When connecting to an Oracle Database, you can use a connect descriptor or a net service name.

For Oracle RAC databases, you can also use the single client access name (SCAN) to connect to any available instance of the Oracle RAC database. Users can access an Oracle RAC database using a client/server configuration or through one or more middle tiers, with or without connection pooling.

• Database Services for an Oracle RAC Database
  Each database is represented by one or more services. A service is identified by a service name, such as sales.example.com.
• Naming Methods and Connect Descriptors
  Each net service name is associated with a connect descriptor. A connect descriptor provides the location of the database and the name of the database service.
• Easy Connect Naming Method
  The Easy Connect naming method eliminates the need to look up service names in the tnsnames.ora file or other repository for TCP/IP environments.
• Understanding SCANs
  The SCAN is a domain name registered to at least one and up to three IP addresses, either in domain name service (DNS) or in Grid Naming Service (GNS).
• About Connecting to an Oracle RAC Database Using SCANs
  Oracle recommends that you configure Oracle RAC database clients to use the SCAN to connect to the database instead of configuring the tnsnames.ora file.
• About Listener Configuration for an Oracle RAC Database
  An Oracle RAC database uses multiple listeners to direct client requests to the available instances.
• About Service Registration for an Oracle RAC Database
  An Oracle Database 19c database service automatically registers with the listeners specified in the database initialization parameters LOCAL_LISTENER and REMOTE_LISTENER.
• How Database Connections are Created When Using SCANs
  Based on the environment, the following actions occur when you use a SCAN to connect to an Oracle RAC database using a service name.