Oracle RAC Scan Listener Connections Test

Oracle RAC 11g release 2 introduces the Single Client Access Name (SCAN), which provides a single name for clients to access Oracle Databases running in a cluster and simplify the database connection strings that an Oracle Client uses to connect.

The RAC components that play a crucial role in SCAN are as follows:

  • SCAN VIP: SCAN VIPs are physical IP addresses that you allocate to a SCAN listener.

  • SCAN Listener: SCAN Listener is an Oracle RAC component, which starts running a service on the port [default 1521] using IP address assigned to SCAN VIPs. The name that is given to a SCAN Listener is referred as SCAN name, which should be registered in DNS server.

  • Node VIP: Each node in the RAC cluster has its own node VIP. It is a high availability resource of RAC that enables a client to initially connect to the database service on its node or to fail over to another cluster node and advise that its node is down.

SCAN listeners can run on any node in the cluster. Typically, each cluster will have a minimum of 1 and a maximum of 3 SCAN listeners, regardless of the number of nodes in the cluster. Three SCAN listeners are recommended considering load balancing and high availability requirements. A SCAN listener can be started only when the SCAN VIPs are online. The script automatically configures the SCAN VIPs and SCAN listeners. When the is run on the first node, all 3 scan IPs are plugged on to the first node’s public interface. The SCAN/VIP combination will failover to another node in the cluster, if the current node fails. Besides performing failover, SCAN also load balances across the instances providing a service, by selecting and routing requests to the least loaded node.

To evaluate the load balancing ability of the SCAN listeners in a cluster and promptly detect inconsistencies, administrators should continuously monitor the connection traffic to each node in the cluster and quickly identify the over-utilized / under-utilized nodes. This can be achieved using the Oracle RAC Scan Listener Connection Traffic test!  This test auto-discovers the nodes in an RAC cluster and reports the rate at which each node successfully services connection requests. In the process, the test pinpoints the following:

  • Nodes handling more connections per second than the rest, and;

  • Nodes handling very few connections when compared to the other nodes

Such anomalies indicate that the SCAN listeners are not performing load balancing efficiently.

Target of the test : Oracle RAC

Agent deploying the test : An internal agent

Outputs of the test : One set of results for every node in the cluster

Configurable parameters for the test
  1. TEST PERIOD - How often should the test be executed.
  2. Host – The host for which the test is to be configured.
  3. Port - The port on which the server is listening.
  4. orasid - The variable name of the oracle instance.
  5. service name - A ServiceName exists for the entire Oracle RAC system. When clients connect to an Oracle cluster using the ServiceName, then the cluster routes the request to any available database instance in the cluster. By default, the service name is set to none. In this case, the test connects to the cluster using the orasid and pulls out the metrics from that database instance which corresponds to that orasid. If a valid service name is specified instead, then, the test will connect to the cluster using that service name, and will be able to pull out metrics from any available database instance in the cluster.

    To know the ServiceName of a cluster, execute the following query on any node in the target cluster:

    select name, value from v$parameter where name =’service_names’

  6. UserName – In order to monitor an Oracle database server, a special database user account has to be created in every Oracle database instance that requires monitoring. A Click here hyperlink is available in the test configuration page, using which a new oracle database user can be created. Alternatively, you can manually create the special database user. When doing so, ensure that this user is vested with the select_catalog_role and create session privileges.

    The sample script we recommend for user creation (in Oracle database server versions before 12c) for eG monitoring is:

    create user oraeg identified by oraeg ;

    create role oratest;

    grant create session to oratest;

    grant select_catalog_role to oratest;

    grant oratest to oraeg;

    The sample script we recommend for user creation (in Oracle database server 12c) for eG monitoring is:

    alter session set container=<Oracle_service_name>;

    create user <user_name>identified by <user_password> container=current default tablespace <name_of_default_tablespace> temporary tablespace <name_of_temporary_tablespace>;

    Grant create session to <user_name>;                                 

    Grant select_catalog_role to <user_name>;

    The name of this user has to be specified here.

  7. Password – Password of the specified database user
  8. Confirm password – Confirm the password by retyping it here.
Measurements made by the test
Measurement Description Measurement Unit Interpretation

Successful connections

Indicates the rate at which this node services connection requests.


Compare the value of this measure across nodes to identify overloaded and poorly loaded nodes in the cluster. The presence of such nodes indicate that the SCAN listeners are not balancing the connection load on the cluster properly. This warrants an investigation.