RAC Operational Practices - Expert Oracle RAC 12c - page 78

While installing Grid Infrastructure software, you can provide the SCAN name and port number. Grid Infrastructure
queries the DNS server to identify IP addresses associated with SCAN names and creates as many SCAN listener
resources as there are SCAN IP addresses. For example, if DNS returns three SCAN IP addresses, then three SCAN IP
resources and three SCAN listener resources are created in the Clusterware.
After the Grid Infrastructure installation, you can modify SCAN IP address and SCAN listener attributes using the
srvctl command. The following configuration of SCAN IP address shows that Clusterware created three sets of SCAN
IP resources and SCAN listeners for the rac-scan.example.com DNS alias.
$ srvctl config scan
SCAN name: rac-scan, Network: 1/10.7.11.0/255.255.254.0/eth1
SCAN VIP name: scan1, IP: /rac-scan.example.com / 10.7.11.110
SCAN VIP name: scan2, IP: /rac-scan.example.com / 10.7.11.111
SCAN VIP name: scan3, IP: /rac-scan.example.com / 10.7.11.112
Enabling client-level TNS trace, you can see that the SCAN IP address retrieved is different for each connection
attempt. This round-robin retrieval of SCAN IP addresses provides a load balancing mechanism among SCAN listeners.
The following lines are shown from SQL*Net trace files, and you can see that the first connection request went
to the SCAN listener with IP address 10.7.11.112 and the second connection request went to the scan listener with
IP address 10.7.11.110.
$ sqlplus apps@po
nttbnd2addr:using host IP address: 10.7.11.112
..
$ sqlplus apps@po
nttbnd2addr:using host IP address: 10.7.11.110
..
The initialization parameter remote_listener is set to the TNS alias of SCAN listener or EZConnect syntax using
the SCAN IP address and port number. The PMON or LREG process register services and propagates service-level
changes to the SCAN listener specified in the remote_listener parameter. EZConnect syntax is as follows.
*.REMOTE_LISTENER= rac-scan.example.com :25000
As a SCAN listener acts as a redirection mechanism for connection requests, reviewing services supported by a
SCAN listener shows that all services supported by a SCAN listener will have a remote server keyword, indicating
that the SCAN listener will redirect the connection to a VIP listener. In the following lsnrctl services command
output, notice the keyword REMOTE SERVER , indicating that the LISTENER_SCAN2 listener will redirect the connection
requests to the rac2 node VIP listener, thereby forwarding the connection to the RAC2 instance.
$ lsnrctl services LISTENER_SCAN2
LSNRCTL for Solaris: Version 11.2.0.2.0 - Production on 19-JAN-2013 16:43:26
Copyright (c) 1991, 2010, Oracle. All rights reserved.
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=LISTENER_SCAN2)))
Services Summary...
Service "PO" has 1 instance(s).
Instance "RAC2", status READY, has 1 handler(s) for this service...
Handler(s):
"DEDICATED" established:1 refused:0 state:ready
REMOTE SERVER
(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)( HOST=rac2.example.com ) (PORT=12000))
(CONNECT_DATA=(SERVICE_NAME=PO)(INSTANCE_NAME=RAC2)))