Messages

Means of communication

Messages are the means by which processes communicate with each other. They are the primary mechanism for exchanging information and coordinating actions in a distributed system.

Messages are sent between processes; consequently, messages are addressed to processes. One (successful) message exchange consists of a send event at the sending process and a receive event at the receiving process.

send-receive-event

The most fine granular model of message exchange between two processes consists of 4 events, a send event at the sending process, a receive event at the network followed by a send event at the network and a receive event at the receiving process. At this level of abstraction, message exchange between processes and the network is reliable, however, the network may reorder, drop, or duplicate messages

The more common model of message exchange between two processes consists of two events, a send event at the sending process and a receive event at the receiving process. At this level of abstraction, message exchange between processes is unreliable, messages may be reordered, dropped, or duplicated.

The only guarantee is that if a process experiences a receive event there is a corresponding send event at a sending process (which may be the same process)