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What is Commit

Introduction to Commit

A Commit in the context of database management systems (DBMS) refers to the action of making changes performed during a transaction permanent. Once a commit operation is executed, all changes are written to the database, and they become visible to other transactions. The concept of commit is integral to ensuring data integrity and consistency within a database system.

Key Characteristics

  • Finality: After a commit, the changes cannot be undone unless there is a compensating transaction.
  • Visibility: Changes made by a committed transaction become visible to other transactions that start after the commit.
  • Durability: Once a transaction is committed, its effects persist even if the system fails immediately after the commit.

ACID Properties and Commit

The commit operation plays a crucial role in maintaining the ACID properties of transactions:

  1. Atomicity: Ensures that all operations within a transaction are completed successfully as a single unit of work; otherwise, the transaction is aborted, and no changes are applied.
  2. Consistency: Guarantees that a transaction brings the database from one valid state to another, maintaining database invariants.
  3. Isolation: Provides the illusion that transactions are processed serially, even though they may be processed concurrently.
  4. Durability: Ensures that once a transaction has been committed, it will remain so, even in the case of a system failure.

How Commit Works

Transaction Lifecycle

  1. Begin Transaction: Starts a new transaction.
  2. Execute Operations: Performs various read/write operations on the database.
  3. Prepare Phase (Optional): Some systems have a prepare phase where they check if the commit can proceed without violating any constraints.
  4. Commit: Finalizes the transaction, making all changes permanent.
  5. End Transaction: Concludes the transaction.

Example Scenario

Consider a banking application that transfers funds between two accounts. This operation involves multiple steps that must all succeed for the transfer to be valid.

BEGIN TRANSACTION;
 
-- Deduct amount from the source account
UPDATE accounts SET balance = balance - 100 WHERE account_id = 1;
 
-- Add amount to the destination account
UPDATE accounts SET balance = balance + 100 WHERE account_id = 2;
 
COMMIT;

In this scenario:

  • If both UPDATE statements succeed, the COMMIT statement makes these changes permanent.
  • If an error occurs before committing, the transaction can be rolled back, undoing any changes made during the transaction.

Benefits of Commit

  • Data Integrity: Ensures that only consistent and complete sets of changes are applied to the database.
  • Concurrency Control: Helps manage concurrent transactions, ensuring that they do not interfere with each other.
  • Recovery: Facilitates recovery mechanisms, allowing databases to restore to a consistent state after failures.

Implementation in Databases

Most relational database management systems provide commands for managing transactions, including commit operations. For example:

  • SQL Server:

    BEGIN TRANSACTION;
-- SQL statements here
COMMIT;

  • MySQL/InnoDB:

    START TRANSACTION;
-- SQL statements here
COMMIT;

  • PostgreSQL:

    BEGIN;
-- SQL statements here
COMMIT;

Practical Example

Using Transactions and Commit

Let's look at how to use transactions and commit in a practical example using PostgreSQL:

-- Start a transaction block
BEGIN;
 
-- Insert a new employee record
INSERT INTO employees (employee_id, first_name, last_name, department)
VALUES (101, 'Alice', 'Smith', 'Engineering');
 
-- Update the department budget to account for the new hire
UPDATE departments SET budget = budget - 60000 WHERE department_name = 'Engineering';
 
-- Commit the transaction to make changes permanent
COMMIT;
 
-- Query to verify changes
SELECT * FROM employees WHERE employee_id = 101;
SELECT * FROM departments WHERE department_name = 'Engineering';

In this example:

  • A transaction block begins with BEGIN.
  • Two operations are performed: inserting a new employee and updating the department's budget.
  • The COMMIT statement finalizes these changes, making them visible to other transactions.
  • Subsequent queries confirm that the changes have been successfully applied.

Conclusion

Understanding the commit operation is essential for managing transactions effectively and ensuring data integrity and consistency in database systems. By properly using commit along with other transaction control commands, you can maintain reliable and robust database applications.

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