There are two terms that, when used in an Oracle context, seem to cause a great deal of confusion: database and instance. To be fair, part of the reason for this is that different database vendors use the same terms to mean different things. In Oracle terminology, a database is a collection of physical operating system files or disks.
When using Oracle Automatic Storage Management (ASM), the database may not appear as individual, separate files in the operating system, but the definition remains the same.
In contrast, an instance is a set of Oracle background processes or threads and a shared memory area, which is memory that is shared across those threads or processes running on a single computer.
This is the place for volatile, nonpersistent stuff, some of which gets flushed to disk. A database instance can exist without any disk storage whatsoever. It might not be the most useful thing in the world, but thinking about it that way definitely helps draw the line between the instance and the database.
The two terms, instance and database, are sometimes used interchangeably, but they embrace very different concepts, especially with the multitenant architecture and Oracle Real Application Clusters (RAC) architecture. In the earliest versions of Oracle, there was usually a one-to-one relationship between an instance and a database.
This is probably why the confusion surrounding the terms arises. However, to be technically accurate, an instance refers to the processes and memory of Oracle. And a database refers to the physical files that hold data.
A database may be accessible from many instances, but an instance will provide access to exactly one database (regardless of database type) at a time. This will become clearer in a moment. Let’s next look at the definition of a single-tenant database.
