MySQL交易的酸特性是什麼？

MySQL事務遵循ACID特性，確保數據庫事務的可靠性和一致性。首先，原子性（Atomicity）保證事務作為不可分割的整體執行，要么全部成功，要么全部失敗回滾，例如轉賬操作中取款和存款必須同時完成或同時不發生；其次，一致性（Consistency）確保事務將數據庫從一個有效狀態轉換到另一個有效狀態，通過約束、觸發器等機制保持數據邏輯正確；第三，隔離性（Isolation）控制多個事務並發執行時的可見性，防止臟讀、不可重複讀和幻讀，MySQL支持Read Uncommitted、Read Committed、Repeatable Read和Serializable四種隔離級別；最後，持久性（Durability）確保事務提交後其更改永久保存，即使系統崩潰也能通過重做日誌恢復，InnoDB存儲引擎通過innodb_flush_log_at_trx_commit等參數控制日誌刷新行為以實現持久性。

MySQL transactions follow the ACID properties , which ensure reliable and consistent database transactions. These properties are crucial for maintaining data integrity, especially in systems where multiple users access and modify data concurrently.

Atomicity

This property ensures that a transaction is treated as a single, indivisible unit of work. Either all operations within the transaction succeed, or none of them do. For example, if you're transferring money from one bank account to another, both the withdrawal and deposit must complete successfully. If either fails, the entire transaction is rolled back, leaving the database unchanged.

• Think of it like an all-or-nothing guarantee.
• In MySQL, this is managed using logs and rollback mechanisms.
• You don't want half of a financial transaction applied — atomicity prevents that.

Consistency

Consistency ensures that a transaction brings the database from one valid state to another. It enforces defined rules (like constraints, triggers, or cascades) so that even after a transaction runs, the data remains logically correct.

• For instance, if you have a foreign key constraint, a transaction can't insert a reference to a non-existent row.
• This also includes maintaining database-specific rules like unique keys or check constraints.
• MySQL uses the underlying storage engine (like InnoDB) to enforce consistency before and after transactions.

Isolation

Isolation determines how visible the effects of one transaction are to others running at the same time. Without proper isolation, issues like dirty reads, non-repeatable reads, or phantom reads may occur.

MySQL supports several isolation levels:

• Read Uncommitted – allows dirty reads
• Read Committed – prevents dirty reads but allows non-repeatable reads
• Repeatable Read – default in MySQL; prevents dirty and non-repeatable reads
• Serializable – highest level, prevents all three types of inconsistencies

You can set the isolation level depending on your application's needs. Higher isolation means better data accuracy but potentially lower performance due to more locking.

Durability

Durability guarantees that once a transaction has been committed, it will remain so, even in the event of a system failure such as a crash or power outage. MySQL achieves durability through its logging system — specifically, the redo log used by the InnoDB storage engine.

• When you commit a transaction, changes are first written to a log file before being applied to the actual database files.
• This log ensures that changes can be recovered even if the server crashes right after the commit.
• You can control durability behavior to some extent with settings like innodb_flush_log_at_trx_commit .

These four ACID properties together ensure that MySQL transactions behave reliably under all conditions. They're fundamental to any serious database system and particularly important when building applications that require strong data integrity, like banking systems or inventory management platforms.

基本上就這些。

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