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Single production instance per multiple apps vs. single instance per single app

In this age of doing more with less, what are your thoughts regarding a company having a single production instance for many (if not all) of their applications instead of the usual vendor-suggested one instance per application. As you can guess, this would reduce hardware and administration costs. What are the upsides and downsides that this scenario presents to you?

It has been the IT business climate these days to squeeze more out of their systems in an effort to cut costs. One of the things that a DBA may consider is to consolidate instances. The first thing that I should tell you is that many experts advocate never running multiple applications against the same Oracle instance. Their advice is to just run multiple instances on the same server instead. While I try to follow this line of reasoning, there are business considerations that often dictate against this advice. So what do you need to consider when combining applications into one Oracle instance?

1. Only one instance to monitor on one server. This can greatly reduce your administration overhead as well as reduce your hardware costs.

2. Performance tuning becomes more difficult. You are sharing resources among applications so each application's performance now affects another application's performance. One application with a terrible SQL statement can impact other applications. When you look at the buffer cache, is it one or more applications that are causing a poor performing buffer cache?

3. One of the biggest headaches concerns your database version and application certification against that version. This is especially important with third party applications. You may have purchased a third party application that is only certified on Oracle 9i. But other third party vendors will only certify their product on Oracle 8i. So you can't upgrade the instance to support one product because the other products aren't ready for the new version. So what do you do? If you have many third party applications, you will find that one of your application's vendor is keeping you on an older release. Is that fair to the other application users?

4. Business rules of your application can cause some problems. One application needs to be up 24x7, but the other applications don't care. This can force you into a backup strategy or do other things to the instance that you wouldn't be constrained to otherwise.

These are just a few of the things that come to mind. But it should start you down the road of determining in consolidation of your Oracle instances.

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