Wednesday 19 September 2012

Single Client Access Name (SCAN) in Oracle 11gR2 RAC


Single Client Access Name (SCAN) is a new Oracle Real Application Clusters (RAC) 11g Release 2 feature that provides a single name for clients to access Oracle Databases running in a cluster. The benefit is that the client’s connect information does not need to change if you add or remove nodes in the cluster. 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. SCAN provides load balancing and failover for client connections to the database. The SCAN works as a cluster alias for databases in the cluster.

SCAN Concepts
  • Single client access name (SCAN) is the virtual hostname to provide for all clients connecting to the cluster (as opposed to the vip hostnames in 10g and 11gR1).  
  • SCAN is a domain name registered to at least one and up to three IP addresses, either in the domain name service (DNS) or the Grid Naming Service (GNS).
  • By default, the name used as the SCAN is also the name of the cluster and must be globally unique throughout your enterprise. The default value for the SCAN is based on the local node name. SCAN name must be at least one character long and no more than 15 characters in length, must be alphanumeric - cannot begin with a numeral and may contain hyphens (-). If you require a SCAN that is longer than 15 characters, then select an Advanced installation.
  • For installation to succeed, the SCAN must resolve to at least one address.
  • SCAN VIP addresses must be on the same subnet as virtual IP addresses and public IP addresses.
  • Oracle strongly recommends that you do not configure SCAN VIP addresses in the hosts file. But if you use the hosts file to resolve SCAN name, you can have only one SCAN IP address.
  • If hosts file is used to resolve SCAN hostname, you will receive Cluster Verification Utility failure at end of installation (see Note: 887471.1 for more details)
  • For high availability and scalability, Oracle recommends that you configure the SCAN to use DNS Round Robin resolution to three addresses.
  • Because the SCAN is associated with the cluster as a whole, rather than to a particular node, the SCAN makes it possible to add or remove nodes from the cluster without needing to reconfigure clients. It also adds location independence for the databases, so that client configuration does not have to depend on which nodes are running a particular database.
  • Clients can continue to access the cluster in the same way as with previous releases, but Oracle recommends that clients accessing the cluster use the SCAN. Clients using the SCAN can also access the cluster using EZCONNECT.
  • Grid Infrastructure will start local listener LISTENER on all nodes to listen on local VIP, and SCAN listener LISTENER_SCAN1 (up to three cluster wide) to listen on SCAN VIP(s); 11gR2 database by default will set local_listener to local LISTENER, and remote_listener to SCAN listener.
  • SCAN listener will be running off GRID_HOME, and by default, in 11gR2 local listener will be running off GRID_HOME as well.

EZconnet sqlplus system/manager@krac-scan:1521/ORCL.INDIA.COM
JDBC connect jdbc:oracle:thin:@krac-scan:1521/ORCL.INDIA.COM


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 interview phase of the Oracle Grid Infrastructure installation, you will be prompted to provide a SCAN name. There are 2 options for defining the SCAN:
1.      Define the SCAN in your corporate DNS (Domain Name Service)
2.      Use the Grid Naming Service (GNS)

Define the SCAN in your corporate DNS (Domain Name Service)

If you choose Option 1, you must ask your network administrator to create a single name that resolves to 3 IP addresses using a round-robin algorithm. 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: “krac-scan’ must be resolvable as opposed to “”). 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.

First nslookup

[root@kracnode2 ~]# nslookup kracnode-scan


Second nslookup

[root@kracnode2 ~]# nslookup kracnode-scan


Note: If your DNS server does not return a set of 3 IPs as shown in figure 3 or does not round-robin, ask your network administrator to enable such a setup. DNS using a round-robin algorithm on its own does not ensure failover of connections. However, the Oracle Client typically handles this. It is therefore recommended that the minimum version of the client used is the Oracle Database 11g Release 2 client. Refer about DNS configuration ClickHere.


If you choose option 2, you only need to enter the SCAN during the interview. During the cluster configuration, three IP addresses will be acquired from a DHCP service (using GNS assumes you have a DHCP service available on your public network) to create the SCAN and name resolution for the SCAN will be provided by the GNS.

