ucsd Lecture 8: Transactions, ACID, 2PC, 2PL, Serializability

ACID Transactions

Traditional database systems have relied upon bundling work into transactions that have the ACID properties. In so doing, they guarantee consistency at the expense of availability and/or partition tolerance.

NOTE: CAP theory

ACID is an acronym for:

1、Atomicity

2、Consistency (serializability)

3、Isolation

4、Durability

1、Acid - "All or nothing"

2、Consistency -- This implies two types of consistency. It implies that a single system is consistent and that there is consistency across systems. In other words, if $100 is moved from one bank account to another, not only is it subtracted from one and added to another on one host -- it appears this way everywhere. It is this property that allows one transaction to safely follow another.

3、Isolation - Regardless of the level of concurrency, transactions must yields the same results as if they were executed one at a time (but any one of perhaps several orderings).

4、Durability - permanance. Changes persist over crashes, &c.

Transactions, Detail and Example

Transactions are sequences of actions such that all of the operations within the transaction succeed (on all recipients) and their effects are permanantly visible, or none of none of the operations suceed anywhere and they have no visible effects; this might be because of failure (unintentional) or an abort (intentional).

NOTE: 简而言之: "All or nothing"

Mommit point

Characterisitically, transactions have a commit point. This is the point of no return. Before this point, we can undo a transaction. After this point, all changes are permanant. If problems occur after the commit point, we can take compensating(补偿；修正；抵消) or corrective action, but we can't wave a magic wand(魔法棒) and undo it.

NOTE: commit后是不能redo的

Distributed Transactions and Atomic Commit Protocols

NOTE: 放到了distributed computing章节中了