Indirect Communication

Time and space uncoupling

Space coupling - Time-coupled: Properties: Communication directed towards a given receiver or receivers; receiver(s) must exist at that moment. Examples: Message passing, remote invocation
Space coupling - Time-uncoupled: Properties: Communication directed towards a given receiver or receivers; sender(s) and receiver(s) can have independent lifetimes. Examples: email
Space uncoupling - Time-coupled: Properties: The sender does not need to know the identity of the receiver(s); the receiver(s) must exist at that moment. Example: IP multicast
Space uncoupling - Time-uncoupled: Properties: The sender does not need to know the identity of the receiver(s); sender(s) and receiver(s) can have independent lifetimes. Examples: Most indirect communication paradigms covered in this chapter

FIFO order: All messages are received in the order sent.
Causal order: If a message m2 is sent as a consequence of a message m1 (i.e., a process has seen m1 and then sends m2), then all members should see m1 before m2.
Total order: All members will see messages in precisely the same order.

Causal ordering does not strictly imply FIFO; a process can send m1 and then m2 but has not yet seen its message m1.

Channel: events are published to a channel that processes can subscribe to.
Topic (Subject): an event is published given one or more topics(#foo). If topics are structured in a hierarchy, processes can choose to subscribe to a topic or a sub-topic.
Content: subscribers specify properties of the content, more general - harder to implement
Type: used by object-oriented languages; subscribe on an event of a particular class

The event routing depends on our subscription model and performance, fault tolerance, availability, and consistency requirements.