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From "Kristian Waagan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-4279) Statement cache deadlock
Date Wed, 27 Jun 2012 11:33:44 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4279?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13402147#comment-13402147
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Kristian Waagan commented on DERBY-4279:
----------------------------------------

Thanks for your continued effort on this problem, Brett.

Without having reviewed the approach used in the patch thoroughly, I can say that suites.All
and derbyall passed with the patch applied in my environment.

BW> So, even though the compile of a SELECT will no longer block DDL from occurring [...]

I'm not convinced this is true. There's another mechanism in place in the data dictionary
(DD) to stop that from happening. For a DDL operation to be allowed, there can't be any ongoing
compilations. Note that the opposite isn't the case; compilations can occur even though a
DDL is in progress. In the latter case the system catalogs are queried directly instead of
going through the/a cache.

Now, this mechanism relies on several state objects and is a bit complicated so I'm not 100%
sure I'm correct. The methods startReading/doneReading and startWriting/transactionFinished
in DataDictionaryImpl are good starting points to investigate this further. Note the use of
the lock and the relevant counters.
                
> 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, 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becomes out-of-date.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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