This layer consists of the actual server-side code we are going to write in order to get the application running. It is also called middleware. In addition to being in the exact center of the architecture diagram, this layer also acts as the controller and middle-man for the other layers.

The HTML pages that form the UI are served through this layer. All the database queries that we were talking about earlier are made here. The code that runs in this layer is responsible for retrieving the sensor readings from our external pins and storing the data in our database. As you will see in later chapters, the middleware can also be further broken down into individual components, each with its own function.