# Design Streaming Speech and Multi-Model Chat Systems Redesign a batch speech-transcription pipeline to process audio incrementally and support multiple speakers. Then design an LLM chat service that streams responses, switches models when rate or token limits are reached, scales behind a load balancer, and stores conversation history safely. ### Constraints & Assumptions - Audio chunks can arrive late, be duplicated, or end mid-utterance. - Partial transcripts may be revised as more context arrives. - A model switch must not silently change the request's safety or tool permissions. - Load balancers cannot rely on one process retaining all stream state. ### Clarifying Questions to Ask - What latency and final-transcript accuracy are required? - Is speaker diarization based on separate channels or mixed audio? - Which provider limits are per request, model, tenant, or time window? ### Part 1 — Streaming transcription Define chunking, ordering, buffering, partial and final transcript states, speaker handling, backpressure, and performance measurement. #### What This Part Should Cover - Sequence IDs and bounded reordering - Revision-aware transcript segments - Diarization, VAD, latency, and throughput diagnostics ### Part 2 — Chat serving Design request routing, streaming through a load balancer, model fallback, rate-limit handling, and conversation persistence. #### What This Part Should Cover - Externalized stream and request state - Capability-aware routing and idempotency - Durable message lifecycle and token accounting ### What a Strong Answer Covers - Ordering and recovery semantics - Backpressure and cancellation - Consistent safety, authorization, and storage across model switches ```hint Make partial output a versioned state A partial transcript or model response is not an append-only final record. Give each segment or message a status and revision so clients can replace provisional text. ``` ### Follow-up Questions - How would you recover a stream after a worker dies? - When should conversation content be written to the database?