(1) Access Objects in the Same Order
If all concurrent transactions access objects in the same order, deadlocks are less likely to occur. For example, if two concurrent transactions obtain a lock on the Supplier table and then on the Part table, one transaction is blocked on the Supplier table until the other transaction is completed. After the first transaction commits or rolls back, the second continues, and a deadlock does not occur. Using stored procedures for all data modifications can standardize the order of accessing objects.
(2) Avoid User Interaction in Transactions
Avoid writing transactions that include user interaction, because the speed of batches running without user intervention is much faster than the speed at which a user must manually respond to queries, such as replying to a prompt for a parameter requested by an application. For example, if a transaction is waiting for user input and the user goes to lunch or even home for the weekend, the user delays the transaction from completing. This degrades system throughput because any locks held by the transaction are released only when the transaction is committed or rolled back. Even if a deadlock situation does not arise, other transactions accessing the same resources are blocked while waiting for the transaction to complete.
(3) Keep Transactions Short and in One Batch
A deadlock typically occurs when several long-running transactions execute concurrently in the same database. The longer the transaction, the longer the exclusive or update locks are held, blocking other activity and leading to possible deadlock situations.
Keeping transactions in one batch minimizes network roundtrips during a transaction, reducing possible delays in completing the transaction and releasing locks.
(4) Use a Lower Isolation Level
Determine whether a transaction can run at a lower isolation level. Implementing read committed allows a transaction to read data previously read (not modified) by another transaction without waiting for the first transaction to complete. Using a lower isolation level, such as read committed, holds shared locks for a shorter duration than a higher isolation level, such as serializable. This reduces locking contention.
(5) Use a Row Versioning-based Isolation Level
When the READ_COMMITTED_SNAPSHOT database option is set ON, a transaction running under read committed isolation level uses row versioning rather than shared locks during read operations.
Snapshot isolation also uses row versioning, which does not use shared locks during read operations. Before a transaction can run under snapshot isolation, the ALLOW_SNAPSHOT_ISOLATION database option must be set ON.
Implement these isolation levels to minimize deadlocks that can occur between read and write operations.
SQL Script:
ALTER DATABASE dbname
SET READ_COMMITTED_SNAPSHOT ON;
ALTER DATABASE dbname
SET ALLOW_SNAPSHOT_ISOLATION ON;
Note:
When run the script, there shall not have other connections to database, otherwise the system may hang forever. The only way I have been able tofix it is to completely stop and restart the database server, thenissue the command and it returns immediately.
SQL Script for verify:
select name, snapshot_isolation_state, is_read_committed_snapshot_on from sys.databases;
(6) Use Bound Connections
Using bound connections, two or more connections opened by the same application can cooperate with each other. Any locks acquired by the secondary connections are held as if they were acquired by the primary connection, and vice versa. Therefore they do not block each other.
(7) Avoid cursors.
(8) Reduce lock time. Try to develop your application so that it grabs locks at the latest possible time, and then releases them at the very earliest time.
(9) If appropriate, reduce lock escalation by using the ROWLOCK or PAGLOCK.
