Concurrency

In a concurrent environment, several transactions are executing simultaneously, and their operations are interleaved. The objective of concurrency control is to ensure that such concurrent execution of transactions leaves the database in a state that could be produced if transactions were executed sequentially. This requirement is referred to as serializability.

Implementing concurrency is essential for any multi-user database system for two primary reasons:

1. Increased System Performance: By allowing multiple transactions to run concurrently, the system can make better use of resources, service more users at once, and generally perform operations more efficiently than if transactions had to wait for exclusive access to the entire database.

2. Improved Transaction Responsiveness: Concurrency can help keep individual transactions from being delayed unnecessarily, making the system more responsive from the point of view of individual users or applications.

However, managing concurrency introduces its challenges because of the need to coordinate concurrent access to the same data. For instance, you could have conflicts where:

• Two transactions are trying to read and update the same data simultaneously, leading to inconsistency (also known as a write-write conflict).

• One transaction is trying to read data that another transaction is updating (read-write or write-read conflict).

To handle these conflicts, various concurrency control techniques have been developed, such as:

• Locking: Shared and exclusive locks prevent multiple transactions from modifying the same data simultaneously. Two-phase locking (2PL) is a popular locking protocol.

• Timestamping: Each transaction is given a unique timestamp when it starts. The DBMS uses these timestamps to determine the order of transactions, thus avoiding conflicts.

• Optimistic Concurrency Control: This method assumes conflicts are rare and allows transactions to proceed without acquiring locks. However, before committing, each transaction checks if any conflicts have occurred. If a conflict is detected, the transaction is rolled back, and the process is repeated.

• Multiversion Concurrency Control (MVCC): Different versions of each data item are maintained. This approach allows for high concurrency and keeps read operations from blocking write operations and vice versa.

The goal of these techniques is to ensure database integrity while maximizing the degree to which users can access and modify the database concurrently. The selection of a suitable concurrency control technique depends on the specific requirements of a given system.