Distributed Databases Questions Long
Distributed concurrency control refers to the management of concurrent access to data in a distributed database system. It ensures that multiple transactions executing concurrently in different nodes of the distributed system do not interfere with each other and maintain the consistency and integrity of the database.
To ensure distributed concurrency control, several techniques and protocols are employed. Some of the commonly used methods are:
1. Locking: Locking is a widely used technique to control concurrent access to data. In distributed databases, distributed lock managers (DLMs) are responsible for granting and releasing locks on data items. Locks can be of different types such as shared locks (read locks) and exclusive locks (write locks). Transactions request locks before accessing data items and are granted only if there is no conflict with other transactions. If conflicts occur, the transaction may be blocked or forced to wait until the conflicting transaction releases the lock.
2. Two-Phase Locking (2PL): Two-Phase Locking is a concurrency control protocol that ensures serializability of transactions. In distributed databases, the protocol is extended to handle distributed transactions. In the first phase (growing phase), transactions acquire locks on data items and in the second phase (shrinking phase), locks are released. The protocol ensures that no transaction releases a lock before it has acquired all the locks it needs, thereby preventing conflicts.
3. Timestamp Ordering: Timestamp ordering is a technique where each transaction is assigned a unique timestamp based on its start time. Transactions are ordered based on their timestamps, and conflicts are resolved by comparing the timestamps. If a transaction with a higher timestamp tries to access a data item locked by a transaction with a lower timestamp, it is forced to wait. This technique ensures serializability and prevents conflicts.
4. Optimistic Concurrency Control (OCC): OCC is a technique that assumes conflicts are rare and allows transactions to proceed without acquiring locks. Transactions are validated at the end to ensure that no conflicts have occurred. If conflicts are detected, the transaction is rolled back and restarted. OCC reduces the overhead of acquiring and releasing locks but requires additional validation steps.
5. Multi-Version Concurrency Control (MVCC): MVCC is a technique where multiple versions of data items are maintained to allow concurrent access. Each transaction sees a consistent snapshot of the database at the start time of the transaction. When a transaction updates a data item, a new version is created, and other transactions continue to access the old version. This technique allows for high concurrency as transactions do not block each other.
These techniques and protocols ensure distributed concurrency control by managing locks, timestamps, and versions of data items. They aim to prevent conflicts, maintain consistency, and ensure the correctness of concurrent transactions in a distributed database system.