VCS uses the cluster interconnect for network communications between cluster systems. Each system runs as an independent unit and shares information at the cluster level. On each system the VCS High Availability Daemon (HAD), which has the decision logic for the cluster, maintains a view of the cluster configuration. This daemon operates as a replicated state machine, which means all systems in the cluster have a synchronized state of the cluster configuration. This is accomplished by the following:
All systems run an identical version of HAD.
HAD on each system maintains the state of its own resources, and sends all cluster information about the local system to all other machines in the cluster.
HAD on each system receives information from the other cluster systems to update its own view of the cluster.
Each system follows the same code path for actions on the cluster.
The replicated state machine communicates over a purpose-built communications package consisting of two components, Group Membership Services/Atomic Broadcast (GAB) and Low Latency Transport (LLT).
Figure: Cluster communications with replicated state machine illustrates the overall communications paths between two systems of the replicated state machine model.