Practice — SQL vs NoSQL & Data Modeling for Scale (6 questions)

You are designing a notifications service: 200 million users, each user accumulates up to a few thousand notifications (read/unread), the dominant query is "give me this user's latest 50 notifications", writes arrive from many producers (likes, comments, follows) at ~30,000/s peak, and there is no requirement for joins or ad-hoc analytical queries on this data.

1. Which storage model best fits this workload, and why?
2. Design the partition key and sort key (or equivalent) for the dominant query.
3. What is the one requirement that, if added later, would force you to reconsider the choice?

A document store models "a user's display name" by denormalizing it directly into every post and comment document that user has ever made, so reads never need a join — a feed query just reads posts, no lookup required. User alice_99 has posted and commented 40,000 times over 3 years. She renames herself once.

What does that single rename cost, write-wise, in this design?