SQL - Database Concurrency Control and Locking in SQL

1. What is Concurrency in Databases?

Concurrency refers to multiple users or processes accessing and modifying the database at the same time.
Modern databases are designed to allow many operations to occur simultaneously without corrupting data.

For example, in an online banking system, many customers may check balances or transfer money at the same time. The database must handle these actions safely.

2. Why is Concurrency Control Important?

  • Prevents data inconsistencies

  • Ensures accuracy of transactions

  • Maintains database integrity

  • Supports multi-user environments

  • Avoids conflicts when data is updated simultaneously

Without proper control, simultaneous operations could produce incorrect or lost data.

3. Common Concurrency Problems

Lost Update
Two users update the same data at the same time, and one update overwrites the other.

Dirty Read
A transaction reads data that has not yet been committed and may later be rolled back.

Non-repeatable Read
A row read twice returns different values because another transaction modified it.

Phantom Read
New rows appear in repeated queries due to concurrent inserts.

4. Locking Mechanism

Locking is a method used by databases to control access to data while it is being used or modified.

Types of locks:

Shared Lock
Allows multiple users to read data but not modify it.

Exclusive Lock
Allows one user to modify data while preventing others from accessing it.

Row-level Lock
Locks a specific row.

Table-level Lock
Locks an entire table.

Databases automatically manage locks during transactions to maintain consistency.

5. Transactions and Isolation Levels

Concurrency control works with transactions, which are sequences of operations treated as a single unit.

Isolation levels define how transactions interact:

Read Uncommitted
Allows reading uncommitted changes.

Read Committed
Prevents dirty reads.

Repeatable Read
Ensures consistent data during transaction.

Serializable
Strictest level, fully isolates transactions.

Higher isolation increases safety but may reduce performance.

6. Deadlocks

A deadlock occurs when two transactions wait for each other’s locked resources, preventing progress.

Example scenario:

  • Transaction A locks resource 1 and waits for resource 2

  • Transaction B locks resource 2 and waits for resource 1

Databases detect and resolve deadlocks automatically by canceling one transaction.

Summary

Concurrency control ensures that multiple database operations can occur safely at the same time. Locking mechanisms and transaction isolation levels protect data integrity by preventing conflicts and inconsistencies. Understanding these concepts is essential for designing reliable multi-user database systems.