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From "Rick Hillegas (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-5443) reduce number of times sequence updater does it work on user thread rather than nested user thread.
Date Fri, 09 Mar 2012 19:06:57 GMT

    [ https://issues.apache.org/jira/browse/DERBY-5443?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13226328#comment-13226328
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Rick Hillegas commented on DERBY-5443:
--------------------------------------

Thanks for continuing this discussion, Mike. The correctness problem may only affect sequences
and not identities. I can not think of a way to force the correctness problem with identity
columns. Sequences, however, have the peculiar behavior that the current value is not affected
by commits and rollbacks. This is described by the SQL Standard, part 2, section 4.21.1. Here's
how it's supposed to work:

1) User A issues a NEXT VALUE FOR and gets value 100.

2) User A rolls back.

3) User A issues another NEXT VALUE FOR. According to the Standard, user A should get a value
greater than 100. However, it is possible to make Derby return a value of 100.

Here is what I think is happening on DERBY-5493:

1a') User A gets a readlock on the SYSSEQUENCES row by selecting the current value of the
sequence generator.

1b') Then user A issues a NEXT VALUE FOR. If pre-allocation has been turned off (or if we
are on the boundary of a pre-allocation range), this requires that  the value in SYSSEQUENCES.CURRENTVALUE
be increased. However, because of the readlock acquired in (1a'), the change to SYSSEQUENCES.CURRENTVALUE
is escalated into the user transaction rather than being immediately committed in a nested
transaction.

2b') Now user A rolls back the transaction. Because the change to SYSSEQUENCES.CURRENTVALUE
was escalated into the user transaction, this rolls back the change to SYSSEQUENCES.CURRENTVALUE.

3') When A issues a NEXT VALUE FOR again, she is right back where she was at the beginning
of (1b'), and she gets the same value. But according to the Standard, she is supposed to get
an incremented value.

Because of the extra-transactional behavior of ANSI/ISO sequences, I do not see how we can
change the value of SYSSEQUENCES.CURRENTVALUE in a user transaction which can be rolled back.

Thanks,
-Rick

                
> 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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