Transaction Management事务管理

Transaction Management Overview • Objectives — schedule queries from multiple simultaneous users efficiently — keep the database in a consistent state • Transaction management involves — performing “logical units of work” (definition of transactions) — controlling concurrency – stop user tasks interfering — resolving conflicts – e.g., simultaneous update attempts — recovering from errors – restore DB to consistent state 事务管理目标是有效安排多个用户同时进行的查询 - 保持数据库处于一致状态 包括 - 执行 "逻辑工作单位"（事务定义） - 控制并发性 - 停止用户任务干扰 - 解决冲突 - 如同时尝试更新 - 从错误中恢复 - 将数据库恢复到一致状态

The “ACID” Requirements For a Transaction A transaction should have certain well-defined properties: • Atomicity – each unit of work is indivisible; “all-or-nothing” (transactions that don’t complete must be undone or “rolled-back”) • Consistency – a transaction transforms the database from one consistent state into another (intermediates may be inconsistent) • Isolation – each transaction effectively executes independently – one transaction should not see the inconsistent/incomplete state of another transaction • Durability – once a transaction is complete, its effects cannot be undone or lost (it can only be “undone” with a compensating transaction) ACID要求 原子性：工作单位不可分割 未完成事务必须撤销 一致性：事务将数据库从一种一致状态转换为另一种（中间状态可能不一致） 隔离性：事务有效独立执行 事务不能看到另一个不一致未完成状态 持久性：事务完成，不可撤销或丢失（只能通过补偿事务 "撤销"）

How Are Transactions Defined? In SQL, by default, each SQL statement is treated as a transaction (even if it affects multiple rows or tables):每条SQL语句都被视为一个事务

This is one atomic unit of work on the database. Multiple statements may be grouped together into a single transaction. Example: A new member of s