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Active Goal

You can design fault tolerance groups with any number of active members. That number is called the active goal.

When a member joins a group, it specifies the active goal as a parameter. Rendezvous fault tolerance software maintains that goal, regulating which members are active by issuing instructions to activate and deactivate. When too few members are active, it instructs inactive members to activate; when too many members are active, it instructs active members to deactivate.

Every member of a group must specify the same value for the active goal parameter. It is an error for members of the same group to specify different values for this parameter. When Rendezvous fault tolerance software detects values that do not agree, it delivers an error advisory message to each member of the group (see PARAM_MISMATCH on page 297).

Example: One Active Member

Broadcast producer programs generally function properly with only one member active at any moment in time; all other members are inactive backup processes. Example Fault-Tolerant Multicast Producer illustrates this kind of program. If the number of active members drops below one, then no information flows to consumers. With more than one active member, duplicate messages flood the network.

As long as one or more member processes exist, Rendezvous fault tolerance software maintains a single active member.

Example: Several Active Members

Consider a data-mirroring program that stores data in several different locations; each process instance of the program stores the data on its host computer. It does not matter which of many possible locations store the copies, as long as the network always has at least 5 complete copies. Such a program satisfies its fault tolerance requirement by setting the active goal to 5.

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