Groups of Applications

Applications can use the group facility to coordinate fault-tolerant operation, or to distribute operating roles among application processes.

Introduction to Groups
A set of application processes can join a group. Each member of a group receives an ordinal, that is, a number representing its current position within the group. Application program logic uses the ordinal to select one of several possible operating roles.
Groups Principles of Operation
Techniques for Programming with Groups
Group Status
Group status advisories report the state of group members.
Group Observer
An application program can join a group as an observer. Observers can monitor the group by subscribing to group status advisories, just as members can. However, the group server does not assign ordinals to observers: observers do not participate in the functional roles of the group.
Programmer’s Checklist Addenda for Groups