            Oracle Universal Installer (OUI) enforces providing a SCAN resolution during the Oracle Grid Infrastructure installation, since the SCAN concept is an essential part during the creation of Oracle RAC 11g Release 2  databases in the cluster. All Oracle Database 11g Release 2 tools used to create a database (e.g. the Database Configuration Assistant (DBCA), or the Network Configuration Assistant (NetCA)) would assume its presence. Hence, OUI will not let you continue with the installation until you have provided a suitable SCAN resolution.
            However, in order to overcome the installation requirement without setting up a DNS-based  SCAN resolution, you can use a hosts-file based workaround. In this case, you would use a typical hosts-file entry to resolve the SCAN to only 1 IP address and one IP address only. It is not possible to simulate the round-robin resolution that the DNS server does using a local host file. The host file look-up the OS performs will only return the first IP address that matches the name. Neither will you be able to do so in one entry (one line in the hosts-file). Thus, you will create only 1 SCAN for the cluster. (Note that you will have to change the hosts-file on all nodes in the cluster for this purpose.)
            This workaround might also be used when performing an upgrade from former (pre-Oracle Database 11g Release 2) releases. However, it is strongly recommended to enable the SCAN configuration as described under “Option 1” or “Option 2” above shortly after the upgrade or the initial installation. In order to make the cluster aware of the modified SCAN configuration, delete the entry in the hosts-file and then issue: "srvctl modify scan -n <scan_name>" as the root user on one node in the cluster. The scan_name provided can be the existing fully qualified name (or a new name), but should be resolved through DNS, having 3 IPs associated with it, as discussed. The remaining reconfiguration is then performed automatically.


                During cluster configuration, several resources are created in the cluster for SCAN. For each of the 3 IP addresses that the SCAN resolves to, a SCAN VIP resource is created and a SCAN Listener is created. The SCAN Listener is dependent on the SCAN VIP and the 3 SCAN VIPs (along with their associated listeners) will be dispersed across the cluster. This means, each pair of resources (SCAN VIP + Listener) will be started on a different server in the cluster, assuming the cluster consists of three or more nodes. In case, a 2-node-cluster is used (for which 3 IPs are still recommended for simplification reasons), one server in the cluster will host two sets of SCAN resources under normal operations. If the node where a SCAN VIP is running fails, the SCAN VIP and its associated listener will failover to another node in the cluster. If by means of such a failure the number of available servers in the cluster becomes less than three, one server would again host two sets of SCAN resources. If a node becomes available in the cluster again, the formerly mentioned dispersion will take effect and relocate one set accordingly.

[oracle@kracnode1 ~]$ srvctl config scan_listener
SCAN Listener LISTENER_SCAN1 exists. Port: TCP:1521
SCAN Listener LISTENER_SCAN2 exists. Port: TCP:1521
SCAN Listener LISTENER_SCAN3 exists. Port: TCP:1521
[oracle@kracnode1 ~]$ srvctl config scan
SCAN name:, Network: 1/
SCAN VIP name: scan1, IP: /
SCAN VIP name: scan2, IP: /
SCAN VIP name: scan3, IP: /
[oracle@kracnode1 ~]$

            For Oracle Database 11g Release 2, SCAN is an essential part of the configuration and therefore the REMOTE_LISTENER parameter is set to the SCAN per default, assuming that the database is created using standard Oracle tools (e.g. the formerly mentioned DBCA). This allows the instances to register with the SCAN Listeners as remote listeners to provide information on what services are being provided by the instance, the current load, and a recommendation on how many incoming connections should be directed to the instance.
            In this context, the LOCAL_LISTENER parameter must be considered. The LOCAL_LISTENER  parameter should be set to the node-VIP. If you need fully qualified domain names, ensure that LOCAL_LISTENER is set to the fully qualified domain name (e.g. By default, a node listener is created on each node in the cluster during cluster configuration. With Oracle Grid Infrastructure 11g Release 2 the node listener run out of the Oracle Grid Infrastructure home and listen on the node-VIP using the specified port (default port is 1521).
Unlike in former database versions, it is not recommended to set your REMOTE_LISTENER parameter to a server side TNSNAMES alias that resolves the host to the SCAN (HOST=kracnode-scan for example) in the address list entry, but use the simplified “SCAN:port”

