Mostly lack of sleep and having to sacrifice a lot of weekends to work on the assignments. I took OMSCS with the goal of learning as much as possible since I did not have a traditional CS degree, so I took all the hard classes in the Computing Systems track (DC, Compilers, ISL:BE, IHPC, etc).

CS7210 (DC) is the hardest class I took. We had to write a correct Paxos implementation, then use it to build a distributed sharded KV store. I remember having to spend 10+ hours on a single day during weekend. It was worth it though, learned a lot about distributed consensus, and how difficult it is to get right (there were test failures that were fixed after hours of debug, and the fix was literally changing the order of some code lol).

Specifically CS7210 is demanding because of its assignments. The assignments come from UW's CSE452, have very little direct connection with the course lectures, and require you to implement a Paxos-like system correctly, basically in one shot, in an environment that is very difficult to debug. So the projects turn into 60-80-hour slogs where students change parameters semi-randomly until something starts working. CS7210 shares that aspect with a number of other courses in the program.