Okay that covers our introduction to Azure Functions. We got one running, we used bindings to interact with remote resources, we called functions from it, triggered functions from it and logged our activities. I spoke briefly earlier about the ILogger Dependency Injection that we were using for that.

Azure Functions 2.0 and later is much, much better at handling Dependency Injection than was version 1.0. And since it's pretty much required in a stateless environment that's a very good thing.

As I mentioned, if you're not familiar with Dependency Injection, that's okay. In this case the point of it is that Azure Functions as a runtime has some built-in tools such as Logging and Configuration that you can use for all of your Azure Function.

And you get access to those tools when you pass the right kind of object types in the Constructor of your Azure Function. Since you haven't created those objects yourself you know that Azure must be doing it for you in the background. Because those background objects are actually controlling much of what happens in your function, Dependency Injection of objects is sometimes called Inversion of Control.

The need for Dependency Injection brings us back to a point I mentioned earlier and again just now, the whole on-demand serverless model is by nature stateless. It's hard to hold state in something that doesn't exist most of the time such as is the case with your Azure Function.

But what happens when you do have several Azure Functions and you would like them to be aware of what the others have done before they execute in a smarter way than we just employed using our bindings and triggers? It's a common enough scenario and it's why Durable Functions were created.

Durable Functions is an extension of Azure Function that lets you orchestrate, or tie together in one way or another, stateful functions in a serverless environment. The orchestration is typically a sequence of logical processing. A Durable Function application is a solution you create that is composed of different Azure Functions, each one playing a different role within that orchestration.