SQL> show parameter listener

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
listener_networks                    string
local_listener                       string      (DESCRIPTION=(ADDRESS_LIST=(AD
remote_listener                      string

Note: if you are using the easy connect naming method, you may need to modify your SQLNET.ORA to ensure that EZCONNECT is in the list when specifying the order of the naming methods used for the client name resolution lookups (the Oracle 11g Release 2 default is NAMES.DIRECTORY_PATH=(tnsnames, ldap, ezconnect)).


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 as discussed. 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 receives a connection request, the SCAN Listener will check for the least loaded instance providing the requested service. It will then re-direct the connection request to the local listener on the node where the least loaded instance is running. Subsequently, the client will be given the address of the local listener. The local listener will finally create the connection to the database instance.

Note: This example assumes an Oracle 11g Release 2 client using a default TNSNAMES. ora:

    (ADDRESS = (PROTOCOL = TCP)(HOST = = 1521))


The successful use of SCAN to connect to an Oracle RAC database in the cluster depends on the ability of the client to understand and use the SCAN as well as on the correct configuration of the REMOTE_LISTENER parameter setting in the database. If the version of the Oracle Client connecting to the database as well as the Oracle Database version used are both Oracle Database 11g Release 2 and the default configuration is used as described in this paper, no changes to the system are typically required.

The same holds true, if the Oracle Client version and the version of the Oracle Database that this client is connecting to are both pre-11g Release 2 version (e.g. Oracle Database 11g Release 1 or Oracle Database 10g Release 2, or older). In this case, the pre-11g Release 2 client would use a TNS connect descriptor that resolves to the node-VIPs of the cluster, while the Oracle pre-11g Release 2 database would still use a REMOTE_LISTENER entry pointing to the node-VIPs. The disadvantage of this configuration is that SCAN would not be used and hence the clients are still exposed to changes every time the cluster changes in the backend. Similarly, if an Oracle Database 11g Release 2 is used, but the clients remain on a former version. The solution is to change the Oracle client and / or Oracle Database REMOTE_LISTENER settings accordingly. The following cases need to be considered:

Oracle Client Version
Oracle Database Version
Oracle Database 11g Release 2
Oracle Database 11g Release 2
No change required.
Oracle Database 11g Release 2
Pre- Oracle Database 11g Release 2
Add the SCAN VIPs as hosts to the
Pre- Oracle Database 11g Release 2
Oracle Database 11g Release 2
Change the client TNSNAMES.ora to include the SCAN VIPs (* see below). IF the database was upgraded using the DBUA from a pre-11g Rel. 2 database, the DBUA will configure the REMOTE_LISTENER parameter to point to the node-VIPs as well as the SCAN.
Pre- Oracle Database 11g Release 2
Pre- Oracle Database 11g Release 2
If you want to make use of SCAN
(recommended): add the SCAN VIPs as hosts to the
AND Change the client TNSNAMES.ora to
include the SCAN VIPs (* see below). Otherwise, no change required.

Sample TNSNAMES.ora for Oracle Database pre- 11g Release 2 Clients


Common Questions Regarding SCAN

The following is a list of commonly asked questions regarding SCAN:

How can we configure the SCAN and SCAN listener?

During Typical installation, you are prompted to confirm the default Single Client Access Name (SCAN), which is used to connect to databases within the cluster irrespective of which nodes they are running on.  If you change the SCAN from the default, then the name that you use must be globally unique throughout your enterprise.

If the SCAN name resolves to one IP address, root script ( or will create the number of SCAN VIP resources( and corresponding SCAN listener resource(ora.LISTENER_SCAN1.lsnr) depend on how many IP address the SCAN name resolves to, i.e.if the SCAN name resolves to two IP addresses, it will create two SCAN VIP resources and two corresponding SCAN listener resource.

