Understand ACID properties: The pillars of a reliable database

Detailed explanation of database ACID properties

ACID attributes are a set of rules that ensure the reliability and consistency of database transactions. They define how database systems handle transactions, and ensure data integrity and accuracy even in case of system crashes, power interruptions, or multiple users concurrent access.

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ACID Attribute Overview

1. Atomicity: A transaction is regarded as an indivisible unit. Any part fails, the entire transaction is rolled back, and the database does not retain any changes. For example, if a bank transfer is deducted from one account but not increased to another, the entire operation is revoked.

<code> begin transaction; update accounts set balance = balance - 100 where accountid = 1; update accounts set balance = balance 100 where accountid = 2; rollback; -- 任何更新失败，都回滚所有更改。</code>

2. Consistency: A transaction converts a database from one valid state to another, maintaining all defined rules such as constraints, triggers, and relationships. For example, if a transaction violates a foreign key constraint, the database will block the operation.

<code> insert into orders (orderid, customerid) values (101, 9999); -- 如果customerid 9999不存在，则失败。</code>

3. Isolation: Transactions are executed independently and do not interfere with each other. The intermediate state of one transaction is invisible to other transactions, preventing problems such as dirty reading, non-repeatable reading and fantasy reading. For example, when one transaction updates a record, another transaction cannot read uncommitted changes.

<code> set transaction isolation level serializable;</code>

4. Durability: Once a transaction is committed, changes are permanently saved and will not be lost due to system failure. Databases are usually guaranteed by writing committed data to persistent storage. For example, after commit, the data is saved even if the system crashes.

<code> commit; -- 数据永久保存。</code>

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ACID attribute practice

Example of atomicity:

 <code>begin transaction; delete from inventory where productid = 10; insert into archive (productid, productname) values (10, 'productx'); if @@error > 0 rollback; else commit;</code>

• If the deletion from inventory fails, the insertion archive will also be cancelled.

Consistency example:

 <code>insert into orders (orderid, customerid, orderdate) values (101, 5, '2024-12-18'); -- 保证外键和日期约束。</code>

Isolation level:

Common isolation levels in SQL:

• Read not submitted: Dirty reading is allowed.
• Submitted reading: Prevent dirty reading.
• Repeatable reading: Ensure that the same data is read multiple times in the transaction.
• Serializable: the strictest level, ensuring complete isolation.

Persistence Example:

 <code>BEGIN TRANSACTION; UPDATE Accounts SET Balance = Balance - 500 WHERE AccountID = 101; COMMIT; -- 保证更改即使崩溃也能持久化。</code>

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The importance of ACID attributes

• Data integrity: Ensure the accuracy and reliability of the database.
• Concurrency control: Prevent concurrent transaction conflicts.
• Error recovery: prevents data from being corrupted by accidental failure.
• Reliability: Build trust in systems that require high data consistency (such as banks and e-commerce platforms).

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The challenge of ACID attributes

• Strict adherence to rules can lead to performance overhead.
• The complexity of distributed transactions increases.

Databases such as MySQL, PostgreSQL, and Oracle all implement ACID properties, ensuring data reliability and accuracy, which is crucial to building robust applications.

Author: Abhay Singh Kathayat

Full-stack developer, proficient in front-end and back-end technologies, and builds efficient, scalable, user-friendly applications using a variety of programming languages ​​and frameworks. Contact email: kaashshorts28@gmail.com

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