# Monitoring deadlocks

The `derby.stream.error.logSeverityLevel` property determines the level of error that you are informed about.

By default, `derby.stream.error.logSeverityLevel` is set to 40000. If `derby.stream.error.logSeverityLevel` is set to display transaction-level errors (that is, if it is set to a value less than 40000), deadlock errors are logged to the `derby.log` file. If it is set to a value of 40000 or higher, deadlock errors are not logged to the `derby.log` file.

The `derby.locks.monitor` property ensures that deadlock errors are logged regardless of the value of `derby.stream.error.logSeverityLevel`. When `derby.locks.monitor` is set to true, all locks that are involved in deadlocks are written to `derby.log` along with a unique number that identifies the lock.

To see a thread's stack trace when a lock is requested, set `derby.locks.deadlockTrace` to true. This property is ignored if `derby.locks.monitor` is set to false.

Note: Use `derby.locks.deadlockTrace` with care. Setting this property can alter the timing of the application, severely affect performance, and produce a very large `derby.log` file.

For information about working with properties, see the Java DB Developer's Guide. For information about the specific properties that are mentioned in this topic, see the Java DB Reference Manual.

Here is an example of an error message when Derby aborts a transaction because of a deadlock:

    --SQLException Caught--

    SQLState: 40001 =
    Error Code: 30000
    Message: A lock could not be obtained due to a deadlock,
    cycle of locks and waiters is: Lock : ROW, DEPARTMENT, (1,14)
    Waiting XID : {752, X} , APP, update department set location='Boise'
    	where deptno='E21'
    Granted XID : {758, X} Lock : ROW, EMPLOYEE, (2,8)
    Waiting XID : {758, U} , APP, update employee set bonus=150 where salary=23840
    Granted XID : {752, X} The selected victim is XID : 752

Note: You can use the `derby.locks.waitTimeout` and `derby.locks.deadlockTimeout` properties to configure how long Derby waits for a lock to be released, or when to begin deadlock checking. For more information about these properties, see the section that discusses controlling Derby application behavior in the Java DB Developer's Guide.
Parent topic: Obtaining locking information