Real-Time Upserts at High QPS (EXPLAINED)
Write buffering, mutable segments, async merge, backpressure, and consistency windows for hot ingestion paths.
TL;DR — Quick Answer
Buffer writes in a WAL or message queue, batch segment flushes, serve queries from merged index plus recent mutable segment, and apply backpressure when merge lag exceeds SLO. Tune concurrent merge workers, avoid synchronous full HNSW rebuild per upsert, and expose time-to-searchable metrics to product teams.
The Interview Question
How do you support real-time vector upserts at high QPS without destroying query latency?
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