Concurrency control is an essential aspect of managing database transactions in a multi-user environment. It ensures that multiple transactions can execute concurrently without interfering with each other, while maintaining the integrity and consistency of the data. In this blog post, we will discuss various concurrency control techniques used in databases.
1. Locking
Locking is one of the most widely used concurrency control techniques in database management systems. It involves acquiring and releasing locks on data objects to prevent conflicting operations from occurring simultaneously. There are two types of locks: shared locks and exclusive locks. Shared locks allow multiple transactions to read the data concurrently, while exclusive locks only allow one transaction to write or modify the data.
Locking can be implemented at different levels, including the granularity of database tables, pages, or even individual data items. However, a higher level of granularity increases concurrency but also increases the chances of conflicts. On the other hand, a lower level of granularity reduces conflicts but also decreases concurrency.
2. Two-Phase Locking (2PL)
Two-Phase Locking is a popular concurrency control protocol that ensures serializability of transactions. It consists of two phases: the growing phase and the shrinking phase. In the growing phase, a transaction acquires locks on the data items it accesses. Once a transaction releases a lock, it cannot acquire any new locks. In the shrinking phase, a transaction releases the locks it holds. This protocol guarantees that no transaction can release a lock before it has acquired all the locks it needs, preventing deadlock situations.
3. Timestamp Ordering
Timestamp ordering is a concurrency control technique that assigns a unique timestamp to each transaction when it enters the system. The system uses these timestamps to order the transactions and enforces a strict schedule based on their timestamps. If transaction T1 has a lower timestamp than transaction T2, T1 must execute before T2. This technique ensures serializability but can result in transaction aborts and rollbacks to maintain the order of timestamps.
4. Optimistic Concurrency Control
Optimistic concurrency control is based on the assumption that conflicts between transactions are rare. It allows multiple transactions to execute concurrently without acquiring any locks initially. However, before committing, each transaction checks if there were any conflicts with other transactions. If conflicts are detected, the transaction is rolled back and restarted. This approach reduces locking overhead but may result in more transaction restarts.
5. Multi-Version Concurrency Control (MVCC)
Multi-Version Concurrency Control is a technique often used in database systems that support read consistency. It allows multiple versions of data items to exist simultaneously, each with its own timestamp. Transactions can access older versions of data items without blocking other transactions that are modifying the same data. This approach improves concurrency and reduces the need for locks.
In conclusion, concurrency control techniques play a vital role in managing database transactions and ensuring data integrity and consistency. Different techniques, such as locking, two-phase locking, timestamp ordering, optimistic concurrency control, and MVCC, offer various trade-offs between concurrency and isolation levels. Database administrators and developers must carefully choose the appropriate technique based on the requirements and characteristics of their applications.
本文来自极简博客，作者：樱花飘落，转载请注明原文链接：Database Transactions
