August 2018
Intermediate to advanced
230 pages
6h 42m
Chinese
Content preview from Flask Web开发：基于Python的Web应用开发实战（第2版）
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46
｜
第
5
章
5.3
使用
SQL
还是
NoSQL
SQL
数据库擅于用高效且紧凑的形式存储结构化数据。这种数据库需要花费大量精力保证
数据的一致性，需要考虑停电或硬件失效。为了达到这种程度的可靠性，关系型数据库
采用一种称为
ACID
的范式，即
atomicity
（原子性）、
consistency
（一致性）、
isolation
（隔
离性）和
durability
（持续性）。
NoSQL
数据库放宽了对
ACID
的要求，从而获得性能上的
优势。
对不同类型数据库的全面分析和对比超出了本书范畴。对中小型应用来说，
SQL
和
NoSQL
数据库都是很好的选择，而且性能相当。
5.4
Python
数据库框架
大多数数据库引擎都有对应的
Python
包，包括开源包和商业包。
Flask
并不限制你使用
何种类型的数据库包，因此你可以根据自己的喜好选择使用
MySQL
、
Postgres
、
SQLite
、
Redis
、
MongoDB
、
CouchDB
或
DynamoDB
。
如果这些都无法满足需求，还有一些数据库抽象层代码包供选择，例如
SQLAlchemy
和
MongoEngine
。你可以使用这些抽象包直接处理高等级的
Python
对象，而不用处理如表、
文档或查询语言之类的数据库实体。
选择数据库框架时，要考虑很多因素。
易用性
如果直接比较数据库引擎和数据库抽象层，显然后者取胜。抽象层，也称为对象关系映
射器（
ORM
）或对象文档映射器（
ODM
），在用户不知不觉的情况下把高层的面向对
象操作转换成低层的数据库指令。
性能
ORM
和
ODM
把对象业务转换成数据库业务时会有一定的损耗。多数情况下 ...
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