n a multi-user database environment, multiple users may access and modify data at the same time. To ensure data consistency and reliability, DBMS uses transactions and concurrency control mechanisms.
What is a Transaction?
A transaction is a sequence of database operations that are treated as a single logical unit of work. A transaction must either complete entirely or not execute at all.
Example:
- Transfer money from one account to another
States of a Transaction
A transaction goes through the following states:
- Active
- Partially Committed
- Committed
- Failed
- Aborted
ACID Properties
Every transaction must satisfy the ACID properties:
- Atomicity – Transaction is fully completed or not executed at all
- Consistency – Database moves from one valid state to another
- Isolation – Transactions do not interfere with each other
- Durability – Committed changes remain permanent
Problems Without Concurrency Control
Without concurrency control, databases may face:
- Lost updates
- Dirty reads
- Unrepeatable reads
- Inconsistent retrievals
What is Concurrency Control?
Concurrency control ensures that multiple transactions execute safely without affecting data consistency. It coordinates simultaneous database operations.
Concurrency Control Techniques
1. Lock-Based Protocols
Locks restrict access to data items.
Types of locks:
- Shared Lock (Read)
- Exclusive Lock (Write)
2. Timestamp-Based Protocols
Each transaction is assigned a timestamp to determine execution order.
Older transactions get priority over newer ones.
3. Validation-Based Protocols
Transactions are validated before committing to ensure no conflicts occurred.
4. Multiversion Concurrency Control (MVCC)
Maintains multiple versions of data for better read performance.
Used in modern DBMS systems.
Deadlock
A deadlock occurs when two or more transactions wait indefinitely for each other to release locks.
Deadlock handling methods:
- Prevention
- Avoidance
- Detection and Recovery
Advantages of Concurrency Control
- Ensures data integrity
- Supports multi-user access
- Prevents data conflicts
- Improves system reliability
Conclusion
Transactions and concurrency control are critical components of a DBMS. They ensure that multiple users can safely access and modify data without causing inconsistencies. Understanding these concepts helps in building reliable and efficient database systems.