[infinispan-dev] Doubts about TxDistributionInterceptor and possible break in transaction isolation

[Dan Berindei]

On Mon, Jun 17, 2013 at 6:35 PM, William Burns <mudokonman at gmail.com> wrote:

>
>
>
> On Mon, Jun 17, 2013 at 11:11 AM, Dan Berindei <dan.berindei at gmail.com>wrote:
>
>>
>>
>>
>> On Mon, Jun 17, 2013 at 3:58 PM, Pedro Ruivo <pedro at infinispan.org>wrote:
>>
>>>
>>>
>>> On 06/17/2013 12:56 PM, Mircea Markus wrote:
>>> >
>>> > On 17 Jun 2013, at 11:52, Pedro Ruivo <pedro at infinispan.org> wrote:
>>> >
>>> >> I've been looking at TxDistributionInterceptor and I have a couple of
>>> >> questions (assuming REPEATABLE_READ isolation level):
>>> >>
>>> >> #1. why are we doing a remote get each time we write on a key? (huge
>>> >> perform impact if the key was previously read)
>>> > indeed this is suboptimal for transactions that write the same key
>>> repeatedly and repeatable read. Can you please create a JIRA for this?
>>>
>>> created: https://issues.jboss.org/browse/ISPN-3235
>>>
>>>
>> Oops... when I fixed https://issues.jboss.org/browse/ISPN-3124 I removed
>> the SKIP_REMOTE_LOOKUP, thinking that the map is already in the invocation
>> context so there shouldn't be any perf penalty. I can't put the
>> SKIP_REMOTE_LOOKUP flag back, otherwise delta writes won't have the
>> previous value during state transfer, so +1 to fixing ISPN-3235.
>>
>>
>>
>>> >>
>>> >> #2. why are we doing a dataContainer.get() if the remote get returns a
>>> >> null value? Shouldn't the interactions with data container be
>>> performed
>>> >> only in the (Versioned)EntryWrappingInterceptor?
>>> > This was added in the scope of ISPN-2688 and covers the scenario in
>>> which a state transfer is in progress, the remote get returns null as the
>>> remote value was dropped (no longer owner) and this node has become the
>>> owner in between.
>>> >
>>>
>>> ok :)
>>>
>>>
>> Yeah, this should be correct as long as we check if we already have the
>> key in the invocation context before doing the remote + local get.
>>
>>
>>
>>> >>
>>> >> #3. (I didn't verify this) why are we acquire the lock is the remote
>>> get
>>> >> is performed for a write? This looks correct for pessimistic locking
>>> but
>>> >> not for optimistic...
>>> > I think that, given that the local node is not owner, the lock
>>> acquisition is redundant even for pessimistic caches.
>>> > Mind creating a test to check if dropping that lock acquisition
>>> doesn't break things?
>>>
>>> I created a JIRA with low priority since it does not affect the
>>> transaction outcome/isolation and I believe the performance impact
>>> should be lower (you can increase the priority if you want).
>>>
>>> https://issues.jboss.org/browse/ISPN-3237
>>>
>>
>> If we don't lock the L1 entry, I think something like this could happen:
>>
>> tx1 at A: remote get(k1) from B - stores k1=v1 in invocation context
>> tx2 at A: write(k1, v2)
>> tx2 at A: commit - writes k1=v2 in L1
>> tx1 at A: commit - overwrites k1=v1 in L1
>>
> This one is just like here: referenced in
> https://issues.jboss.org/browse/ISPN-2965?focusedCommentId=12779780&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-12779780
>
>
Yep, it's the same thing.


> And even locking doesn't help in this case since it doesn't lock the key
> for a remote get only a remote get in the context of a write - which means
> the L1 could be updated concurrently in either order - causing possibly an
> inconsistency.  This will be solved when I port the same fix I have for
> https://issues.jboss.org/browse/ISPN-3197 for tx caches.
>

I thought the locking happened for all remote gets, and that's how I think
it should work.

We don't have to keep the lock for the entire duration of the transaction,
though. If we write the L1 entry to the data container during the remote
get, like you suggested in your comment, then we could release the L1 lock
immediately and remote invalidation commands would be free to remove the
entry.



>>
>> >>
>>> >> After this analysis, it is possible to break the isolation between
>>> >> transaction if I do a get on the key that does not exist:
>>> >>
>>> >> tm.begin()
>>> >> cache.get(k) //returns null
>>> >> //in the meanwhile a transaction writes on k and commits
>>> >> cache.get(k) //return the new value. IMO, this is not valid for
>>> >> REPEATABLE_READ isolation level!
>>> >
>>> > Indeed sounds like a bug, well spotted.
>>> > Can you please add a UT to confirm it and raise a JIRA?
>>>
>>> created: https://issues.jboss.org/browse/ISPN-3236
>>>
>>> IMO, this should be the correct behaviour (I'm going to add the test
>>> cases later):
>>>
>>> tm.begin()
>>> cache.get(k) //returns null (op#1)
>>> //in the meanwhile a transaction writes on k and commits
>>> write operation performed:
>>> * put: must return the same value as op#1
>>> * conditional put //if op#1 returns null the operation should be always
>>> successful (i.e. the key is updated, return true). Otherwise, the key
>>> remains unchanged (return false)
>>> * replace: must return the same value as op#1
>>> * conditional replace: replace should be successful if checked with the
>>> op#1 return value (return true). Otherwise, the key must remain
>>> unchanged (return false).
>>> * remote: must return the same value as op#1
>>> * conditional remove: the key should be removed if checked with the op#1
>>> return value (return true). Otherwise, the key must remain unchanged
>>> (return false)
>>>
>>> Also, the description above should be valid after a removal of a key.
>>>
>>> >
>>> > Cheers,
>>> >
>>> _______________________________________________
>>> infinispan-dev mailing list
>>> infinispan-dev at lists.jboss.org
>>> https://lists.jboss.org/mailman/listinfo/infinispan-dev
>>>
>>
>>
>> _______________________________________________
>> infinispan-dev mailing list
>> infinispan-dev at lists.jboss.org
>> https://lists.jboss.org/mailman/listinfo/infinispan-dev
>>
>
>
> _______________________________________________
> infinispan-dev mailing list
> infinispan-dev at lists.jboss.org
> https://lists.jboss.org/mailman/listinfo/infinispan-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.jboss.org/pipermail/infinispan-dev/attachments/20130618/15404f2c/attachment.html