The Group Membership Services and Atomic Broadcast protocol (GAB) is responsible for the following cluster membership and cluster communications functions:

• Cluster Membership

  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.

• Cluster Communications

  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.