Concurrency Control in DBMS
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Concurrency Control in DBMS
Concurrency control is an essential aspect of database management systems (DBMS) that ensures that multiple transactions occurring simultaneously do not interfere with each other’s operations. The goal of concurrency control is to maintain the consistency of data by ensuring that transactions do not read or write data that has been modified by other transactions in an inconsistent state.
Concurrency control mechanisms use locks, timestamps, and validation to manage concurrent transactions in a DBMS. Lock-based concurrency control is the most commonly used approach, where transactions acquire and release locks on data items to access them. A lock is a mechanism that prevents other transactions from accessing the same data item simultaneously. Two types of locks are used in lock-based concurrency control: shared locks and exclusive locks. A shared lock allows multiple transactions to read a data item, while an exclusive lock allows only one transaction to write to the data item.
Timestamp-based concurrency control uses timestamps to order transactions and ensure that a transaction reads data that is consistent with the database state at the time the transaction began. In timestamp-based concurrency control, transactions are assigned unique timestamps, and the database keeps track of the timestamp of the last transaction that read or wrote each data item. A transaction can read a data item only if its timestamp is earlier than the timestamp of the last transaction that wrote to the data item.
Validation-based concurrency control is used in optimistic concurrency control, where transactions execute without acquiring any locks on data items. Validation checks are performed at the end of a transaction to ensure that it has not violated any constraints in the database. If a violation is detected, the transaction is aborted and must be restarted.
In addition to these mechanisms, DBMSs also use various isolation levels to control how transactions interact with each other. The most commonly used isolation levels are read uncommitted, read committed, repeatable read, and serializable. These isolation levels determine the degree to which a transaction can see changes made by other transactions and affect the performance and concurrency of the DBMS.
Concurrency control is essential to ensure that a DBMS can handle multiple transactions efficiently without violating the consistency and integrity of the data. The choice of concurrency control mechanism and isolation level depends on the requirements of the application and the performance characteristics of the DBMS. DBMSs employ sophisticated techniques for concurrency control to provide efficient and reliable access to data in multi-user environments.
Concurrency Control in DBMS
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