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单实例和RAC打造的Oracle Stream

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Jun 07, 2016 pm 05:02 PM

前天按照《大话ORACLE RAC》一书中介绍RAC和STREAM相关章节的实验步骤在HP rx8640上搭建了类似的实验环境做实验，结果失败了。昨

前天按照《大话ORACLE RAC》一书中介绍RAC和STREAM相关章节的实验步骤在HP rx8640上搭建了类似的实验环境做实验，结果失败了。昨天跟着网上搜来的一篇《一步一步学ORACLE STREAM 》的文章以两个单实例搭建实验平台初获成功，晚上粗略研读了一下Oracle的官方文档《Streams Concepts and Administration 》，无奈太过繁琐，没有耐心和勇气继续读下去。今天闲来无事，硬着头皮再次挑战单实例和RAC下的ORACLE STREAM实现，按照昨天的思路，居然成功了，心里真是高兴。

细想开来，其实单实例也好，RAC 也好，数据库始终只有一个，区别只是实例多少的不同，实际操作步骤还是大同小异的。这让我想起前段时间操作的ORACLE RAC数据库版本升级（从10.2.0.1升级到10.2.0.4），我执行了两次升级，第一次当然是顺利的，第二次升级失败那是必然的了，因为数据库只有一个，升级也只需要一次即可，再次升级就是画蛇添足。这里又联想起无论是参加ORACLE原厂OCP培训还是看到李丙洋的《涂抹ORACLE》里面都着重指出一个容易混淆的概念：实例是实例，数据库是数据库，如果混为一谈就不好了。我之所以犯错，归根结底还是对数据库的概念理解不够透彻。

ORACLE官方文档《concepts》第四十一页里面有一句话说得很清楚：

"An oracle database server consists of an oracle database and an oracle insance. Every time a database is started, a system global area(SGA) is allocated and oracle background processes are started. Then combination of the background processes and memory buffers is called an oracle instance."

这句话里面有两层意思，第一，数据库是由数据文件和实例两部分组成；第二，实例只是一组内存结构，由内存缓冲区和后台进程组成。我们常常对单个数据库进行操作，所以容易混淆数据库和实例的概念，【　Linux公社　】认为实例就是数据库，数据库就是实例，实际根本不是这么回事。

理解了这个概念，然后再回过头来做实验，就相对比较容易了。其实无论是单实例数据库也好，ORACLE RAC也好（多实例数据库），只要认清了数据库只有一个，无论是升级也好，构建高可用性也好，按照单个节点的思路走下去就行。就比如说RAC 数据库升级，所不同的无非就是需要额外修改一下参数文件里面的个别参数，操作起来也挺简单的。基本概念很重要，这也是修房子的时候为什么要先打地基，地基打得牢，房子才盖得高，，想想学技术也是这个道理。

下面言归正传，回到此次实验。

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