Group communication

 A multicast operation is more appropriate – this is an operation that sends a single message from one process to each of the members of a group of processes, usually in such a way that the membership of the group is transparent to the sender. There is a range of possibilities in the desired behaviour of a multicast. The simplest multicast rotocol provides no guarantees about message delivery or ordering.

 Multicast messages provide a useful infrastructure for constructing distributed systems with the following characteristics:

 Fault tolerance based on replicated services: A replicated service consists of a group of servers. Client requests are multicast to all the members of the group, each of which performs an identical operation. Even when some of the members fail, clients can still be served.

 Discovering services in spontaneous networking: Section 1.3.2 defines service discovery in the context of spontaneous networking. Multicast messages can be used by servers and clients to locate available discovery services in order to register their interfaces or to look up the interfaces of other services in the distributed system.

 Better performance through replicated data: Data are replicated to increase the performance of a service – in some cases replicas of the data are placed in users’ computers. Each time the data changes, the new value is multicast to the processes managing the replicas.

 Propagation of event notifications: Multicast to a group may be used to notify processes when something happens. For example, in Facebook, when someone changes their status, all their friends receive notifications. Similarly, publishsubscribe protocols may make use of group multicast to disseminate events to subscribers.