Oracle Deadlock

A deadlock in Oracle occurs when two or more transactions are blocked, each waiting for the other to release a lock. This situation creates a circular dependency where neither transaction can proceed, and the database system needs to resolve the deadlock. Oracle employs a mechanism to detect and resolve deadlocks automatically.

For example, Transaction A might hold a lock on some rows in the Accounts table and needs to update some rows in the Orders  table to finish. Transaction B holds locks on those very rows in the Orders  table but needs to update the rows in the Accounts table held by Transaction A. Transaction A cannot complete its transaction because of the lock on Orders. Transaction B cannot complete its transaction because of the lock on Accounts .All activity comes to a halt and remains at a standstill forever unless the DBMS detects the deadlock and aborts one of the transactions.

Oracle Deadlock Detection:

Oracle uses a mechanism known as cyclic wait detection to identify deadlocks. This involves checking the wait-for graph to see if there is a cycle. If a cycle is detected, a deadlock is confirmed.

Oracle Deadlock Resolution:

Once a deadlock is detected, Oracle needs to resolve it. Oracle resolves deadlocks by automatically rolling back the statement of the transaction that is less likely to be rolled back. This allows the other transaction to proceed.

Factors considered in deciding which transaction to roll back may include:

Transaction Work: The amount of work done by each transaction.

Transaction Time: The time each transaction has been waiting.

Heuristics: Oracle may use heuristics to make an informed decision.

 Handling Deadlocks Programmatically:

While Oracle can automatically resolve deadlocks, it's also possible to handle them programmatically in your application code. Strategies may include:

Retrying the Transaction: Catching the deadlock exception and retrying the transaction after a delay.

Transaction Timeouts: Setting a timeout for transactions to avoid waiting indefinitely for a resource.

Monitoring and Identifying Deadlocks:

Oracle Alert Logs: Deadlock information may be recorded in the Oracle alert log.

DBA_HIST_ACTIVE_SESS_HISTORY: Querying this view can provide historical information about sessions involved in deadlocks.

V$LOCK and V$SESSION Views: Monitoring locks and sessions to identify the sessions involved in a deadlock.

Preventing Deadlocks:

Transaction Design: Design transactions to acquire locks in a consistent order to reduce the likelihood of deadlocks.

Avoid Long Transactions: Long-running transactions increase the chances of encountering deadlocks.

Optimistic Concurrency Control: Use techniques like optimistic locking to minimize lock contention.

It's essential to design your application with deadlock prevention and handling strategies to minimize their occurrence and impact on your system's performance.