Add System overview

Author: alexmojaki

README.md

@@ -83,3 +83,11 @@ To explore the course more freely:
 At the beginning of the course only the shell is available to encourage quick exploration. After a few pages an editor is introduced to allow running full programs.
 
 The course provides three debuggers to specially run code: snoop, PythonTutor, and birdseye. Each should only become available starting from a specific page which introduces that tool. No such page has been written yet for birdseye, so for now it's immediately available when the editor is introduced.
+
+## System overview
+
+The UI is written in React. It communicates with the web server using the `rpc` function, e.g. `rpc("run_code", {code, source}, onSuccess)`. This eventually reaches a method in the `API` class, e.g. `def run_code(self, code, source):`.
+
+Running code specifically sends a request from the web server to the workers master server. This forwards the request to a process associated with that user's ID, starting a new process if necessary. Every user has their own process, which holds the state of the shell or the currently running program (which may be awaiting `input()`). The processes are isolated from each other and everything else, they can easily be terminated, and they have limitations on CPU time usage and file access.
+
+After the code finishes running, it checks the `Page` and `Step` that the user is currently on, and calls the `Step.check` method. In most cases this is a `VerbatimStep` - the user is supposed to enter exactly the code in the text, using the AST to check for equality. Next most common is an `ExerciseStep` where a function has to pass tests and produce the same output as a given solution. The result of `Step.check` determines if the user succeeded and advances to the next step. It may also return a message to show the user, e.g. if they made a common mistake.