Message Passing — Indirect Communication

Messages are directed and received through mailboxes. Each mailbox has a unique ID. Processes can only communicate if they share a mailbox.

The mailboxes here form the communication link, and can be shared between multiple processes. Each pair of processes may share several communication links. This link may be either uni-directional or bi-directional.

The OS provides operations to:

• Create mailboxes (ports).
• $send(A, m)$ — Send $m$ to mailbox $A$.
• $receive(A, m)$ — Receive $m$ from mailbox $A$.
• Delete mailboxes.

The main issue with a shared link such as this is determining which process receives a messsage. There are three main solutions to this:

• Allowing a link to be associated with only two processes.
• Allow one process at a time to receive.
• Arbitrarily select receiver, sender is then notified of who received their message.