Question:  I hear that Oracle 11g release 2 has a new RAC feature called SCAN (Single Client Access Name).  How does SCAN replace TAF in a RAC cluster?  Should I use SCAN instead of manual load balancing?

Answer:  According to reliable sources on the web, SCAN provides a single domain name via DNS), allowing and-users to address a RAC cluster as-if it were a single IP address. SCAN works by replacing a hostname or IP list with virtual IP addresses (VIP). 

Because SCAN determines the actual node and listener which accepts the connection, SCAN provides location independence for the databases.  A SCAN database connection does not require any details about a particular node on instance.  SCAN is part of the 11g release 2 movement toward "RAC Virtualization".  Virtualization is great for some RAC shops, not so good for others.

Expert Note:  SCAN is an automatic load balancing tool that uses a relatively primitive least-recently-loaded algorithm.  Most Fortune 50 mission critical RAC systems will not use an automated load balancer in favor of intelligent RAC load balancing., where you direct like-minded transactions to like-minded nodes.  This approach greatly reduces the load on the cache fusion payer because less blocks must be sent across the RAC interconnect.

According to Oracle, there are two benefits for SCAN:

• Fast RAC failover:  If a node fails, Oracle detects the loss of connection to the VIP and redirects new connects to the surviving VIP's.  This is an alternative to the transparent application failover. (TAF) for automatic load balancing. 
  
  
• Easier maintenance for Grid RAC systems:  For Grid systems that gen-in and gen-out blade servers frequently, SCAN offers easier change control for the RAC DBA.  As RAC nodes are added or deleted, the DBA does not have to change the configuration files to reflect the current list of RAC node IP addresses (or hostnames).  In a nutshell, SCAN allows a single cluster alias for all instances in the cluster.

Configuring RAC to use SCAN

The set-up for SCAN is implemented as follows:

STEP 1 - Add a new DNS entry for SCAN (e.g. /etc/hosts).  This will create a single DNS domain name that resolves to all of the IP addresses in your RAC cluster (one for each node).  This will use a round-robin algorithm

root> cat /etc/hosts

myscan.mydomain.com IN A 122.22.22.22 IN A 122.22.22.23 IN A 122.22.22.24

STEP 2 - Create a SCAN VIP for each IP address in  step 1.

STEP 3 - Create a SCAN listener for each VIP that was created in step 2.

STEP 4 - Change the tnsnames.ora file.  This entry will point SCAN to the single DNS hostname created in step 1.  Prior to the introduction of SCAN (11g r1 and earlier using TAF or intelligent load balancing ), the tnsnames.ora file contained a list of hostnames, one for each node in the cluster:

SCANDB =
(DESCRIPTION =
  (ADDRESS = (PROTOCOL = TCP)(HOST = myscan1.mydomain.com)(PORT = 1522))   
  (ADDRESS = (PROTOCOL = TCP)(HOST = myscan2.mydomain.com)(PORT = 1522))
  (ADDRESS = (PROTOCOL = TCP)(HOST = myscan3.mydomain.com)(PORT = 1522))
   (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = SCANDB)
   )
)

STEP 5 - Set the remote_listener and local_listener parameters:  You want to set your remote_listener parameter to point to a tnsnames.ora entry for SCAN as defined in step 4.

remote_listener=myscan.mydomain.com:1522

The SCAN listener for RAC

This single domain addressing is implemented via a "scan listener" and the status can be checked with the standard lsnrctl command, passing the SCAN listener name as an argument:

oracle> lsnrctl service

oracle> lsnrctl stat myscan_listener

LSNRCTL for UNIX: Version 11.2.0.1.0  . .

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=IPC)(KEY=LISTENER_MYSCAN1)))

STATUS of the LISTENER . . .

Server control commands for SCAN

According to Karen Reidford, there are several new server control (srvctl) commands for SCAN:

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