Problems / AI systems, medium

Model provider outage

Your chat product depends on one model provider. At 20 s it is down for 15 seconds.

Solve it in the labTake it as a mock interview

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Your goals

Success ≥ 99% through the outage.

Test cases

Like a coding problem, a solution has to pass every case, not just the one it was built for. Submit in the lab runs all 5; 2 of them are hidden until then.

  1. As given. The problem’s own traffic, scheduled failures and seed.
  2. Peak hour. 25% more traffic than the problem states, with the same failures.
  3. A slow dependency. The main data store or model answers twice as slowly for part of the run.

The traffic

40 requests a second for 45 seconds: 100% chat (reads).

The system

What each component does in this design, and what calls it. The simulator mimics each one from these settings.

ComponentIts job hereSettings
Load balancerLoad balancerTraffic enters here.Calls AI gateway (keys, routing, retries).Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header.–
AI gateway (keys, routing, retries)App serviceLoad balancer forwards to it.Calls Prompt cache.Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s.2 replicas × 400 slots · timeout 20 s
Prompt cachePrompt cacheAI gateway (keys, routing, retries) calls it.Calls Primary model (provider A).Providers can cache a long repeated prompt prefix, cutting cost and time to first token.2000 slots · 20% hit rate
Primary model (provider A)LLM endpointPrompt cache calls it.Latency has two parts: time to the first token, then a steady stream of tokens per second.400 streams · 3,000 calls/min

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