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Re: terminology

From: Hans Forbrich <forbrich_at_telusplanet.net>
Date: Fri, 23 May 2003 16:05:09 GMT
Message-ID: <3ECE4595.C51DE2BD@telusplanet.net>


Peter Morris wrote:

> I'm having trouble understanding certain ORACLE
> terminology. For example, I don't understand the
> difference between an Oracle database and an Oracle
> instance. Also, what's the difference between
> start/stop a database , mount/unmount a database
> and open/close a database?

Yes, the Oracle manuals can be overwhelming. My general approach is to plan to read them 3 times before using them as reference - once to get a list of words that don't make sense; once to try to understand those words; once to try to understand the meaning of the document.

I recommend the book "Oracle Essentials" (see http://oracle.oreilly.com for details) as a prerequisite to the manuals specifically to give a lightweight intorduction to the terminology.

There are several ways to answer your question, depending on your general computing experience and understanding of different operating systems. Following gives a high level answer and may make sense if you are comfortable with the [UNIX] distinction between program and process.

Briefly on instance vs database ...
- the database is the collection of data files, the definitions (schemas) that makes the contents of the data files useful and the data itself. The database uses disk resources. - the instance is the running collection of programs supplied by Oracle that coordinates the user's request to manipulate the database (update, extract, etc.). The instance uses CPU and memory resources. - one reason for the difference is that in a clustered environment, several instances on several servers can coordinate activity to one database. This can provide scalability, high availability and a lot of other benefits (as well as mass migration by DBAs to the local pharmacy for headache remedies)

On the stop/start - mount/unmount and open/close can be viewed as stop/start states. One convenient way of looking at this is mount links the database and the instance, whereas the world access to the linked and operational database is handled by the open/close state. You start the instance, link it to a specific database and then open it for access.

Hopefully that should give you enough to start grokking the manuals. Received on Fri May 23 2003 - 11:05:09 CDT

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