The SID_LIST_LISTENER heading in the Oracle Network Services configuration plays a pivotal role in the establishment and management of connections between the Oracle database and its users.
The SID_LIST_LISTENER section is part of the listener.ora configuration file, which is the configuration file for the Oracle Net Listener. This section contains entries that map Oracle system identifiers (SIDs) to the Oracle instances they represent.
Each SID_LIST_LISTENER entry contains one or more SID_DESC entries that specify:
- The global database name (GLOBAL_DBNAME), which is usually the same as the SID.
- The Oracle home directory (ORACLE_HOME) for the database.
- The program used for dedicated connections (PROGRAM), typically the Oracle executable (oracle).
When a client sends a connection request to the listener, the client provides the SID for the desired database. The listener then uses the SID_LIST_LISTENER entries in its configuration file to determine how to route the connection request.
For example, for a client requesting a connection to a database with the SID "ORCL", the listener would look up "ORCL" in the SID_LIST_LISTENER section of its configuration file and use the corresponding ORACLE_HOME and PROGRAM settings to establish a dedicated server process for the connection.
In Oracle 11g and later, dynamic service registration eliminates the need for static SID_LIST_LISTENER entries for instances running on the same machine as the listener. When an instance starts, it automatically registers itself and its services with the listener, supplying all of the information that the listener needs to manage connections to that instance.
However, SID_LIST_LISTENER entries may still be needed for some configurations, such as those using Oracle's external procedures, Oracle Connection Manager, or when instances are running on a different machine from the listener. The appropriate configuration under these circumstances ensures successful connectivity and efficient performance of the Oracle network services.
