Understanding the Differences Between Oracle RAC SCAN Listener and LOCAL_LISTENER

Oracle Real Application Clusters (RAC) is a powerful technology that allows multiple instances of an Oracle database to run on different servers, providing high availability and scalability. When managing Oracle RAC environments, understanding how clients connect to the database is crucial. Two important components in this connection process are the SCAN Listener and the LOCAL_LISTENER. These terms often confuse database administrators and developers alike, but knowing their differences and roles can improve RAC configuration and troubleshooting.

This article explains what the SCAN Listener and LOCAL_LISTENER are, how they work, and when to use each. It also covers practical examples and best practices to help you manage Oracle RAC connections effectively.

Oracle RAC Scan listener and local listener

What is the SCAN Listener in Oracle RAC?

The Single Client Access Name (SCAN) Listener is a key feature introduced in Oracle RAC 11g Release 2 to simplify client connection management. SCAN provides a single network name for clients to connect to the RAC database, regardless of how many nodes the cluster has.

How SCAN Listener Works

• Single Network Name: SCAN is a DNS name that resolves to multiple IP addresses, typically three. These IPs correspond to SCAN listeners running on different nodes in the cluster.

• Load Balancing: When a client connects using the SCAN name, the DNS returns one of the SCAN IP addresses, distributing connections evenly across the cluster.

• Failover: If a SCAN listener or node fails, the DNS automatically resolves to other available SCAN IPs, ensuring continuous client access.

• Simplified Client Configuration: Clients only need to know one name (the SCAN name) instead of individual node names or IPs.

  
  

Benefits of SCAN Listener

• Reduced Client Configuration Complexity: No need to update client connection strings when nodes are added or removed.

• Improved Load Distribution: SCAN listeners balance incoming connections across nodes.

• High Availability: Automatic failover without client-side changes.

  
  

What is LOCAL_LISTENER in Oracle RAC?

LOCAL_LISTENER is a database initialization parameter that specifies the address of the listener local to the database instance. Each RAC node runs its own listener process, and LOCAL_LISTENER tells the database how to register itself with that listener.

How LOCAL_LISTENER Works

• Instance Registration: Oracle instances register their services with the listener specified by LOCAL_LISTENER.

• Node-Specific: Each RAC node has its own local listener, usually listening on a unique port or IP.

• Parameter Configuration: LOCAL_LISTENER can be set to a listener name or a full network address (TNS alias or EZCONNECT string).

  
  

Why LOCAL_LISTENER Matters

• Service Registration: Ensures that the database instance is reachable through the local listener.

• Load Balancing and Failover: Helps the listener know which services are available on which node.

• Custom Listener Setup: Useful when listeners run on non-default ports or when multiple listeners exist on a node.

  
  
  
  

Key Differences Between SCAN Listener and LOCAL_LISTENER

| Aspect | SCAN Listener | LOCAL_LISTENER |

|----------------------|---------------------------------------------------|------------------------------------------------|

| Purpose | Provides a single network name for client access | Specifies the local listener for instance registration |

| Scope | Cluster-wide | Node-specific |

| Client Connection | Used by clients to connect to the RAC database | Used internally by the database for service registration |

| Number of Listeners | Typically 3 SCAN listeners across the cluster | One listener per node |

| Configuration Impact | Simplifies client connection strings | Controls how instances register with listeners |

| Failover Handling | DNS-based failover and load balancing | Listener-level service registration and failover |

How SCAN Listener and LOCAL_LISTENER Work Together

In an Oracle RAC environment, SCAN and LOCAL_LISTENER complement each other:

• Clients connect using the SCAN name, which directs them to one of the SCAN listeners.

• Each RAC instance registers its services with the local listener specified by LOCAL_LISTENER.

• The local listener forwards client connections to the appropriate database instance.

• SCAN listeners provide a stable, cluster-wide entry point, while local listeners manage node-specific service registrations.

  
  

This layered approach improves connection management, balancing, and failover capabilities.

Configuring SCAN Listener and LOCAL_LISTENER

Configuring SCAN Listener

1. DNS Setup: Create a DNS entry for the SCAN name that resolves to three IP addresses.

2. Listener Configuration: On each node, configure the SCAN listener to listen on the SCAN IP addresses.

3. Start SCAN Listeners: Ensure SCAN listeners are running on the nodes.

4. Client Connection Strings: Use the SCAN name in client TNS entries or EZCONNECT strings.

   
   

Example TNS entry using SCAN:

```plaintext

MYDB =

(DESCRIPTION =

(ADDRESS = (PROTOCOL = TCP)(HOST = mydb-scan.example.com)(PORT = 1521))

(CONNECT_DATA =

(SERVICE_NAME = mydb_service)

)

)

```

Configuring LOCAL_LISTENER

1. Identify Local Listener: Determine the listener name or address on each node.

2. Set LOCAL_LISTENER Parameter: Use `ALTER SYSTEM SET LOCAL_LISTENER` to specify the listener.

   
   

Example:

```sql

ALTER SYSTEM SET LOCAL_LISTENER='(ADDRESS=(PROTOCOL=TCP)(HOST=node1.example.com)(PORT=1521))' SCOPE=BOTH;

```

1. Register Services: Force instance registration with the listener using:

   
   

```sql

ALTER SYSTEM REGISTER;

```

1. Verify Registration: Check listener status with `lsnrctl status` and database views like `V$ACTIVE_SERVICES`.

   
   

Oracle RAC cluster infrastructure with SCAN and LOCAL_LISTENER setup

Practical Examples and Use Cases

Example 1: Client Connection Using SCAN

A client application connects to the RAC database using the SCAN name `rac-scan.example.com`. The DNS resolves this to one of three SCAN IPs, distributing the load. The client does not need to know the individual node names or IPs, simplifying connection management.

Example 2: LOCAL_LISTENER with Custom Port

Suppose a RAC node runs its listener on port 1522 instead of the default 1521. The LOCAL_LISTENER parameter must be set to reflect this:

```sql

ALTER SYSTEM SET LOCAL_LISTENER='(ADDRESS=(PROTOCOL=TCP)(HOST=node2.example.com)(PORT=1522))' SCOPE=BOTH;

ALTER SYSTEM REGISTER;

```

This ensures the instance registers correctly with the local listener, allowing clients to connect through SCAN or directly if needed.

Example 3: Troubleshooting Connection Issues

If clients cannot connect using the SCAN name, check:

• DNS resolution of the SCAN name.

• Status of SCAN listeners on all nodes.

• LOCAL_LISTENER settings on each instance to ensure proper registration.

• Listener logs for errors.

  
  
  
  

Best Practices for Managing SCAN Listener and LOCAL_LISTENER

• Always use SCAN for client connections to simplify configuration and improve availability.

• Set LOCAL_LISTENER explicitly if listeners run on non-default ports or multiple listeners exist.

• Regularly verify listener status and service registration using `lsnrctl` and database views.

• Monitor DNS entries for SCAN to ensure they resolve correctly to all SCAN IPs.

• Document listener configurations for each node to avoid confusion during maintenance.

  
  

Understanding the roles and differences between Oracle RAC SCAN Listener and LOCAL_LISTENER helps maintain a stable and efficient RAC environment. SCAN simplifies client access by providing a single network name, while LOCAL_LISTENER ensures each instance registers correctly with its local listener. Together, they support load balancing, failover, and seamless client connectivity.