In a multi-user environment, deadlocks can occur when multiple transactions compete for the same resources. Oracle, being a database management system, provides deadlock detection and recovery mechanisms to handle such situations effectively.
1. Deadlock Detection
Oracle employs a wait-for graph to detect deadlocks. This graph represents the dependencies between transactions and resources. When a transaction requests a resource that is already being held by another transaction, it waits until the resource becomes available. Deadlock occurs when a circular dependency is formed within the wait-for graph.
Oracle periodically checks for circular dependencies in the wait-for graph using the deadlock detection process. If a deadlock is detected, Oracle initiates the deadlock recovery process.
2. Deadlock Recovery
Once a deadlock is detected, Oracle follows a recovery process to resolve it. The recovery process consists of the following steps:
2.1 Selecting a Victim
Oracle selects one of the transactions involved in the deadlock as the victim. The selection is based on predefined criteria, such as the amount of work done or the transaction’s priority. The victim is chosen to minimize the impact on the system.
2.2 Rolling back the Victim
Oracle rolls back the victim transaction, undoing all its changes and releasing the resources held by it. By rolling back the victim, Oracle breaks the circular dependency and resolves the deadlock.
2.3 Notifying the Applications
After rolling back the victim transaction, Oracle notifies the applications or users of the transaction’s failure. This allows them to take appropriate actions, such as resubmitting the transaction or handling the failure gracefully.
2.4 Resuming the Surviving Transactions
Once the victim’s transaction is rolled back, the surviving transactions involved in the deadlock are allowed to proceed. These transactions now have access to the resources previously held by the victim.
Conclusion
Oracle’s deadlock recovery processes play a crucial role in maintaining the integrity and availability of data in a multi-user environment. By periodically detecting deadlocks and efficiently resolving them, Oracle ensures that transactions can proceed smoothly without significant disruptions.
For more information on deadlock recovery in Oracle, refer to the official Oracle documentation here.