Group Membership Services/Atomic Broadcast (GAB)

GAB is responsible for Cluster Membership and Cluster Communications as described below.

Cluster Membership

All nodes in a distributed system, such as Storage Foundation for Oracle RAC, must be aware of each node currently participating in the cluster. Nodes can leave or join the cluster because of shutting down, starting up, rebooting, powering off, or faulting processes.

Storage Foundation for Oracle RAC uses LLT heartbeats to determine cluster membership. When systems no longer receive heartbeats from a peer for a predetermined interval, a protocol excludes the peer from the current membership. When systems start receiving heartbeats from a peer that is not part of the membership, a protocol enables the peer to join the current membership.

Storage Foundation for Oracle RAC delivers the new membership information consistently to all nodes and initiates actions specific to each module. For example, if a node faults, CVM initiates volume recovery and CFS performs a parallel file system check.

Cluster Communications

The second function of GAB is to provide reliable cluster communications for Storage Foundation for Oracle RAC modules. GAB provides guaranteed delivery of point-to-point messages and broadcast messages to all nodes. Point-to-point messaging involves sending and acknowledging the message. Atomic-broadcast messaging ensures all systems within the cluster receive all messages. If a failure occurs while transmitting a broadcast message, GAB ensures all systems have the same information after recovery.

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