Get on the Bus— The Azure Service Bus

Azure Service Bus is Microsoft's cloud offering for developing message-based applications. In this article, I'll explore some fundamental Azure Service Bus techniques like how to configure queues, how to send and receive messages, and how to make sure you're properly handing exceptions and poison messages. But first, let's take a moment to think about why you'd want to build a message-based application in the first place.

- Matthew Hess

What's a Message-Based Architecture For?

The purpose of a message-based architecture is to decouple the components of a system. What does that really mean? To see what decoupling is all about, let's first look at the opposite: a synchronous, tightly coupled system.

A Tightly Coupled System

Imagine you're feeling hungry. You walk over to a nearby vending machine, put in some money, and push a button. Voila! The vending machine gives you your snack. This is a traditional, tightly coupled call sequence, as shown in Figure 1.

If you think of this in terms of software, perhaps the vending machine is a web service that exposes an “OrderSnack” endpoint. Your customer interaction with the web service is tightly coupled and synchronous. You POST a request message to the service and get back a response, your snack (or maybe a “400: Snack not found” error).

A Loosely Coupled, Message-Based System

Compare this to the following sequence: now you are really hungry, so you walk into your favorite cafe, go to the counter, and order a full breakfast. Back in the kitchen, someone prepares your meal and then someone brings it out to you. In this sequence, the process of placing an order and preparing the order are decoupled and are connected by a message, as shown in Figure 2.

The key feature of this diagram is the presence of the Order Queue. This queue decouples the Counter and the Kitchen. The Counter doesn’t cook your food or even interact with the Kitchen directly. Instead, the Counter enqueues the order and later a cook in the Kitchen dequeues it and starts preparing it. (In a real kitchen, there's likely a second “delivery queue,” i.e., a shelf with heat lamps and a bell, to signal that there is food ready to be brought out).