ANN index updates and deletes
Design the operational lifecycle of an ANN (approximate nearest neighbor) index for a semantic-retrieval service holding 500M item embeddings, serving 20k QPS at p99 < 30ms, where ~5M items are added/changed and ~2M removed every day. The hard part isn't the query — HNSW handles that — it's keeping the index correct and fresh: HNSW graphs don't support efficient deletes, full rebuilds take hours, and a stale index silently returns retired/deleted items. Design the update and rebuild strategy.
What a strong answer looks like
Clarify scale and constraints first. Propose a clean component breakdown, then go deep on the hard parts (data model, bottlenecks, consistency, failure modes) and name the trade-offs you are making.
Clarify5:30 left
Estimate5:30 planned
Design16:30 planned
Deep dive13:30 planned
Failure9:00 planned
Which questions mattered is sealed until you submit. Telling you now would just be handing over the edge cases.
Run or narrate your approach, then ask the coach.