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Database transaction

Transaction是DBMS中的核心概念，DBMS中的如下内容的基石:

1) unit of work

2) unit of concurrency control，在Theory\Concurrency-Control-in-DBMS中进行了描述

stackoverflow How do ACID and database transactions work?

A

ACID is a set of properties that you would like to apply when modifying a database.

  • Atomicity
  • Consistency
  • Isolation
  • Durability

A transaction is a set of related changes which is used to achieve some of the ACID properties. Transactions are tools to achieve the ACID properties.

Atomicity means that you can guarantee that all of a transaction happens, or none of it does; you can do complex operations as one single unit, all or nothing, and a crash, power failure, error, or anything else won't allow you to be in a state in which only some of the related changes have happened.

Consistency means that you guarantee that your data will be consistent; none of the constraints you have on related data will ever be violated.

Isolation means that one transaction cannot read data from another transaction that is not yet completed. If two transactions are executing concurrently, each one will see the world as if they were executing sequentially, and if one needs to read data that is written by another, it will have to wait until the other is finished.

Durability means that once a transaction is complete, it is guaranteed that all of the changes have been recorded to a durable medium (such as a hard disk), and the fact that the transaction has been completed is likewise recorded.

So, transactions are a mechanism for guaranteeing these properties; they are a way of grouping related actions together such that as a whole, a group of operations can be atomic, produce consistent results, be isolated from other operations, and be durably recorded.

wikipedia Database transaction

A database transaction symbolizes(象征、表示) a unit of work performed within a database management system (or similar system) against a database, and treated in a coherent(连贯的、一致的) and reliable way independent of other transactions. A transaction generally represents any change in a database.

NOTE: **unit of work**是理解上面这段话的关键所在，需要以文章《Unit》中描述的思想来看待transaction:

1、transaction是DB中work的单位，即在DB中，以transaction为单位来度量work，显然，它是比SQL语句更大的一个量级

2、显然transaction是一个非常好的抽象，它抽象地表示了database work，符合DB的诸多需求，下面进行了描述

Purpose

Transactions in a database environment have two main purposes:

1、To provide reliable units of work that allow correct recovery from failures and keep a database consistent even in cases of system failure, when execution stops (completely or partially) and many operations upon a database remain uncompleted, with unclear status.

2、To provide isolation between programs accessing a database concurrently. If this isolation is not provided, the programs' outcomes are possibly erroneous.

NOTE: 如何实现isolation呢？

Property

A database transaction, by definition, must be atomic (it must either complete in its entirety or have no effect whatsoever), consistent (it must conform to existing constraints in the database), isolated (it must not affect other transactions) and durable (it must get written to persistent storage).[1] Database practitioners often refer to these properties of database transactions using the acronym ACID.

NOTE: 在Theory\Database-transaction\ACID中进行了专门的介绍。

Distributed transactions

NOTE: 放到了distributed computing章节中了

Transactional filesystems

NOTE: 这其实是将transaction概念使用到filesystem中