Fundamentals of a Messaging System

When talking about messages and messaging systems, there are four fundamental elements to take into consideration:

• The direction of the communication, which can be one-way only or a request/reply exchange

• The purpose of the message, which also determines its content

• The timing of the message, which can be sent and received in-context (synchronously) or out-of-context (asynchronously)

• The delivery of the message, which can happen directly or via a broker

One-way vs. request/reply patterns

The most fundamental aspect of a messaging system is the direction of the communication, which often also determines its semantics.

The simplest communication pattern is when the message is pushed one way from a source to a destination; this is a trivial situation, and it doesn’t need much explanation:

A typical example of one-way communication is an email or a web server that sends a message to a connected browser using WebSockets, or a system that distributes tasks to a set of workers.

On the other side, we have the Request/Reply exchange pattern, where the message in one direction is always matched (excluding error conditions) by a message in the opposite ...