How do databases handle concurrent transactions?

Databases handle concurrent transactions through a process known as concurrency control.

Concurrency control is a fundamental concept in database systems, designed to ensure that transactions are executed safely and concurrently without interfering with each other. This is crucial in a multi-user database environment where multiple transactions are often executed simultaneously. The main goal of concurrency control is to maintain consistency, isolation, and durability of transactions, collectively known as the ACID properties.

There are several methods used to manage concurrent transactions, including locking, timestamping, and optimistic concurrency control.

Locking is a common method used to control concurrency. In this method, a transaction locks the data it accesses, preventing other transactions from modifying it until the transaction is complete. There are two types of locks: shared locks and exclusive locks. Shared locks allow multiple transactions to read (but not write) the same data simultaneously. Exclusive locks, on the other hand, allow one transaction to both read and write data, blocking all other transactions.

Timestamping is another method used to control concurrency. Each transaction is given a unique timestamp when it starts. The database uses these timestamps to determine the order in which transactions should access data. If a transaction tries to access data that has been accessed by a newer transaction, the database will either abort the older transaction or make it wait.

Optimistic Concurrency Control (OCC) is a method that assumes conflicts between transactions are rare. It allows multiple transactions to access the same data without locks. However, before a transaction can commit, it must verify that no other transaction has modified the data it accessed. If a conflict is detected, the transaction is aborted and restarted.

In summary, databases handle concurrent transactions by implementing various concurrency control techniques. These techniques help maintain the integrity of the database and ensure that all transactions are executed in a safe and consistent manner.