Problems / RAG and retrieval, medium

Semantic cache too strict

A semantic cache in front of a RAG assistant hits 10%: its similarity threshold is 0.99. The model quota can’t take the rest.

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Your goals

Success ≥ 99%, time to first token p99 ≤ 2.5 s.

The traffic

50 requests a second for 30 seconds: 100% request (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 app.Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header.–
AI appApp serviceLoad balancer forwards to it.Calls Semantic cache (checked first), Model.Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s.3 replicas × 500 slots · timeout 30 s
ModelLLM endpointAI app calls it.Latency has two parts: time to the first token, then a steady stream of tokens per second.400 streams · 4M input tokens/min
Semantic cacheSemantic cacheAI app checks it first.A semantic cache matches questions by meaning using embeddings, not exact text.10% hit rate

Hints

Try it first. Open one hint at a time if you get stuck.

Hint 1

Many questions are paraphrases of each other.

Hint 2

Loosen the threshold carefully.

One way to solve it

Raise the semantic cache hit rate.

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