In most Oracle configurations, there is a one-to-one correspondence between the database (disk files) and the instance (memory structures). When an Oracle database opens, a lock is placed on these datafiles. If another instance attempts to open with any of these datafiles, it won't be able to obtain the lock that has been placed on these files. So an error is raised, and the second instance will not start.
The exception to this is Oracle Parallel Server (OPS). This has been renamed to Real Application Clusters (RAC) for 9i. OPS and RAC let you have multiple instances connected to the same database.
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This was first published in June 2002