[infinispan-dev] Transactional consistency of query

[Adrian Nistor]

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
>
>