What is a Resource in Oracle RAC?

 

In Oracle Real Application Clusters (RAC), resources are components or entities that are managed by Oracle Clusterware to ensure high availability and proper coordination across the nodes in the cluster. Let's address your query step by step.

1. What is a Resource in Oracle RAC?

A resource in Oracle RAC refers to any service, database instance, application, or other entity that is managed by Oracle Clusterware. 

Clusterware monitors and manages these resources to ensure they are available and functioning as intended. If a resource fails, Clusterware can restart it on the same node or relocate it to another node to maintain availability.

2. Why is it Called a Resource?

The term "resource" is used because these are critical assets or components required for the proper functioning of the RAC environment. They represent managed entities within the cluster that:

• Provide specific services or functionalities.
• Are finite and require monitoring and allocation.

For instance:

• A database instance is a resource because it provides access to the Oracle database.
• A VIP (Virtual IP) is a resource because it facilitates client connectivity.
• A listener is a resource because it handles client connection requests.

Since these components are managed by Clusterware for availability, stability, and scalability, they are collectively termed as "resources."

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3. List of Resources in Oracle RAC

Below are some common resources managed in an Oracle RAC environment:

Database Resources:

1. Database Instances - The actual database instances running on each node (e.g., ora.<DB_NAME>.db).
2. Cluster Services - Services defined in the database to handle client connections.
3. Redo Threads - Associated with redo log management in RAC.

Network Resources:

4. VIPs (Virtual IPs) - Each node has a VIP for client failover in case of node failure (e.g., ora.<hostname>.vip).
5. SCAN (Single Client Access Name) - A cluster-wide name for database access.
6. SCAN Listener - Listeners for the SCAN VIPs.

Infrastructure Resources:

7. Oracle Clusterware Daemons - Processes like CRSD, OCSSD, etc., which form the backbone of Clusterware.
8. OCR (Oracle Cluster Registry) - Stores cluster configuration information.
9. Voting Disk - Manages cluster membership.

Application Resources:

10. User-Defined Applications - Applications that you configure to run on the cluster.
11. Custom Scripts - Scripts that perform specific functions for your environment.

Other Resources:

12. ASM Instances - Automatic Storage Management instances (e.g., ora.asm).
13. Disk Groups - Managed under ASM.
14. ACFS (Oracle Cluster File System) - File systems for storing data.
15. GIMR (Grid Infrastructure Management Repository) - For monitoring and diagnostic purposes.

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4. How Oracle Manages Resources?

Oracle Clusterware uses a resource management framework to:

• Define the attributes of each resource (e.g., start, stop, check actions, dependencies).
• Monitor resource status (e.g., using crsctl or srvctl commands).
• Automate failover and failback for resources.
• Maintain dependencies (e.g., a listener resource depends on the VIP resource).

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5. Commands to View Resources

You can use the following commands to view resources in your Oracle RAC environment:

• To List All Resources:

  crsctl stat res -t

  This displays all resources and their current status in a tree format.

• To Get Details of a Specific Resource:

  crsctl stat res <resource_name> -p

• To List Only Database Resources:

  srvctl config database -d <db_name>

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Summary:

A resource in Oracle RAC is any entity managed by Oracle Clusterware for high availability. The term "resource" highlights its role as an essential and finite entity that Clusterware monitors and allocates. Examples include database instances, VIPs, SCAN listeners, ASM, and user-defined applications. These resources are crucial for the operation of an Oracle RAC environment, ensuring scalability and fault tolerance.