Servers, Instances, and Databases

DB2 sees the world as a hierarchy of objects. Workstations (or servers) on which DB2 9 has been installed occupy the highest level of this hierarchy. During the installation process, program files for a background process known as the DB2 Database Manager are physically copied to a specific location on the server, and an instance of the DB2 Database Manager is created. Instances occupy the second level in the hierarchy and are responsible for managing system resources and databases that fall under their control. Although only one instance is created initially, several instances can exist. Each instance behaves like a separate installation of DB2 9, even though all instances within a system share the same DB2 Database Manager program files (unless each instance is running a different version of DB2). And although multiple instances share the same binary code, each runs independently of the others and has its own environment, which can be modified by altering the contents of its associated configuration file.

Every instance controls access to one or more databases. Databases make up the third level in the hierarchy and are responsible for managing the storage, modification, and retrieval of data. Like instances, databases work independently of each other. Each database has its own environment (also controlled by a set of configuration parameters), as well as its own set of grantable authorities and privileges to govern how users interact with the data and database objects it controls. From a user's perspective, a database is a collection of tables (preferably related in some way) that are used to store data. However, from a database administrator's viewpoint, a DB2 database is much more; a database is an entity that is composed of many physical and logical components. Some of these components help determine how data is organized, while others determine how and where data is physically stored. Figure 4-1 shows the hierarchical relationship between systems, instances, and databases.

Figure 4-1: Hierarchical relationship between systems, instances, and databases.