Oracle Database includes the following components: the Oracle Database instance, which is a collection of processes and memory, and a set of disk files that contain user data and system data.
Each instance has an instance ID, also known as a system ID (SID). Because there can be multiple Oracle instances on a host computer, each with its own set of data files, you must identify the instance to which you want to connect. For a local connection, you identify the instance by setting operating system environment variables. For a remote connection, you identify the instance by specifying a network address and a database service name. For both local and remote connections, you must set environment variables to help the operating system find the SQL*Plus executable and to provide the executable with a path to its support files and scripts.
To manage objects that are shared by the multitenant container database and its pluggable databases (PDBs), such as control files, redo log files, or archived redo log files, connect to the CDB root. Objects such as tablespaces, data files, or temp files can be created in the CDB root or a PDB. To manage such objects, connect to the container that owns the object.
In the remainder of this book, connecting to the database means connecting to the CDB root.
