By Sangamesh B Satihal, Oracle DBA
SCAN is a new feature of 11gR2 that provides a user or client to connect to the cluster(RAC database) using sinlge virtual hostname. This virtual hostname is called as SCAN and this will work as cluster alias for a database in the cluster. This SCAN needs to be resolved to minimun one IP address but oracle strongly recommends three IP addresses.
In previous releases(10g and 11gR1), whenever user has to connect RAC database, he was supposed to connect using virtual IPs through VIP listener and each instance in a cluster had it's own virtual IP to connect to the database. But note that even we could connect RAC database using public IP also but our purpose of high availability will be lost if we use public IP. So if there are n number of instances, we had n number of VIPs and same should be used by a client in his TNS entry. It impies that when we add or remove a node or nodes from a cluster, then client has to update his tns enty in his tnsnames.ora file. In order to avoid this, Oracle introduced SCAN concept. By using which client can use only one host to connect to cluster database, independenet of how many instances are running on that cluster. One more advantage of SCAN is it automatically provides both failover and load balancing of connects, where the new connection will be directed to the least busy instance in the cluster by default. Having a single name to access the cluster allows clients to use the EZConnect client and the simple JDBC thin URL to access any database running in the cluster, independently of which server(s) in the cluster the database is active. RAC Instances register to SCAN listeners as remote listeners. Example of EZconnect method in oracle and JDBC connection is shown below.
EZconnet :sqlplus system/manager@testdb-scan:1521/testdb
( EZconnecxt format is CONNECT username/password@[//]host[:port][/service_name])
JDBC connect: jdbc:oracle:thin:@testdb-scan:1521/testdb
Note : Since EZconnect is used with SCAN, the SQLNET.ora file should include EZconnect as one of the naming methods, for example:
How SCAN works
For clients connecting using Oracle SQL*Net 11g Release 2, three IP addresses will be received by the client by resolving the SCAN name through DNS. The client will then go through the list it receives from the DNS and try connecting through one of the IPs received. If the client receives an error, it will try the other addresses before
returning an error to the user or application. This is similar to how client connection failover works in previous releases when an address list is provided in the client connection string. When a SCAN Listener listening on a SCAN IP address receives a connection request, Because all services on the cluster are registered with the SCAN listener, the SCAN listener checks with VIP listener(local listener) and replies with the address of the local listener on the least-loaded node (Each scan listener keeps updated cluster load statistics) where the service is currently being offered to the client. Finally, the client establishes connection to the service through the listener on the node where service is offered. All of these actions take place transparently to the client without any explicit configuration required in the client.
How we can configure SCAN
The SCAN is configured during the installation of Oracle Grid Infrastructure that is distributed with Oracle Database 11g Release2. Oracle Grid Infrastructure is a single Oracle Home that contains Oracle Clusterware and Oracle Automatic Storage Management. You must install Oracle Grid Infrastructure first in order to use Oracle RAC 11g Release 2. During the installation phase of the Oracle Grid Infrastructure installation, you will be prompted to provide a SCAN name. The default value for SCAN is cluster_name.GNS_sub_domain, or, cluster_name-scan.domain_name if GNS is not used. For example, in a cluster that does not use GNS, if your cluster name is salesRAC, and your domain is example.com, then the default SCAN address is salesRAC-scan.example.com:1521.
During the installation of Oracle Grid Infrastructure, several Oracle Clusterware resources are created for SCAN:
A SCAN VIP is created for each IP address that SCAN resolves to
A SCAN listener is created for each SCAN VIP
A dependency on the SCAN VIP is configured for the SCAN listener
SCAN VIPs function like node VIPs, except that SCAN VIPs can run on any node in the cluster. Also, if you have three SCAN VIPs but only two nodes in your cluster, you can have two SCAN VIPs running on the same server. Clients (users or applications) that connect using SCAN instead of the node VIPs do not have to update the list of VIP addresses in their local tnsnames.ora file when nodes are added to or removed from the cluster.
