[infinispan-dev] L1 Consistency with Sync Caches

[William Burns]

Trying to leave my points that would most likely have responses to
second email so we can try to get back to a single thread :)

On Thu, Jun 27, 2013 at 4:12 PM, Dan Berindei <dan.berindei at gmail.com> wrote:
>
> On Thu, Jun 27, 2013 at 4:18 PM, William Burns <mudokonman at gmail.com> wrote:
>>
>> First off I apologize for the length.
>>
>> There have been a few Jiras recently that have identified L1 consistency
>> issues with both TX and non TX sync caches.  Async caches with L1 have their
>> own issues as well, but I only wanted to talk about sync caches.
>>
>> https://issues.jboss.org/browse/ISPN-3197
>> https://issues.jboss.org/browse/ISPN-2965
>> https://issues.jboss.org/browse/ISPN-2990
>>
>> I have proposed a solution in
>> https://github.com/infinispan/infinispan/pull/1922 which should start L1
>> consistency down the right track.  There are quite a few comments on it if
>> you want to look into it more, but because of that I am moving this to the
>> dev mailing list.
>>
>> The key changes in the PR are the following (non-tx):
>>
>> 1. Concurrent reads for a key that can retrieve a remote value are
>> "corralled" into a single thread of execution for that given key.  This
>> would reduce network traffic with concurrent gets for the same key.  Note
>> the "corralling" only happens on a per key basis.
>
>
> Get commands on owners should not be serialized. Get commands on non-owners
> should not be serialized either, if the key already exists in L1. So I'd say
> L1ReadSynchronizer should be L1WriteSynchronizer instead :)

You are suggesting to check the context to see if the key is present
before attempting the synchronizer right?  Reading your second email
that seems the case :)

>
>>
>> 2. The single thread that is doing the remote get would update the L1 if
>> able (without locking) and make available the value to all the requests
>> waiting on the get.
>
>
> Well, L1ReadSynchronizer does prevent other threads from modifying the same
> key, so we are locking the key - just not using LockManager.
> It would also require StateTransferLock.acquireSharedTopologyLock() to make
> sure it doesn't write an L1 entry after the node became a proper owner.

Agree, when I was saying locking I was meaning through the use of the
lock manager.

>
>>
>> 3. Invalidations that are received would first check to see if there is a
>> current remote get occurring for it's keys.  If there is it will attempt to
>> cancel the L1 write(s) before it occurs.  If it cannot cancel the L1 write,
>> then it must also wait on the current remote get completion and subsequently
>> run the invalidation.  Note the cancellation would fail when the remote get
>> was done and it is in the middle of updating the L1, so this would be very
>> small window.
>
>
> I think it would be clearer to describe this as the L1 invalidation
> cancelling the remote get, not the L1 update, because the actual L1 update
> can't be cancelled.

When I say L1 update I meant the write to the data container after the
remote get.  The invalidation can't stop the remote get, all it does
is tell the caller that "Hey don't write the remote value you
retrieved into the L1."

>
> We also have to remove the logic in AbstractLockingInterceptor that skips L1
> invalidation for a key if it can't acquire a lock with a 0 timeout.
>
>>
>> 4. Local writes will also do the same thing as the invalidation with
>> cancelling or waiting.  Note that non tx local writes only do L1
>> invalidations and don't write the value to the data container.  Reasons why
>> I found at https://issues.jboss.org/browse/ISPN-3214
>
>
> I didn't know about ISPN-3214 or that non-tx writes don't write to L1, but
> it sounds fair.

Yeah I really wanted that to work, but without some additional checks
such as versioned data, I don't see a way to do this without locking
at the primary node like tx caches.

>
>>
>> 5. Writes that require the previous value and don't have it in the L1
>> would also do it's get operations using the same "corralling" method.
>
>
> The remoteGetBeforeWrites are a bit different - they don't happen on
> non-owners, they only happen on writeCH-owners that didn't receive that
> entry via state transfer yet. They put the value in the InvocationContext,
> but they don't write it to the data container - nor do they invalidate the
> L1 entry, if it exists.

Ah yes that is true, but only for non tx caches it seems.

>
>>
>>
>> 4/5 are not currently implemented in PR.
>>
>> This approach would use no locking for non tx caches for all L1
>> operations.  The synchronization point would be done through the
>> "corralling" method and invalidations/writes communicating to it.
>>
>> Transactional caches would do almost the same thing as non-tx.  Note these
>> changes are not done in any way yet.
>>
>> 1. Gets would now update the L1 immediately after retrieving the value
>> without locking, but still using the "corralling" technique that non-tx
>> does.  Previously the L1 update from a get was transactional.  This actually
>> would remedy issue [1]
>>
>> 2. Writes currently acquire the remote lock when committing, which is why
>> tx caches are able to update the L1 with the value.  Writes would do the
>> same cancellation/wait method as non-tx.
>>
>> 3. Writes that require the previous value and don't have it in the L1
>> would also do it's get operations using the same method.
>
>
> Just like for non-tx caches, I don't think these remote gets have to be
> stored in L1.

Tx caches do the remote get and could cache the L1 value immediately.
This would help if the transaction is rolled back or a conditional
operation failed etc.  There are some locking concerns, here but I
will leave that the other post.

>
>>
>> 4. For tx cache [2] would also have to be done.
>>
>> [1] -
>> https://issues.jboss.org/browse/ISPN-2965?focusedCommentId=12779780&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-12779780
>> [2] - https://issues.jboss.org/browse/ISPN-1540
>>
>> Also rehashing is another issue, but we should be able to acquire the
>> state transfer lock before updating the L1 on a get, just like when an entry
>> is committed to the data container.
>>
>
> The same for L1 invalidations - we don't want to remove real entries from
> the data container after the local node became an owner.

Yeah actually as you mentioned this, it sounds like a hole currently
even.  I don't know if this case can happen, but what if you received
a L1 invalidation and then got a rehash event right before it was
committing it to the container?  It seems the L1 commit to the
container would block until it could get the shared topology lock and
after it could then removes the value.  I probably need to dig into
the state transfer stuff deeper to know myself.

>
>>
>> Any comments/concerns would be appreciated.
>>
>> Thanks,
>>
>>  - Will
>>
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>
>
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