There are quite a bit of details missing.
- No mention about Load Balancing
- What kind of cache to use? For example, should we use Redis or Memcached? which suits better in this UC based on the type of data structure we are going to use.
- Which Data Structure is better. Why there is no mention about graph use. In use cases like this will graph not be a better data structure?
- Why there is no mention of No-SQL DB to be used. The meta data is better stored in no-sql so that we can later use the meta data for other analytical processing. Analytical Processing is better performed on no-sql than a structured db.
Over all, this System Designing course is more for people who are just trying to learn concepts, like a Product Manager who is interested to get to know a 10,000 feet of the architecture.