Understand oracle's "lsnrctl status"

Let's take a look at the output information of lsnrctl status:

Services Summary...
Service "PLSExtProc" has 1 instance (s ).
Instance "PLSExtProc", status UNKNOWN, has 1 handler (s) for this
Service...
Service "catadb" has 2 instance (s ).
Instance "catadb", status UNKNOWN, has 1 handler (s) for this
Service...
Instance "catadb", status READY, has 1 handler (s) for this service...
Service "catadbXDB" has 1 instance (s ).
Instance "catadb", status READY, has 1 handler (s) for this service...
The command completed successfully

Two questions are raised here:

1. Where are the services displayed in the output defined?
2. Why does a service have two instances?

Let's take a look at the second question. After oracle version 8.1, instances and listener can automatically discover each other when no entries are pre-defined for the database in the listener. ora file. However, it is customary to define a SID_DESC entry for each database in the listener. ora file for use as needed. Therefore, the "catadb" service has two instances:

An instance in the UNKNOWN state is statically defined in the listener. ora file, and an instance in the READY state is automatically registered to the listener by the PMON process when the database is started. Register each minute after the database is started. When using tools such as OEM, you must have a static definition of the listener.

Let's take a look at how these services are defined. Oracle has three listening methods:

Database	Provides network access to database instances
PLSExtProc	How to access executable programs in the operating system using PL/SQL packages
Executable	Provides network access to executable programs in the operating system

Service "PLSExtProc" is configured by default for the database instance in the listener. ora file, allowing PL/SQL packages to access external programs;
Service "catadb" is also configured in the listener. ora file, which is the standard mode used for each database connection;
Service "catadbXDB" is the default xml db feature for database creation in Versions later than Oracle 9i. It provides http, ftp, and other services and is defined in the initialization file spfile (or pfile;