The Role of Transactions in Databases
Transactions are essential for ensuring that databases remain consistent and reliable, even in the face of concurrent access or system failures.
NoteA transaction is a sequence of one or more operations (such as inserts, updates, or deletes) that are executed as a single, indivisible unit.
The ACID Properties: Ensuring Reliable Transactions
The ACID properties are a set of principles that guarantee the reliability and integrity of database transactions:
- Atomicity
- Consistency
- Isolation
- Durability
The ACID properties are essential for maintaining data integrity, especially in environments where multiple users or systems access the database simultaneously.
Atomicity: All or Nothing
- Atomicity ensures that a transaction is treated as a single, indivisible unit.
- If any part of the transaction fails, the entire transaction is rolled back, leaving the database unchanged.
Imagine a bank transfer where money is deducted from one account and added to another. If the second operation fails, atomicity ensures that the first operation is also reversed, preventing inconsistencies.
Consistency: Maintaining Valid States
Consistency ensures that a transaction brings the database from one valid state to another, adhering to all defined rules and constraints.
ExampleIf a transaction violates a constraint (such as a foreign key relationship), it is rolled back to maintain consistency.
Isolation: Independent Transactions
- Isolation ensures that concurrent transactions do not interfere with each other.
- Each transaction appears to execute in isolation, even if others are running simultaneously.
Without isolation, two transactions updating the same record could lead to inconsistent data.
Durability: Permanent Changes
Durability guarantees that once a transaction is committed, its changes are permanent, even in the event of a system failure.
ExampleAfter a bank transfer is completed, the updated balances are safely stored, ensuring they are not lost due to a power outage or system crash.
Transaction Control Language (TCL) Commands
TCL commands are used to manage transactions and enforce ACID properties.
- BEGIN TRANSACTION
- COMMIT
- ROLLBACK
