Problems / Real world, medium

Search autocomplete on every keystroke

Autocomplete queries the search index on every keystroke: 1,500 queries a second, 4 ms of CPU each, on a 4-vCPU index.

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Your goals

Success ≥ 99%, p99 ≤ 300 ms.

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; 1 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.
  4. A server dies. One replica of the service with the most replicas is killed for part of the run.

The traffic

1,500 requests a second for 30 seconds: 100% 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 Suggest API.Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header.–
Suggest APIApp serviceLoad balancer forwards to it.Calls Prefix index.Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s.3 replicas × 200 slots · timeout 4.0 s
Prefix indexSearch indexSuggest API calls it.Search engines build an inverted index: each word points to the documents that contain it.32 slots

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