Worked design answers

One file per design, each a full transcript rather than a diagram: clarify, capacity math, API contract, data model and partition key, architecture, the deep dive on the hardest component, failure modes and degradation, then tradeoffs and what changes at ten times the scale.

The two designs already fully worked in the source document (the distributed rate limiter and RAG with document-level access control) were the length and depth target for every other design here. All nineteen are now written, and each holds to that bar.

Read them in any order; they are cross-linked where one design's mechanism answers another's problem. If you are working through them systematically, the most transferable ideas are the capacity arithmetic in step 2 of each (which derives the architecture rather than decorating it) and the clarifying question each one opens with, since interviewers score the clarification round and most candidates skip it.

What this chapter covers

Source: §43, §44, §45.