浅析Oracle 的体系架构及内存分配机制
Oracle Server ：Oracle服务器，一般可以看作是一个摸得着看的见的机器，我们可以称之为服务器。也可以看作是一套运行在服务器上
首先讲 Oracle 包含 的 三个部分：
1： Oracle Server ：Oracle服务器，一般可以看作是一个摸得着看的见的机器，我们可以称之为服务器。也可以看作是一套运行在服务器上的软件。
在Oracle官方网站是这样定义的：
An Oracle server is a database management system that provides an open ,comperhensive ,integrated approach to information management 。
An Oralce server consits of an Oracle instance and an Oracle database;
从上面的一段文字我们可以看出Oracle server是一个比较大的概念 是一个数据库管理系统，提供开放的，复杂的集成的信息管理系统。里面包含两个一个就是我们通常所说的Instance（Oracle实例）另外一个就是Oracle数据库（Oracle database）。
图1：Oracle Server图解
2：Oracle Instance (Oracle 实例)
定义：A Oracle Instance is means to access an Oracle database;（是Oracle访问数据哭的一种手段）
Alway open one and only one database.这句话就是说Oralce的一个Instance对应一个也只能一个database来访问。而database可以有多个Instance与之对应并访问，在一般的方式下Oracle Instance 与Oracledatabase是 1:1 的关系 而在集群的时候是 n:1 的关系。
另外，，Oracle Instance 是由内存以及oracle的后台进程构成（详见下图2）。
图2：The structure of Oralce Instance
Connection：Oracle客户端与进程建立的TCP连接。
Server Process:服务进程，主要是用于处理客户端和后台连接
在通过安全验证后，Oracle链接就建立起了一个Session（session） 3:Oracle Database
An Oracle database is a collection of data that is threaded as an unit.It. Constists of three file type.
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