Oracle RAC Scan Listener Status 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.

Without SCAN, till Oracle Clusterware 11g release 1, every client and server that participated in the RAC environment needed to have a tnsnames.ora file, which contains a single entry for every node in the cluster. This meant that, whenever the cluster configuration changes - say, a node is added/removed from the cluster - the tnsnames.ora file of every client had to be updated with this change. SCAN on the other hand, completely eliminates the need for these edits! In short, with SCAN, clients can access any database in the cluster, independent of the number of databases or servers running in the cluster, or the server(s) on which the requested database is actually active.

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 root.sh script automatically configures the SCAN VIPs and SCAN listeners. When the root.sh 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. Likewise, SCAN also load balances across the instances providing a service, by selecting and routing requests to the least loaded node. Also, if a single SCAN listener IP stops running on a node, connection requests will automatically failover to another SCAN listener IP address on that node.

This means that, if all SCAN listeners in a cluster are down or are not enabled, then users will not be able to connect to the database service. To avoid this, administrators must track the status of each SCAN listener in an RAC cluster, promptly identify the listener that is down, and quickly start it. This can be achieved using the Oracle RAC SCAN Listener Status test! This test auto-discovers the SCAN listeners in a target cluster, and reports the status of each listener. In the process, the test alerts administrators to listeners that are not enabled or are not running. Detailed diagnostics point administrators to the exact node on which the failure has occurred, thereby easing troubleshooting.

Target of the test : Oracle RAC

Agent deploying the test : An internal agent

Outputs of the test : One set of results for every SCAN listener.

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. Detailed diagnosis - To make diagnosis more efficient and accurate, the eG Enterprise embeds an optional detailed diagnostic capability. With this capability, the eG agents can be configured to run detailed, more elaborate tests as and when specific problems are detected. To enable the detailed diagnosis capability of this test for a particular server, choose the On option. To disable the capability, click on the Off option.

    The option to selectively enable/disable the detailed diagnosis capability will be available only if the following conditions are fulfilled:

    • The eG manager license should allow the detailed diagnosis capability
    • Both the normal and abnormal frequencies configured for the detailed diagnosis measures should not be 0.
Measurements made by the test
Measurement Description Measurement Unit Interpretation

Is listener enabled?

Indicates whether this SCAN listener is enabled or not.

 

The values that is measure reports and their corresponding numeric values are listed in the table below:

Measure Value Numeric Value
Yes 1
No 0

Note:

This measure reports the Measure Values listed in the table above to indicate whether/not a listener is enabled. In the graph of this measure however, the same is indicated using the numeric equivalents only.

If this measure reports the value No for a listener, then use the following command to enable that listener:

srvctl enable scan_listener [-i ordinal_number]

Example: srvctl enable scan_listener -i 2

Also, check whether the SCAN VIP is available in the corresponding node.

Is listener running?

Indicates whether/not this SCAN listener is currently running.

 

The values that is measure reports and their corresponding numeric values are listed in the table below:

Measure Value Numeric Value
Yes 1
No 0

Note:

This measure reports the Measure Values listed in the table above to indicate whether/not a listener is running. In the graph of this measure however, the same is indicated using the numeric equivalents only.

If this measure reports the value No for a listener, then use the detailed diagnosis of the measure to quickly identify the node on which the listener is not running.

Listener count

Indicates the number of listeners that are successfully running in the cluster.

Number

This measure is reported only for the Summary descriptor.