Communications

* Types of Communication
- Loosely coupled comm.
- Tightly coupled comm.

* Approaches
Shared Data Structure
- Resource sync needed.
Message Passing (Message Queue)
- Provide sync and comm together
- No owner and mutual exclusion implemented internally
- Value passing for small messages and reference passing for large messages
- Semaphore implementation: a mailbox of one valid message

Usage for Comm
- Loosely coupled one-way comm: ONE message queue
- Tightly coupled one-way comm: ONE message queue and ONE binary semaphore
- Tightly coupled two-way comm (Rendezvous): TWO message queues with one or multiple entries
Note: the task model here could be "one to one", "multiple to one" and "one to multiple", etc.