Analysis of ACID Properties in SQL Transactions

Problem Description
ACID is an acronym for the four core properties that ensure the correct execution of database transactions: Atomicity, Consistency, Isolation, and Durability. Interviews often require explaining the meaning of each property and describing how they are implemented in database systems.

Solution Process

1. Understanding the Basic Concepts of Transactions

   • A transaction is the smallest logical unit of database operations, consisting of one or more SQL statements (e.g., a transfer operation: deducting from account A and depositing into account B).
   • Goal of transactions: To ensure data correctness under concurrent operations and failure scenarios.

2. Breaking Down ACID Properties

   • Atomicity

     • Meaning: All operations in a transaction must either all succeed and be committed, or all fail and be rolled back. Partial execution is not allowed.
     • Analogy: Similar to atoms in chemistry being indivisible, a transaction is an indivisible whole.
     • Implementation Mechanism: Achieved through the database's Undo Log. For example, before modifying data during transaction execution, the original values are recorded in the log. If the transaction fails, the system rolls back to the initial state based on the log.

   • Consistency

     • Meaning: After a transaction is executed, the database must transition from one consistent state to another consistent state (e.g., constraints and trigger rules are not violated).
     • Note: Consistency is maintained jointly by the application layer and the database layer. For example, the total account balance should remain unchanged before and after a transfer.
     • Implementation Mechanism: Ensured by the database's integrity constraints (e.g., primary keys, foreign keys) and business logic.

   • Isolation

     • Meaning: Concurrent transactions are isolated from each other, meaning the operations of one transaction will not be interfered with by other transactions.
     • Problem Scenarios: Lack of isolation may lead to dirty reads (reading uncommitted data), non-repeatable reads (different results from the same query), and phantom reads (new data causing result changes).
     • Implementation Mechanism: Achieved through locking mechanisms or Multi-Version Concurrency Control (MVCC) to implement different isolation levels (e.g., Read Uncommitted, Read Committed, Repeatable Read, Serializable).

   • Durability

     • Meaning: After a transaction is committed, its modifications to data are permanent and will not be lost even in the event of a system failure.
     • Implementation Mechanism: Achieved through the Redo Log. When a transaction is committed, modifications are first recorded in the log and persisted to storage. During database recovery from a failure, the log is replayed to ensure data durability.

3. Practical Application Example

   • Taking a bank transfer as an example:
     1. Atomicity: Both deduction and deposit must either execute or not execute at all.
     2. Consistency: The total account balance remains unchanged before and after the transfer.
     3. Isolation: During the transfer, other transactions cannot read intermediate states (e.g., only deduction without deposit).
     4. Durability: After the transfer is successful, the data will not be lost even if the system crashes.

4. Common Interview Extensions

   • Comparison of Isolation Levels: Explain how different levels balance performance and consistency (e.g., Repeatable Read avoids non-repeatable reads through MVCC).
   • Log Technical Details: How Redo Log and Undo Log collaborate to achieve crash recovery.
   • Distributed Transactions: How to ensure ACID in distributed systems through Two-Phase Commit (2PC).

By gradually understanding the characteristics and implementation principles of ACID, one can deeply grasp the core mechanisms of database transactions and address data consistency challenges in practical scenarios.