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Donald K. Burleson

Oracle RAC Tips

Failover Database Clusters

As we noted earlier, many enterprises use failover (FO) database clusters to meet their high availability requirement. With this method, if a primary node fails, the database instance running on the failed node fails over to a backup node. In other words, the instance is restarted on the surviving node.

The behavior of a typical parallel database cluster such as Oracle RAC is much different. An Oracle RAC instance is not failed over, but causes the reconfiguration of resources from the failed instance to the non-failed instance(s) to take place.

In this section, we will explore the functionality of the failover database cluster. We’ll also look at cluster resources, resource groups, and failover mechanisms.

Resources, Resource Types

Resources are hardware or software entities, such as disks, network interface cards (NIC), IP addresses, applications, and database instances controlled by cluster software. Controlling a resource means bringing it online (starting) and taking it offline (stopping), as well as monitoring the health or status of the resource.

The cluster software controls the status (starting and stopping activities) through a script or an agent, depending on the type of resource. For example, mounting involves starting a file system resource. Starting an IP resource involves configuration of the IP address on a network interface card. Monitoring a resource means testing it to determine if it is online or offline. How cluster software monitors a resource is also specific to the resource type. For example, a file system resource tests as online if mounted, and an IP address tests as online if configured.

Resource Groups

A resource group is a set of resources working together to provide application services to clients. A resource group is sometimes called a ‘service group’ or a ‘package’; different cluster vendors refer to them with different names.

For example, as shown in Fig 3.8, a database resource group might consist of:

  • Disk groups on which the physical storage data volumes are located.
  • Volumes built in the disk group storage.
  • A file system using the volume.
  • Network interface card or cards used.
  • One or more IP addresses associated with the network card(s).
  • A Listener Process.
  • A Database Instance.

A cluster software script or agent executes operations on resources, including starting, stopping, restarting, and monitoring at the resource group level. Resource group operations initiate administrative operations for all resources within the group. For example, when a resource group is brought online, all the resources within that group are brought online. When failover occurs, resources never failover individually, rather the entire resource group fails.

Thus, the resource is the unit of failover. If there is more than one group defined on a server, one group may failover without affecting the other group(s) on the server.

Fig 3.8 Cluster Resource Group

From a cluster standpoint, there are two significant aspects to this view of an application resource group as a collection of resources:

  • If a resource group is to run on a particular server, all of the resources it requires must be available to the server.
  • The resources comprising a resource group have interdependencies; that is, some resources (e.g., volumes) must be operational before other resources (e.g., the file system) can be made operational.

For more information, see the book Oracle 11g Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

Written by top Oracle experts, this RAC book has a complete online code depot with ready to use RAC scripts.  



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