General Meaning of a Database Instance
In general, a database instance describes a complete database environment and all of its components. This system includes multiple parts, including the relational database management system (RDBMS) software, table structure, stored procedures, and other functionality. Database administrators might create multiple instances of the same database for different purposes. For example, an organization with an employees database might have three instances:
Production instances: Administrators use these instances to contain live data.Pre-production instances: Developers use pre-production instances to test new functionality before releasing features into production.Development instances: Database developers use development instances to create new functionality before testing it in pre-production.
Oracle Database Instances
If you have an Oracle database, you know that a database instance means a very specific thing. While the database includes the application data and metadata stored in physical files on a server, an instance is a combination of the software and memory used to access that data. For example, if you sign in to an Oracle database, your login session is an instance. If you log off or shut down your computer, your instance disappears, but the database and your data remain intact. An Oracle instance can access only one database at a time, while multiple instances can access an Oracle database.
SQL Server Instances
A SQL Server instance usually means a specific installation of SQL Server. It’s not the database; instead, it’s the software used to create the database. Maintaining multiple instances might be useful when managing server resources. You can configure each instance for memory and CPU usage, which is something you can’t do for individual databases within a SQL Server instance.
Database Schema vs. Database Instance
It may also be helpful to think of an instance in context with a database schema. The schema is the metadata that defines the database design and determines the methods for data organization. It includes a database’s tables and their columns and any rules that govern the data. For instance, an employee table in a database might have columns for name, address, employee ID, and job descriptions. This arrangement is the structure, or schema, of the database. While the database schema defines the organization of data, an instance of the database is a snapshot of the content at any given moment. This picture includes the data and its relationship to other data in the database at a particular time.
