Database Concurrency Control: Principles and Locking

Posted on Jul 25, 2025 in Visual Arts and Dance

Database Concurrency Control: Core Concepts

In a Database Management System (DBMS), allowing transactions to run concurrently offers significant advantages, such as better system resource utilization and higher throughput. However, it is crucial that these transactions do not conflict. The ultimate goal is to ensure database consistency and accuracy. For instance, if two users try to book the last available seat on a flight at the same time, the system must ensure that only one booking succeeds.

Concurrency control is a critical mechanism in DBMS that ensures the consistency and integrity of data when multiple operations are performed simultaneously. It provides mechanisms to handle concurrent execution in a way that maintains ACID properties (Atomicity, Consistency, Isolation, Durability). By implementing concurrency control, a DBMS allows transactions to execute concurrently while avoiding issues such as deadlocks, race conditions, and conflicts between operations.

Challenges of Concurrent Execution

Concurrent execution can lead to various challenges:

  • Dirty Reads: One transaction reads uncommitted data from another transaction, leading to potential inconsistencies if the changes are later rolled back.
  • Lost Updates: When two or more transactions update the same data simultaneously, one update may overwrite the other, causing data loss.
  • Inconsistent Reads: A transaction may read the same data multiple times during its execution, and the data might change between reads due to another transaction, leading to inconsistency.

Concurrency Control Protocols

Concurrency control protocols are sets of rules designed to solve concurrency issues in databases. They ensure that concurrent transactions execute properly while maintaining database consistency. These protocols provide atomicity, consistency, isolation, durability (ACID properties), and serializability for concurrent transaction execution.

Lock-Based Concurrency Control

In a Database Management System (DBMS), lock-based concurrency control (LCC) is a method used to manage how multiple transactions access the same data. This protocol ensures data consistency and integrity when multiple users interact with the database. It uses locks to manage data access, preventing transaction clashes and ensuring smooth, consistent operations.

What is a Lock?

A lock is a variable associated with a data item, indicating whether it is currently in use or available. Locks are essential for managing data access during concurrent transactions. When one transaction accesses or modifies a data item, a lock prevents other transactions from interfering, maintaining data integrity and preventing conflicts. This process, known as locking, is a widely used method to ensure smooth and consistent database operations.

Lock-Based Protocols

Lock-Based Protocols in DBMS ensure that a transaction cannot read or write data until it acquires the necessary lock. Here’s how they work:

  • These protocols prevent concurrency issues by allowing only one transaction to access a specific data item at a time.
  • Locks help multiple transactions work together smoothly by managing access to database items.

Types of Locks

  1. Shared Lock (S): Also known as a read-only lock. As the name suggests, it can be shared between transactions because, while holding this lock, the transaction does not have permission to update the data item. An S-lock is requested using the lock-S instruction.
  2. Exclusive Lock (X): Allows both read and write operations on the data item. It is exclusive and cannot be held simultaneously on the same data item by multiple transactions. An X-lock is requested using the lock-X instruction.