Subscribing to a group

When it is desirable for a process to monitor a group without playing a part in the control of the group's state, the GSAPI allows the process to subscribe to the group. A subscriber may subscribe to receive approved membership changes, approved state changes, or both.

Subscribers do not participate in any of the voting protocols. In fact, they are not notified that any such activity is taking place. If a state or membership change is not approved, no notification is sent to the subscribers.

Notifications to all subscribers to any single group are serialized, so all subscribers receive all notifications in the same order. However, it is not guaranteed that all subscribers will receive any one notification before any other subscribers receive subsequent notifications. No notifications or protocol proposals are made when subscribers join or leave a group.

Subscription allows one group to maintain a loose synchronization with one or more other groups. For example, a subscriber could be used to monitor and display information about the state of a number of groups and the members of those groups. As the subscribed-to groups change state or membership, the monitor can collect the changes and display or log the updated information.