Flex Cluster - 12c Grid Infrastructure and RAC New Features

With oracle 12c clusterware, "Flex clusters" are built to scale up to 2000 nodes that you can use Flex cluster to manage large pools of high availability and failover protection application resources with multiple databases and application running in one cluster.

• Hub Nodes:  The main component of a Flex Cluster is a group of Hub Nodes.  There is only one group of Hub nodes in a Flex cluster deployment, and each Hub Node must be connected to Shared storage across the group of Hub Nodes. 
• Leaf Nodes:  Zero or more Leaf Nodes can be connected to a Flex Cluster through a Hub Node. Leaf nodes are loosely coupled and they are associated with a single Hub Node and periodically hub nodes exchange heartbeat messages with their associated Leaf Nodes.  Failure of hub node or network results in leaf node eviction.  It does not require direct access to shared storage and on the same networks (public and private) as the Hub nodes.
• Only the Hub Nodes have direct access to the OCR and voting disks
• Hub-and-spoke topology is the key architecture feature that segments the cluster into groups of nodes.  Two fundamental impacts:  1)  limiting the size of the hub to reduce contention to OCR and voting disks and 2) less heartbeats network traffic exchange between the nodes
• Clients on Leaf Nodes use GNS (Grid Naming Service) to locate Hub Node services.  This requires access to GNS through a fixed VIP running on one of the nodes so that Leaf Node clients have reliable naming service within the cluster.
• You can disable or enable Flex cluster functionalities.  By the default, Flex cluster functionality is disabled.

To convert from a standard cluster to a Flex Cluster, you need to ensure that GNS is configured with a fixed VIP and then set the cluster cluster mode via "crsctl set cluster mode flex" .  To covert from a Flex cluster to a standard cluster "crsctl set cluster mode standard".

To show the current node role:
crsctl get node role status -node glsn02
Node glsn02 active role is 'hub

Administrator can explicitly specified the node role as hub or leaf:
crsctl set node role leaf -node glsn02

Administrator can also set the node role to auto.  This allows the cluster to decide which role a node will perform based on the composition of the cluster.  The auto role works with cluster hubsize setting.  If Hub Nodes is smaller than hubsize, the nodes joins the cluster as a Hub Node.  Else, it joins the cluster as a Leaf Node.

The leafmisscount setting defines the threshold (in seconds) duration for tolerable communication failure between a Hub Node and associated leaf Node, if more than the defined value, the leaf node is evicted from the cluster.  By default, leafmisscount setting is 30 seconds.

Flex cluster and Node Failure:

• Nodes that are evicted from the cluster do not require a restart; only a cluster software restart
• If a Hub Node fails:   1)  the node is evicted from the cluster (services on the hub node are relocated to another Hub nodes)  2) Leaf nodes can reconnect to other Hub nodes within the grace period otherwise they are evicted from the cluster.
• If a Leaf Node fails:  the node is evicted from the cluster, the cluster attempts to relocate services running on the leaf node to other leaf nodes connected to the same Hub node.