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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-4279) Statement cache deadlock
Date Thu, 28 Jun 2012 18:47:44 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13403353#comment-13403353
] 

Mike Matrigali commented on DERBY-4279:
---------------------------------------

commenting on RowLocation.  The system is designed such that one could implement multiple
types of conglomerates.  Currently there are only 2 types, btree and heap - but a lot of structure
to allow for more.  So in keeping with this callers know they need a RowLocation but it is
up to each implementation to provide the "hidden" actual implementation, in this case the
HeapRowLocation.  Getting
this row location from an open conglomerate seemed a natural place as almost all work with
a conglomerate already has an open conglomerate of some sort. 

If it helps to not have to open a conglomerate to get this RowLocation then it would be best
to just add a new interface.  I think code could be written to just return the right kind
of RowLocation based
on the conglomerate id.  At least with the current conglomerate inplementations there is nothing
ddl related that would change the RowLocation object returned.  There might be future implementations
that might require a lock, but I would just add comments about the current problems with getting
locks in this operation and the JIRA issue and let future implementers think about that. 


New interface would look something like:  TransactionController.newRowLocationTemplate(conglomerateId)

If it looks like this is needed let me know, I would be happy to submit a patch with this
new interface - if the testing shows this to be helpful.
                
> Statement cache deadlock
> ------------------------
>
>                 Key: DERBY-4279
>                 URL: https://issues.apache.org/jira/browse/DERBY-4279
>             Project: Derby
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 10.0.2.1, 10.1.3.1, 10.2.2.0, 10.3.3.0, 10.4.2.0, 10.5.1.1, 10.8.1.2
>         Environment: Windows Vista, OS X 10.5+
>            Reporter: Jeff Stuckman
>              Labels: derby_triage10_5_2
>         Attachments: Derby4279.java, client_stacktrace_activation_closed.txt, no-lock-experiment.diff,
patch4279.txt, patch4279_2.txt, stacktrace.txt
>
>
> Due to a design flaw in the statement cache, a deadlock can occur if a prepared statement
becomes out-of-date.
> I will illustrate this with the following example:
> The application is using the embedded Derby driver. The application has two threads,
and each thread uses its own connection.
> There is a table named MYTABLE with column MYCOLUMN.
> 1. A thread prepares and executes the query SELECT MYCOLUMN FROM MYTABLE. The prepared
statement is stored in the statement cache (see org.apache.derby.impl.sql.GenericStatement
for this logic)
> 2. After some time, the prepared statement becomes invalid or out-of-date for some reason
(see org.apache.derby.impl.sql.GenericPreparedStatement)
> 3. Thread 1 begins a transaction and executes LOCK TABLE MYTABLE IN EXCLUSIVE MODE
> 4. Thread 2 begins a transaction and executes SELECT MYCOLUMN FROM MYTABLE. The statement
is in the statement cache but it is out-of-date. The thread begins to recompile the statement.
To compile the statement, the thread needs a shared lock on MYTABLE. Thread 1 already has
an exclusive lock on MYTABLE. Thread 2 waits.
> 5. Thread 1 executes SELECT MYCOLUMN FROM MYTABLE. The statement is in the statement
cache but it is being compiled. Thread 1 waits on the statement's monitor.
> 6. We have a deadlock. Derby eventually detects a lock timeout, but the error message
is not descriptive. The stacks at the time of the deadlock are:
> This deadlock is unique because it can still occur in a properly designed database. You
are only safe if all of your transactions are very simple and cannot be interleaved in a sequence
that causes the deadlock, or if your particular statements do not require a table lock to
compile. (For the sake of simplicity, I used LOCK TABLE in my example, but any UPDATE statement
would fit.)

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