[infinispan-dev] Transactional consistency of query

Adrian Nistor anistor at redhat.com
Mon Jul 31 06:30:28 EDT 2017


Yup, I also meant 'eventually consistent' when saying such 
inconsistencies should be acceptable. At some point in time after 
transactions have been committed and topology changes have been handled 
(state transfer completed) and we have a steady state we should see a 
consistent index when querying.

On 07/31/2017 11:41 AM, Gustavo Fernandes wrote:
> IMO, indexing should be eventually consistent, as this offers the best 
> performance.
>
> On tx-caches, although Lucene has hooks to be enlisted in a 
> transaction [1], some backends (elasticsearch) don't
> expose this, and Hibernate Search by design doesn't make use of it. So 
> currently we must deal with inconsistencies
> after the fact: checking for nulls, mismatched types and so on.
>
> [1] 
> https://lucene.apache.org/core/6_0_1/core/org/apache/lucene/index/TwoPhaseCommit.html
>
>
> On Fri, Jul 28, 2017 at 1:59 PM, Adrian Nistor <anistor at redhat.com 
> <mailto:anistor at redhat.com>> wrote:
>
>     My feeling regarding this was to accept such inconsistencies, but
>     maybe
>     I'm wrong. I've always regarded indexing as being async in
>     general, even
>     though it did behave as if being sync in some not so rare
>     circumstances,
>     which probably made people believe it is expected to be sync in
>     general.
>     I'm curious what Sanne and Gustavo have in mind.
>
>     Please note that updating the index synchronously during tx commit was
>     always regarded as a performance bottleneck, so it was out of the
>     question.
>
>     And that would not always work anyway, it all depends on the
>     underlying indexing technology. For example when using HS with elastic
>     search you have to accept that elastic indexing is always async.
>
>     And there might not be an index at all. It's very possible that the
>     query runs unindexed. In that case it will use distributed streams
>     which
>     have their own transaction issues.
>
>     In the past we had some bugs were a matching entry was deleted/evicted
>     right before the search results were returned to the user, so
>     loading of
>     those values failed in a silent way. Those queries mistakenly returned
>     some unexpected nulls among other valid results. The fix was to just
>     filter out those nulls. We could enhance that to double check that the
>     returned entry is indeed of the requested type, to also cover the
>     issue
>     that you encountered.
>
>     Adrian
>
>     On 07/28/2017 01:38 PM, Radim Vansa wrote:
>     > Hi,
>     >
>     > while working on ISPN-7806 I am wondering how should queries
>     work with
>     > transactions. Right now it seems that updates to index are done
>     during
>     > either regular command execution (on originator [A]) or prepare
>     command
>     > on remote nodes [B]. Both of these cause rolled-back
>     transactions to be
>     > seen, so these must be treated as bugs [C].
>     >
>     > If we index the data after committing the transaction, there
>     would be a
>     > time window when we could see the updated entries but the index
>     would
>     > not reflect that. That might be acceptable limitation if a
>     > query-matching misses some entity, but it's also possible that we
>     > retrieve the query result key-set and then (after retrieving full
>     > entities) we return something that does not match the query. One
>     of the
>     > reproducers for ISPN-7806 I've written [1] triggers a situation
>     where
>     > listing all Persons could return Animal (different entity type),
>     so I
>     > think that there's no validity post-check (though these reproducers
>     > don't use transactions).
>     >
>     > Therefore, I wonder if the index should contain only the key;
>     maybe we
>     > should store an unique version and invalidate the query if some
>     of the
>     > entries has changed.
>     >
>     > If we index the data before committing the transaction, similar
>     > situation could happen: the index will return keys for entities that
>     > will match in the future but the actually returned list will contain
>     > stale entities.
>     >
>     > What's the overall plan? Do we just accept inconsistencies? In that
>     > case, please add a verbose statement in docs and point me to that.
>     >
>     > And if I've misinterpreted something and raised the red flag in
>     error,
>     > please let me know.
>     >
>     > Radim
>     >
>     > [A] This seems to be a regression after moving towards async
>     > interceptors - our impl of
>     > org.hibernate.search.backend.TransactionContext is incorrectly
>     bound to
>     > TransactionManager. Then we seem to be running out of
>     transaction and
>     > are happy to index it right away. The thread that executes the
>     > interceptor handler is also dependent on ownership (due to remote
>     > LockCommand execution), so I think that it does not fail the
>     local-mode
>     > tests.
>     >
>     > [B] ... and it does so twice as a regression after ISPN-7840 but
>     that's
>     > easy to fix.
>     >
>     > [C] Indexing in prepare command was OK before ISPN-7840 with
>     pessimistic
>     > locking which does not send the CommitCommand, but now that the
>     QI has
>     > been moved below EWI it means that we're indexing before storing the
>     > actual values. Optimistic locking was not correct, though.
>     >
>     > [1]
>     >
>     https://github.com/rvansa/infinispan/commit/1d62c9b84888c7ac21a9811213b5657aa44ff546
>     <https://github.com/rvansa/infinispan/commit/1d62c9b84888c7ac21a9811213b5657aa44ff546>
>     >
>     >
>
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