Minimizing Deadlocks
Although deadlocks cannot be completely avoided, following certain coding conventions can minimize the chance of generating a deadlock. Minimizing deadlocks can increase transaction throughput and reduce system overhead because fewer transactions are:
Rolled back, undoing all the work performed by the transaction.
Resubmitted by applications because they were rolled back when deadlocked.
To help minimize deadlocks:
Access objects in the same order.
Avoid user interaction in transactions.
Keep transactions short and in one batch.
Use a lower isolation level.
Use a row versioning-based isolation level.
Set READ_COMMITTED_SNAPSHOT database option ON to enable read-committed transactions to use row versioning.
Use snapshot isolation.
Use bound connections.
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.
.gif "Diagram showing deadlock avoidance")
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.
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.
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.
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.
Note
Some applications rely upon locking and blocking behavior of read committed isolation. For these applications, some change is required before this option can be enabled.
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.
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.