The Group Membership Services and Atomic Broadcast protocol (GAB) is responsible for the following cluster membership and cluster communications functions:
GAB maintains cluster membership by receiving input on the status of the heartbeat from each node by LLT. When a system no longer receives heartbeats from a peer, it marks the peer as DOWN and excludes the peer from the cluster. In VCS, memberships are sets of systems participating in the cluster.
VCS has the following types of membership:
A regular membership includes systems that communicate with each other across more than one network channel.
A jeopardy membership includes systems that have only one private communication link.
A visible membership includes systems that have GAB running but the GAB client is no longer registered with GAB.
GAB's second function is reliable cluster communications. GAB provides guaranteed delivery of point-to-point and broadcast messages to all nodes. The VCS engine uses a private IOCTL (provided by GAB) to tell GAB that it is alive.