VCS

Service Group: It is a logical grouping of resources and resource dependencies.

Types of Service Groups:

01. Failover Service Groups
02. Parallel Service Groups
03. Hybrid Service Groups
04. ClusterService Group

The ClusterService group is a special purpose service group, which contains resources required by VCS components. The group contains resources for Cluster Manager (Web Console), Notification, and the wide-area connector (WAC) process used in global clusters.

The ClusterService group can fail over to any node despite restrictions such as “frozen.” It is the first service group to come online and cannot be autodisabled. The group comes online on the first node that goes in the running state. The VCS engine discourages taking the group offline manually.

High-Availability Daemon (HAD)
The high-availability daemon, or HAD, is the main VCS daemon running on each system. It is responsible for building the running cluster configuration from the configuration files, distributing the information when new nodes join the cluster, responding to operator input, and taking corrective action when something fails. It is typically known as the VCS engine. The engine uses agents to monitor and manage resources. HAD operates as a replicated state machine (RSM). This means HAD running on each node has a completely synchronized view of the resource status on each node

Group Membership Services/Atomic Broadcast (GAB)
The Group Membership Services/Atomic Broadcast protocol (GAB) is responsible for cluster membership and cluster communications.