Tuesday, October 10, 2017

Oracle Flex Cluster

Oracle Real Application Cluster is a feature that was first introduced in Oracle 9i (Oracle Parallel Servers was a similar feature available before 9i, but not as robust as RAC) and has been an essential part of Oracle’s Maximum Availability Architecture. Since its advent in Oracle 9i, it has evolved a lot. Oracle 9i onward, every new release of Oracle has introduced something new in RAC which made it even more resilient, fault tolerant, and easy to manage architecture.
    
Starting 12c, Oracle has introduced two very interesting features to be added in MAA (Maximum Availability Architecture), Oracle Flex Cluster and Oracle Flex ASM. In this article I will be explaining Oracle flex cluster and its architecture.

In Oracle Flax Cluster, the concept of Hub and Leaf node is introduced. Previously when we configured a RAC, all nodes were Hub nodes technically, but they were not called Hub Nodes until the phenomena of Leaf Nodes introduced in 12c. All hub nodes share the storage, and directly access the OCR and Vote disks from the shared storage just like conventional RAC. Leaf nodes are the nodes that sit behind the Hub nodes. When we do a fresh RAC installation or if we add new nodes to an existing cluster later down the road, we can specify a node as Hub or Leaf node (each Leaf Node sits behind a specific Hub node). Hub nodes are tightly coupled, whereas leaf nodes are loosely coupled and access the shared resources via a Hub node behind which they sit.

Example of a leaf node is to add an application server in the cluster by installing Grid Infrastructure on it and defining it as a leaf node. Technically this application server would be part of the cluster and would be treated as such, but it would not have direct access to OCR, Vote disk or shared storage, and it would access all these resource through its Hub node; leaf nodes sitting behind different Hub Nodes are not directly connected to each other; hub nodes are directly connected to each other through private interconnect.

Following picture explains the Flex Cluster architecture. This is quite clear to understand how leaf nodes connect to the RAC. The resources shown inside the oval are tightly coupled and form a conventional RAC. Adding leaf nodes behind the hub nodes makes it an example of Flex cluster



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