IMPORTANTS OF SCAN IN RAC ENVIRONMENT

 IMPORTANT SCAN LISTNER SETUP

1.What is SCAN listener?

A. scan listener is something that additional to node listener which listens the incoming db connection requests from the client which got through the scan IP, it got end points configured to node listener where it routes the db connection requests to particular node listener.

SCAN IP can be disabled if not required. However SCAN IP is mandatory during the RAC installation. Enabling/disabling SCAN IP is mostly used in oracle apps environment by the concurrent manager (kind of job scheduler in oracle apps).
Steps to disable the SCAN IP,
i. Do not use SCAN IP at the client end.
ii. Stop scan listener
srvctl stop scan_listener
iii.Stop scan
srvctl stop scan (this will stop the scan vip's)
iv. Disable scan and disable scan listener
srvctl disable scan



2.  How to setup SCAN in 11gr2 and how it works?

A. SCAN is a single name defined in DNS that resolves to three IP addresses using a round-robin
algorithm. The three IP addresses are on the same subnet as RAC’s default public network in the cluster.

How to setup SCAN:
In order to successful install grid infrastructure you need to configure your DNS prior to installing the grid
infrastructure to resolve the name accordingly. Oracle requires at least one IPs to be configured for the
scan name.
You can have two options to define SCAN name
1) Define it in your DNS (Domain Name Service)
2) Use GNS (Grid Naming Service)

How SCAN works:
1) Client sends connection to SCAN name SCAN name resolved to SCAN IP address returned by DNS.
SCAN IPs are returned in round-robin process. [If GNS is used for SCAN ip management then DNS
delegates the request to GNS services and GNS services in turn return a SCAN IP address ( again in
round-robin fashion)
2) The TNS request is now forwarded by SCAN IP to the SCAN Listeners. Remember that the
remote_listener parameter is already made to point to SCAN tnsnames.ora entry and local_listener uses
VIP Listener entry.
3) SCAN listeners in turn forward the request to local listeners (least loaded one).  The remote listeners
which points to SCAN listeners will do the load balancing and local listeners will take care of new process
spawning and connection to database.
4) Local listeners take care of client request.
PMON registers with SCAN listener as defined in parameter ‘remote_listener’ setting and also with the
node listener depending on the local listener settings. On the basis of PMON provided details SCAN will
choose the least loaded node to forward the request received from client.

3.  What are benefits of SCAN?

A) NO NEED TO RECONFIGURE CLIENT: the SCAN makes it possible to add or remove nodes from the
cluster without needing to reconfigure clients (Because the SCAN is associated with the cluster as a
whole, rather than to a particular node). Before SCAN, if the cluster changed, then the client
TNSNAMES.ORA files (or other tns connect strings like Easy Connect strings) would need to change.

B) LOCATION INDEPENDENCE: SCAN can connect from one node to any node . This is  location
independence for the databases, so that client configuration does not have to depend on which nodes
are running a particular database.
C) LOAD BALANCING: Round-robin on DNS level allows for a connection request load balancing across
SCAN listeners floating in the cluster.

New features of SCAN in database 12c:
1. SCAN and Oracle Clusterware managed VIPs now support IPv6 based IP addresses
2. SCAN is by default restricted to only accept service registration from nodes in the cluster
3. SCAN supports multiple subnets in the cluster (one SCAN per subnet)