You've covered a lot of ground in this chapter, looking at not only user authentication in Express applications but also microservice development.
Specifically, you covered session management in Express, using Passport for user authentication, including Twitter/OAuth2, using router middleware to limit access, creating a REST service with Restify, and when to create a microservice.
In the next chapter, we'll take the Notes application to a new level—semi-real-time communication between application users. To do this, we'll write some browser-side JavaScript and explore how the Socket.io package can let us send messages between users.