Database ServicesAn Oracle database is represented to clients as a service; that is, the database performs work on behalf of clients. A database can have one or more services associated with
A database can have multiple services associated with it. One database may two different services for Web clients. One service,
say,book.us.acme.com, is dedicated to clients making book purchases. The other service,
soft.us.acme.com,say, is dedicated to clients making software purchases.
A database has one or more instances. An instance is comprised of a memory area called the System Global Area (SGA) and Oracle processes. The memory and processes of an instance efficiently manage the associated database's data and serve the database users.
When the listener receives the client request, it selects one of the service handlers that were previously registered. Depending on the type of handler selected, the communication protocol used, and the operating system of the database server, the listener performs one of the following actions:
- Hands the connect request directly off to a dispatcher.
- Sends a redirect message back to the client with the location of the dispatcher or dedicated server process. The client then connects directly to the dispatcher or dedicated server process.
- Spawns a dedicated server process and passes the client connection to the dedicated server process.
Once the listener has completed the connection operation for the client, the client communicates with the Oracle database server without the listener's involvement. The listener resumes listening for incoming network sessions.