数据库编程知识点:ACID

ACID (atomicity, consistency, isolation, and durability)

■■ Atomicity:

All changes made during a transaction are made successfully,
or in the case of failure, none are made. If any operation fails during the
transaction, then the entire transaction is rolled back, leaving your data in
the state it was before the transaction was started. For example, suppose
Jack is making a transfer of $500 from his checking account to a savings
account. Sometime between the withdrawal of the $500 from the checking
account and the deposit of $500 to the savings account, the software running
the banking system crashes. Jack’s $500 has disappeared! With atomicity,
either the entire transfer would have happened, or none of it would
have happened, leaving Jack a much happier customer than he is now.

■■ Consistency:

 All operations transform the database from one consistent
state to another consistent state. Consistency is defined by how the database
schema is designed and whether integrity constraints such as foreign
keys are used. The database management system is responsible for ensuring
that transactions do not violate the database schema or integrity constraints.
For example, the bank’s database developers have declared in the
database schema that the balance of an account cannot be empty, or “null.”
If any transaction attempts to set the balance to an empty value, the transaction
will be aborted and any changes rolled back.

■■ Isolation:

A transaction’s changes are not made visible to other transactions
until they are committed under the atomicity rule described earlier.
This is best demonstrated by what happens when month-end reports are
generated. Let’s say that Jack is performing the transfer transaction outlined
in the atomicity example, and at the same time you are generating his
Why Use an RDBMS? 3
monthly statement. Without isolation, the monthly statement might show
the withdrawal from the checking account but not the deposit into the savings
account. This discrepancy would make it impossible for Jack or the
bank to balance their books.

■■ Durability:

 Once completed, a transaction’s changes are never lost
through system or hardware crashes. If Jill has paid for $50 worth of groceries
with her debit card at the grocery store and the transaction succeeds,
even if the database software crashes immediately after the
transaction competes, it won’t forget that her checking account balance is
$50 lower

原子性，一致性，不相关性和持久性

　

ACID是一种缩写，它代表了操作（也称为操作管理器）的四个主要性质：

原子性：在一个操作中涉及两个或两个以上独立的信息，这些信息要么全部提交，要么一个也不提交。

一致性：操作要么创建新的有效的数据状态，要么（如果发生任何错误的话），将所有数据返回到初始状态。

不相关性：操作在没有提交并不与别的任何操作发生任何关系。

持久性：即使发生错误重新启动，保存过的已经提交的数据在正确状态下仍然有效。