During Oracle Grid Infrastructure installation, SCAN listeners are created for as many IP addresses as there are addresses assigned to resolve to the SCAN. Oracle recommends that the SCAN resolves to three addresses, to provide high availability and scalability. If the SCAN resolves to three addresses, then there are three SCAN listeners created.
The addresses for the SCAN listeners resolve with either the domain name service (DNS), or within the cluster for the Grid Naming Service (GNS), using a round-robin method. SCAN listeners can run on any node in the cluster.
The database parameter LOCAL_LISTENER specifies the listening endpoint of the local database listener, and the database parameter REMOTE_LISTENER parameter identifies the SCAN listeners. The database registers with the local and SCAN listeners by using the connect description information contained in these parameters. Oracle Database 11g release 2 and later instances only register with SCAN listeners as remote listeners. Upgraded databases register with SCAN listeners as remote listeners, and also continue to register with all node listeners.
Starting with Oracle Database 11g release 2, the REMOTE_LISTENER parameter is always set to the SCAN address. Do not set it to an Oracle Net alias that has a single address that uses SCAN for the host name (HOST=scan). For example, if SCAN for the cluster is myscan, and the GNS subdomain for the cluster is mycluster.example.com, then the REMOTE_LISTENER parameter has the following value:
IP Addresses required for RAC 11gr2 configuration are
A Public IP on each node
A Local VIP for each noe
A Privated IP for each node for Interconnect
SCAN IP for the cluster. At least one, recommended three.
A Public IP for GNS VIP
You can configure SCAN Name resolution using :
1. Define the SCAN in your corporate DNS (Domain Name Service)
2. Use the Grid Naming Service (GNS)
1. Define the SCAN in your corporate DNS (Domain Name Service)
Ih this method, hostname which is used for SCAN will be resolved to 3 IP addresses using a round-robin algorithm in DNS server. This will be taken care by system admin or network admin. Three IP addresses are recommended considering load balancing and high availability requirements regardless of the number of servers in the cluster. The IP addresses must be on the same
subnet as your public network in the cluster. The name must be 15 characters or less in length, not including the
domain, and must be resolvable without the domain suffix (for example: “sales1-scan’ must be resolvable as opposed
to “scan1-scan.example.com”). The IPs must not be assigned to a network interface (on the cluster), since Oracle
Clusterware will take care of it.
You can check the SCAN configuration in DNS using “nslookup”. If your DNS is set up to provide round-robin access to the IPs resolved by the SCAN entry, then run the “nslookup” command at least twice to see the round-robin algorithm work. The result should be that each time, the “nslookup” would return a set of 3 IPs in a different order.
testdb1-scan.example.com IN A 22.214.171.124
IN A 126.96.36.199
IN A 188.8.131.52
2. Use the Grid Naming Service (GNS): GNS is a Grid naming service that services subdomain. This subdomain is a portion of the corporate DNS domain. The only requirement is that a GNS Sub domain must be made and the DNS must be configured so that each request for this Sub Domain will be delegated to the GNS Sub domain, so that GNS can handle the request. DNA administrator (Network admin) creates a subdomain and delegates the subdomain name resolution to GNS.
This DNS subdomain must be created before grid installation. GNS listens on any fixed IP address of the RAC node within the cluster for the resolution of the name for that subdomain. The GNS VIP address is the ip address of the server that will host the GNS. You need to make sure this one is available for use. This GNS is managed by a CRS agent. GNS and GNS VIP run on any one node of the cluster and it listens on default port 53. If the node on which GNS was running fails, then Oracle clusterware fails GNS and along with the GNS VIP to another node in the cluster.
During GRID installation, the option "Configure GNS" is checked, as shown below, if we are using GNS. If the option "Configure GNS" is not checked, then DNS will be used for SCAN name resolution.
After installation you will find three listener processes running (separated on all cluster nodes). For each SCAN listener there will be a dedicated network interface running with the same IP as configured in DNS:
Client Connection to database “TESTDB” using SCAN would use this tnsnames entry:
The ”old fashioned” way, we can connect to db as below
Connecting to a named instance: