What is a: 1) database instance? 2) database listener
In Oracle-speak, the database is made of the disk files that hold your data. These datafiles may hold tables, indexes, passwords, configuration information or information about the database in the control files.
When you start an Oracle database, you allocate memory on the server and start up various processes that manage the database for you. These processes and the allocated memory are referred to as the Oracle INSTANCE. Until you have an instance running, you just have datafiles with unaccessible data. Once you start an instance of the database, you can then begin to access the data.
Many times, users and applications connect to the database from a machine outside of the database server. These users or applications request a connection be made to the database. The database listener runs on the database server listening for connections. Once the listener detects a connection request, it sets up any needed processes on the server to get the connection up and running. Once the user or application is connected to the database, the listener hangs around listening for new connection request. This is similar to a Web server listening for HTTP requests.
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