10:35 a.m.
On Mon, Jun 17, 2013 at 11:11 AM, Dan Berindei <dan.berindei(a)gmail.com>wrote:
On Mon, Jun 17, 2013 at 3:58 PM, Pedro Ruivo <pedro(a)infinispan.org> wrote:
>
>
> On 06/17/2013 12:56 PM, Mircea Markus wrote:
> >
> > On 17 Jun 2013, at 11:52, Pedro Ruivo <pedro(a)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@A: remote get(k1) from B - stores k1=v1 in invocation context
tx2@A: write(k1, v2)
tx2@A: commit - writes k1=v2 in L1
tx1@A: commit - overwrites k1=v1 in L1
This one is just like here: referenced in
https://issues.jboss.org/browse/ISPN-2965?focusedCommentId=12779780&p...
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.
>>
> >> 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,
> >
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