November 2020
Beginner to intermediate
214 pages
5h 10m
Chinese
Content preview from 数据库可靠性工程：数据库系统设计与运维指南
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178
｜
第
12
章
节。每个用户或事务都在寻找数据的一个子集，这意味着查询模式倾向于在大型集合中查
找和访问特定的小数据集。高效的索引、隔离和并发是关键，这就是通常由关系型系统实
现的原因。
前端数据存储的另一个特点是，其数据主要是由用户输入的。还有一些面向用户且主要用
于分析的数据存储，通常称为
OLAP
（
on-line analytics processing
，联机分析处理），稍后
讨论。
前面讨论了大部分数据存储的各种特性：存储结构，数据模型，
ACID/BASE
范式，可用
性、一致性以及延时之间的权衡。此外，还必须考虑整体的可操作性，以及与生态系统其
他部分之间的集成。这方面需要考虑的典型特性有：
•
低延时的写入和查询；
•
高可用；
• MTTR
较短；
•
在线可扩展；
•
易于和应用程序及运维服务集成。
对于任何单一架构而言，要满足这些需求都是非常困难的；在架构中如果没有其他组件的
协助，几乎无法满足这些需求，稍后会阐述这一点。
12.1.2
数据访问层
应用程序通常分为展现层和业务逻辑层。数据访问层（
data access layer
，
DAL
）位于业务
逻辑层。这一层为应用程序访问持久化数据存储提供了简便的方式。这通常表现为一组对
象，这些对象包含相关存储过程或查询的属性和方法。这种抽象向软件工程师隐藏了数据
存储的复杂性。
数据访问层的一个例子是
DAO
。
DAO
通过将应用程序调用映射到数据库，来提供数据库
访问接口。通过单独存放持久性逻辑，软件工程师可以单独测试数据访问；类似地，通过
提供
mock
接口而非数据库 ...
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