Avoiding Locking Scenarios

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Avoiding Locking Scenarios • Best Practices – Application – Use least restrictive isolation level that maintains the data integrity requirements of the application – Reduce Isolation level of specific statements by using statement level isolation (i.e., WITH clause) – CLOSE cursors WITH RELEASE to free locks prior to end of transaction – Perform updates as close to the end of the transaction as possible, to reduce exclusive lock duration – COMMIT frequently to release locks – Avoid multiple applications accessing the same tables, but acquiring locks in different orders (Access patterns should be similar) – Avoid having multiple processes that access the same...

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Avoiding Locking Scenarios • Best Practices – Application – Use least restrictive isolation level that maintains the data integrity requirements of the application – Reduce Isolation level of specific statements by using statement level isolation (i.e., WITH clause) – CLOSE cursors WITH RELEASE to free locks prior to end of transaction – Perform updates as close to the end of the transaction as possible, to reduce exclusive lock duration – COMMIT frequently to release locks – Avoid multiple applications accessing the same tables, but acquiring locks in different orders (Access patterns should be similar) – Avoid having multiple processes that access the same table for both reads and writes within the same transaction
Avoiding Locking Scenarios • Best Practices – Database – Avoid lock escalations by increasing DB CFG parameters LOCKLIST and/or MAXLOCKS – Avoid lock timeouts: • Adjust the DB CFG parameter LOCKTIMEOUT or use the SET CURRENT LOCK TIMEOUT command – Avoid deadlocks: • Reduce row blocking during index and table scans: – DB2_SKIPINSERTED to skip/ignore uncommitted inserted rows – DB2_SKIPDELETED to skip/ignore uncommitted deleted rows – DB2_EVALUNCOMMITTED to defer locking until row is known to satisfy query. Uncommitted data will be evaluated. Skips deleted rows on table scans.
More Useful Registry Variables for Locking • DB2_KEEPTABLELOCK – allows DB2 to maintain the table lock when an uncommitted read or cursor stability isolation level is closed. The table lock is released at the end of the transaction • DB2_MAX_NON_TABLE_LOCKS – defines the maximum number of NON table locks a transaction can have before it releases these locks. Because transactions often access the same table more than once, retaining locks and changing their state to NON can improve performance • DB2LOCK_TO_RB – specifies whether lock timeouts cause the entire transaction to be rolled back, or only the current statement