There are currently four durable function types in Azure Functions, Activity, Orchestrator, Entity, and Client. But we'll start by looking at just the first two, Activity and Orchestrator. 

Activity Functions are pretty much basic Azure Functions written more or less as usual with the one important difference being the requirement of an ActivityTrigger attribute that indicates that this function can be triggered from your Orchestrator function.

Your Orchestrator orchestrates the various Activity Functions by executing them as you specify. The Orchestrator function type requires an OrchestrationTrigger as we'll soon see.

When you start your Durable function, it creates the necessary control queues for each of the Activity Functions that it finds, and one worker item queue. Take note that although your Orchestrator looks like it is directly calling the function, under the covers it is instead actually sending a message to that work-item queue.

Your Activity Functions will receive a message from the queue through implementation of your ActivityTrigger and then execute any of its internal logic. When the Activity Function completes its execution, the function sends a response message to the control queue. The Orchestrator function then receives that response message via its OrchestrationTrigger, and the full behavior of the Durable Function is complete.

So an example might be you have a total of order items that you need to pass to another function to get taxes, or you have area parameters for something that then you need to calculate the total volume. Basically, any situation in which you have a value that you would like to pass on to another separate function to return a dependent value would be a use case for Durable Functions orchestrating your Azure Functions.

So let's take a look at how this works in code by jumping back into our Visual Studio project.