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From "Jeff Stuckman (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-4279) Statement cache deadlock
Date Fri, 19 Jun 2009 23:30:07 GMT
Statement cache deadlock

                 Key: DERBY-4279
                 URL: https://issues.apache.org/jira/browse/DERBY-4279
             Project: Derby
          Issue Type: Bug
          Components: SQL
    Affects Versions:
         Environment: Windows Vista
            Reporter: Jeff Stuckman

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becomes out-of-date.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
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 After some time, the prepared statement becomes invalid or out-of-date for some reason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is in the statement cache but it is out-of-date.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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