I'm a Scala software developer interested in knowing more about the theory behind Distributed Systems. One of the things that I want to do this year is to learn the topic by sharing what I learned in form of a course.

I have the following material to start:

- An introduction to distributed systems by Kyle Kingsbury (https://github.com/aphyr/distsys-class/blob/master/README.markdown)

- Distributed Systems for fun and profit by Mikito Takada (http://book.mixu.net/distsys/)

- Designing Data intensive Applications by Martin Klepmann (https://dataintensive.net/)

I have references of papers authored by Leslie Lamport and some of his theory (CAP Theorem, Clocks).

My questions are: which topics are the "essentials" for an introductory course? And what other materials could you recommend?

Thanks!