Most mobile apps rely on data that comes from an external source. Think of apps for reading books, watching movies, sharing pictures with your friends, reading the news, or writing emails: all these apps use data taken from an external source. When an app consumes external data, usually, there is a backend service that provides that data for the app: a web service or web API.
What happens is that your app (frontend or client) connects to a web service over HTTP and requests some data. The backend service then responds by sending the data to the app, usually in .json or .xml format.
For this app, we will create an app that reads and writes data from a web service. As creating a web API is well beyond the scope of this book, we will use a mock service, called MockLab, that will simulate the behavior of a real web service, but will be extremely easy to set up and use. In a later chapter, you will also see another way of creating a real-world backend...
