In database management systems, concurrent transactions refer to multiple transactions that are executing simultaneously in a multi-user environment. When multiple transactions run concurrently, there is a need to ensure data consistency and integrity while maintaining high concurrency and performance.
Recovery in the context of concurrent transactions involves the ability to restore the database to a consistent state after a system failure or an abnormal termination. The recovery process typically involves two phases: undo and redo.
- Undo Phase: During the undo phase, the system identifies the transactions that were active but not committed at the time of the failure. These transactions may have made changes to the database that need to be rolled back. The system uses transaction logs, which contain a record of all the changes made by each transaction, to determine which changes need to be undone. The changes are then applied in reverse order to restore the database to its state before the transactions began.
- Redo Phase: After the undo phase, the system moves on to the redo phase. In this phase, the system identifies the transactions that were committed but not yet recorded in the database due to the failure. The transaction logs are used again to identify these transactions and the changes they made. The changes are then reapplied to the database to bring it up to date.
Recovery with concurrent transactions adds complexity to the process because multiple transactions may have made changes to the same data items simultaneously. To ensure data consistency, the system needs to manage dependencies and conflicts between concurrent transactions. This is typically achieved through locking and concurrency control mechanisms.
Locking: Locks are used to control access to data items. When a transaction wants to read or modify a data item, it must acquire the appropriate lock. Locks prevent conflicting operations from executing concurrently. For example, if one transaction has acquired a lock to modify a data item, other transactions must wait until the lock is released before they can access the same item.
Concurrency Control: Concurrency control mechanisms ensure that transactions are executed in a way that maintains data consistency. They include techniques such as locking, optimistic concurrency control, and multiversion concurrency control. These mechanisms coordinate the execution of concurrent transactions, ensuring that conflicts are resolved correctly and maintaining data integrity during recovery.
In summary, recovery with concurrent transactions involves the undo and redo phases to restore the database to a consistent state after a system failure. It requires the use of transaction logs, locking, and concurrency control mechanisms to manage dependencies and conflicts between concurrent transactions, ensuring data consistency and integrity.