wrote:
>
>
> On 06/24/2014 05:11 PM, Mircea Markus wrote:
>>
>> On Jun 24, 2014, at 16:50, Galder Zamarreño <galder(a)redhat.com> wrote:
>>
>>>
>>> On 24 Jun 2014, at 16:51, Mircea Markus <mmarkus(a)redhat.com> wrote:
>>>
>>>>
>>>> On Jun 24, 2014, at 15:27, Galder Zamarreño <galder(a)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.
I think the WriteCommands only modify wrapped entries.
It would be really bad if a put(k, v) in a read-committed tx cache could
modify the cache entry before the tx is committed. Making this distinction
clear would be another reason to make InternalCacheEntry immutable...
> 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?).
Nitpicking: copy() only exists in the CopyableDeltaAware interface, and the
commands themselves should not access the DataContainer.
This sounds like it might work, and in fact it's quite similar to
Takayoshi's initial proposal (though he targeted it specifically to
HotRod's replaceIfUnmodified). It's a bit scary because
CacheEntry.setValue() is called in so many places, but I think everything
but InternalEntryFactoryImpl.update() should be working on context entries,
so they don't need synchronization. Still, ISPN-2956...
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.
Agree, it would be a significant change. Perhaps context entries in
read-committed caches could hold a reference to the actual
EquivalentConcurrentHashMapV8.MapEntry if the entry was local, but that
would add more complexity. So I would rather see it removed completely.
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.
It would still affect every write operation. And we would need a way to
distinguish between GetKeyValueCommands for getCacheEntry and generated by
ClusteredGetCommands, since both kinds return InternalCacheEntries.
On the other hand, I think we have the same race with the write skew check
in optimistic caches. We read the value and version for write skew checking
without holding the key lock, so in theory it's possible to read an
outdated value and the current version. So we need some kind of
synchronization when reading the entry for optimistic mode, anyway.
But instead of a synchronized block, I propose making the value and
metadata references volatile. It would require care to always read the
metadata first and write it last, but reads would still be "free" on x86.
HotRod would still need transactions, but it wouldn't need
FORCE_WRITE_LOCK, so this might be a little faster than Galder's version.
>
>>
>>>>>
>>>>> 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...
>>>>> [4]
https://github.com/galderz/infinispan/tree/t_4424
>>>>> [5]
https://github.com/infinispan/infinispan/blob/master/core/src/main/java/o...
>>>>> --
>>>>> Galder Zamarreño
>>>>> galder(a)redhat.com
>>>>>
twitter.com/galderz
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> infinispan-dev mailing list
>>>>> infinispan-dev(a)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(a)lists.jboss.org
>>>>
https://lists.jboss.org/mailman/listinfo/infinispan-dev
>>>
>>>
>>> --
>>> Galder Zamarreño
>>> galder(a)redhat.com
>>>
twitter.com/galderz
>>>
>>>
>>> _______________________________________________
>>> infinispan-dev mailing list
>>> infinispan-dev(a)lists.jboss.org
>>>
https://lists.jboss.org/mailman/listinfo/infinispan-dev
>>
>> Cheers,
>>
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