File server modules can be used to present resources on an external endpoint as if they were files sitting on the Master. The default file server module, called roots, does in fact use local files on the Master. Many file server modules cache files locally on the Master, to avoid making too many calls to the external source, but this is not always appropriate.
There are a number of functions inside a file server module, which work in concert to present a file-server-like interface. Some of these functions cannot be tested directly, but they can still be tested in tandem with other functions that do have a direct external interface.
Despite all of the functions involved, file server modules are relatively easy to write. In the next chapter, we'll talk about cloud modules, which have even more required functions, but which are even easier to write.