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questions about # ofinstances

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Deanna Tran
Frequent Advisor

questions about # ofinstances

I'd like to know if more than one instance can
access the same database?
and why or why not?
or can 1 databse have multiple instances
5 REPLIES
Solution

Re: questions about # ofinstances

In order to have more than one instance accessing a database, you will need to run Real Application Clusters (RAC), which is an option on top of the normal database product.

In order to deploy RAC, you will need to have a clustered environment of 2 or more servers, running with Shared Storage of some sort, and MC/ServiceGuard, at least at the HP-UX level. Each server will run an instance accessing the central database.

RAC can also be deployed on Linux and Win2K environments, which still need the Shared Storage, but the cluster software is supplied on the CD supplied by Oracle.

Obviously, RAC can be deployed on other flavours of UNIX, but this is an HP forum:-)

Hope this helps,
Jeanine Kone
Trusted Contributor

Re: questions about # ofinstances

On the less complex, single server side of things, no. An instance and a database, while not synonomous, have a one to one match. Basically, the instance comprises all of the background processes and the database is the physical files and data. You can have more than one instance on a machine (i.e totally separate databases).
David LaPoint
Occasional Advisor

Re: questions about # ofinstances

What Andrew and Jeanine said is right. But I would also added that you can have multiple instances (memory processes/user processes) going against the same database (data in files) if you also use Oracle Parallel Server (OPS). OPS can be used for load balancing or HA. But it's an addon feature for Oracle at additional cost. You will also need to buy a lock manager. This solves the main reason you can't just have multiple instances going against the same database files. The lock manager is responsible for which instance has control of a particular piece of data. If two users go to update or select the same data, but each in a different instance, the lock manager is the final judge of who has control of that row.

Does that help explain why one can just start multiple instances for the same database?

Dave
You want to ask me a quick question? Sure! You can have the quick answer too, 42!
Aashish Raj
Valued Contributor

Re: questions about # ofinstances

Although you can specify maxinstances more than 1 while creating database, it is ignored by oracle if you are not using OPS or RAC in 9i.

Thanks
AR
Deanna Tran
Frequent Advisor

Re: questions about # ofinstances

Thank you all..
You have given me all the vital informations that I need and help me to understand how
oracle uses instances...