[infinispan-dev] L1 Consistency with Sync Caches

[William Burns]

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.
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.
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.
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
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.

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.
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.

Any comments/concerns would be appreciated.

Thanks,

 - Will
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