The Oracle® Hacker's Handbook: Hacking and Defending Oracle

1.1. Architecture

When we talk about an Oracle database server what do we actually mean? A database refers to all of a system's data — such as on the disk. A database instance refers to all the running processes and memory that enable that data to be queried. Normally, one database is serviced by one instance, but in the case of clustering there can be many instances running for a single database. The System Identifier, or SID, is the name given to the database instance. When I use the term host or server I'm generally referring to the machine upon which the database server software is running — regardless of the instance or database.

There are two major components to the software: the TNS Listener and the relational database management system (RDBMS) itself. The TNS Listener is the hub of all Oracle communications. When a database instance is brought up or started, it registers with the TNS Listener. When a client wishes to access the database, it first connects to the Listener and the Listener then redirects the client to the database server. When the RDBMS wants to launch an external procedure, it connects first to the Listener, which in turns launches a program called extproc. This is fully covered in the Database Hacker's Handbook so won't be repeated here. One exception to this is the external jobs launched by the database's job scheduler. Here the RDBMS connects directly to the external job server. You'll learn more about this later, too.