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) are the length and depth target for every other design here. If a design in this chapter is shorter than those, it is not finished.

What this chapter covers

• [done] Design a distributed rate limiter
• [done] Design RAG with document-level access control
• [done] Design an LLM gateway
• [done] Design multilingual semantic search
• [todo] Agent platform with sandboxed tools and full replay
• [todo] Evaluation pipeline that gates CI
• [todo] Cost attribution for LLM spend across 40 teams
• [done] Design a news feed
• [done] Design a notification and fan-out system
• [done] Design a distributed job scheduler
• [done] Design a metrics pipeline
• [done] Design ad click aggregation
• [done] Design a payments ledger
• [done] Design ride-hailing dispatch
• [todo] Object storage and file sync
• [done] Design ticketing under contention
• [done] Design a multi-region active-active KV store
• [done] Design recommendation serving on 50 ms
• [todo] Inference batching for one GPU under a latency SLA

Source: §43, §44, §45.