Follower network crawler

The end game for this chapter is to build a crawler to explore GitHub's follower graph. We have already outlined how we can do this in a single-threaded manner earlier in this chapter. Let's design an actor system to do this concurrently.

The moving parts in the code are the data structures managing which users have been fetched or are being fetched. These need to be encapsulated in an actor to avoid race conditions arising from multiple actors trying to change them concurrently. We will therefore create a fetcher manager actor whose job is to keep track of which users have been fetched and which users we are going to fetch next.

The part of the code that is likely to be a bottleneck is querying the GitHub API. We therefore want to be able to scale the number of workers doing this concurrently. We will create a pool of fetchers, actors responsible for querying the API for the followers of a particular user. Finally, we will create an actor whose responsibility is to...