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https://issues.apache.org/jira/browse/DERBY-5443?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13226400#comment-13226400
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Mike Matrigali commented on DERBY-5443:
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thanks for the clear explanation of the DERBY-5493 problem, that is exactly
what I was looking for. Probably worth copying over to the other issue
also. Also important for all the current open sequence issue is to clearly
note whether they affect identity or not. I have been assuming in 10.9
they do given the conversion to using same underlying code and have been
thinking of each a 10.9 blocker if they affect identity which is used so
much in existing applications.
I will take time with new information, now I see clearer why the extra
mechanisms. It would be sad to add all this added complexity to defend
against an application doing a select count(*) from sys.syssequences with rs;
The standard behavior is interesting and has the feel of trying to make it
easier rather than harder for implementers, but has not worked out for
us.
If we fix DERBY-5443 to only escalate in the self deadlock cases and not in the
normal conflicts with other threads using the sequences, then DERBY-5493 seems
less important to me. I wonder if throwing an error
in those cases would be ok rather than escalating and possibly return a wrong
result saying that SEQUENCE use has been blocked by in transaction ddl or user
initiated locking of an internal system catalog. I'd rather just make it work,
so will continue to think about this. For this solution we would
just always do the syssequences catalog update in a nested transaction, or
throw an error. I have to think about this more, but if you go
down this route for sequences I think you just always wait on the lock in the
nested transaction and throw an error in all cases if you timeout. You don't
need any special new locking. I would keep existing behavior for identity
columns.
> reduce number of times sequence updater does it work on user thread rather
> than nested user thread.
> ---------------------------------------------------------------------------------------------------
>
> Key: DERBY-5443
> URL: https://issues.apache.org/jira/browse/DERBY-5443
> Project: Derby
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 10.9.0.0
> Reporter: Mike Matrigali
> Priority: Minor
> Attachments: blockingDDL.sql
>
>
> Currently the Sequence updater tries to do the system catalog update as part
> of the user thread, but in a nested user transaction. When this works
> all is well as the nested user transaction is immediately committed and thus
> the throughput of all threads depending on allocating sequences is
> optimized.
> In order to be able to commit the nested writable transaction independently
> the lock manager must treat the parent and nested transactions as two
> independent transactions and locks held by the parent will thus block the
> child. And in effect any lock that is blocked by the parent is a deadlock,
> but the lock manager does not understand this relationship and thus only will
> timeout and not recognize the implicit deadlock.
> Only 2 cases come to mind of the parent blocking the child in this manner for
> sequences:
> 1) ddl like create done in transaction followed by inserts into the table
> requiring sequence update.
> 2) users doing jdbc data dictionary lookups in a multistatment transaction
> resulting in holding locks on the system catalog rows and subsequently
> doing inserts into the table requiring sequence updates.
> The sequence updater currently never waits for a lock in the nested
> transaction and assumes any blocked lock is this parent deadlock case. It
> then falls back on doing the update in tranaction and then the system catalog
> lock remains until the user transaction commits which could then
> hold hostage all other inserts into the table. This is ok in the above 2
> cases as there is not any other choice since the user transaction is already
> holding the system hostage.
> The problem is the case where it was not a deadlock but just another thread
> trying to do the sequence update. In this case the thread should
> not be getting locks on the user thread.
> I am not sure best way to address this project but here are some ideas:
> 1) enhance lock manager to recognize the deadlock and then change to code to
> somehow do an immediately deadlock check for internal
> nested transactions, no matter what the system default is. Then the code
> should go ahead and use the system wait timeout on this lock
> and only fall over to using user transaction for deadlock (or maybe even
> throw a new "self deadlock" error that would only be possible for
> internal transactions).
> 2) somehow execute the internal system catalog update as part of a whole
> different transaction in the system. Would need a separate context.
> Sort of like the background daemon threads. Then no self deadlock is
> possible and it could just go ahead and wait. The downside is that then
> the code to "wait" for a new sequence becomes more complicated as it has
> to wait for an event from another thread. But seems like it could
> designed with locks/synchonization blocks somehow.
> 3) maybe add another lock synchronization that would only involve threads
> updating the sequences. So first an updater would request the
> sequence updater lock (with a key specific to the table and a new type)
> and it could just wait on it. It should never be held by parent
> transaction. Then it would still need the catalog row lock to do the
> update. I think with proper ordering this would insure that blocking on
> the catalog row lock would only happen in the self deadlock case.
> Overall this problem is less important as the size of the chunk of sequence
> is tuned properly for the application, and ultimately best if derby
> autotuned the chunk. There is a separate jira for auto tuning: DERBY-5295
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