SCAN VIP and the corresponding SCAN listener works like a pair, when SCAN VIP fails over to other node, the corresponding SCAN listener will also be failed over to the same node.
When SCAN VIP fails over happens, it will always select a node with least running SCAN VIP, i.e., if SCAN VIP runs on node1, node2 and node3 of a 4-node cluster, if node3 goes down, the SCAN VIP and corresponding SCAN listener will be failed over to node4 as the other two nodes already have one SCAN VIP running on each node.

Also we can use 'srvctl' to add/modify the scan resource and the listeners.  Please refer to "Real Application Clusters Admin and Deployment Guide" or Note 1053147.1 for more information.

Do we still need to configure local listeners on each node?

Yes, you would need to configure independent local listeners for each node.  SCAN listeners are not replacements for the node listeners.

A new set of cluster processes called scan listeners will run on three nodes in a cluster (or all nodes if there are less than 3).  If you have more than three nodes, regardless of the number of nodes you have, there will be at most three scan listeners.  The database registers with the SCAN listener through the remote listener parameter in the init.ora/spfile.  If any of these clustered processes fail, they are automatically restarted on a new node.

How does SCAN work ?

"When a client submits a request, the SCAN listener listening on a SCAN IP address and the SCAN port is contracted on a client's behalf. Because all services on the cluster are registered with the SCAN listener, the SCAN listener 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. 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."

[oracle@kracnode1]$ srvctl STATUS SCAN_LISTENER
SCAN Listener LISTENER_SCAN1 is enabled
SCAN listener LISTENER_SCAN1 is running on node kracnode1
SCAN Listener LISTENER_SCAN2 is enabled
SCAN listener LISTENER_SCAN2 is running on node kracnode2
SCAN Listener LISTENER_SCAN3 is enabled
SCAN listener LISTENER_SCAN3 is running on node kracnode3

Instead of DNS or GNS, Can we use '/etc/hosts' to resolve SCAN?

Oracle strongly recommends that you do not configure SCAN VIP addresses in the hosts file. But if you use the hosts file to resolve SCAN name, you can have only one SCAN IP address.
failure at end of installation (See
NOTE 887471.1 for more details) 

Can we use the previous method (Using VIP) for client connection?

Clients can continue to access the cluster in the same way as with previous releases. Vips are still used internally, and can still be used for connections. But Oracle strongly recommends that clients accessing the cluster use the SCAN. Clients using the SCAN can also access the cluster using EZCONNECT.

Is it mandatory to use SCAN?

It's highly recommended to use SCAN unless there's strong business reason preventing it from being used.

Is it supported to remove SCAN?

SCAN is an elementary part of 11gR2 Grid Infrastructure, it's not supported to remove SCAN.
Is it recommended to use COST feature?

As a Best Practice, Oracle recommends using the COST feature to restrict instance registration with SCAN listeners as part of your standard listener configuration.  Refer to note 1340831.1 for more details.
Sample TNS entry for SCAN


Sample TNS Entry without SCAN


Related Blogs:


Hope! This helps...


  1. V nice and detailed Description of Scan Names!, solved many of my doubts..

  2. Excellent article abt SCAN ....Hats off...Keep up the good work

  3. very useful and brilliantly explained .. keep up the good posting ..

    expecting much more from your blog..

  4. Very useful to know about SCAN in detail...Good Article kavin :)

  5. Great work, Easy to understand, Keep up the good work.

  6. excellent...........................

  7. Awesome explanation .....

  8. Excellent article ..Appreciate ur efforts ...Thanks

  9. Can u please tell the database connection strings that an Oracle Client uses to connect to access Oracle Databases running in a cluster using

  10. In SCAN CONFIGURATION IN THE CLUSTER ... you should to do ... srvctl modify scan_listener –u

  11. very nice can you update the client TNSNAME.ora

  12. Very nice explanation, Thanks a heap for sharing :)

  13. Great help solve all my doubts regrading. I wish I could find this doc in earlier time

  14. Thank you, definitely learned quite a bit.

  15. Nice explanation. Really helpful .

  16. This is an excellent piece of work. Thanks a ton.


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