Problems / Agents and tools, medium

Online evals silently lose trajectories

An agent service samples 5% of its trajectories, 12 a second, into a stream that keeps 20 seconds of data. LLM-judge consumers score them for regressions, but four consumers manage only about 7 a second.

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Your goals

No trajectories lost to retention; success ≥ 99.9%.

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

The traffic

12 requests a second for 60 seconds: 100% trajectory (writes).

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 Agent service (5% sampled).Layer 4 balancers route by IP and port; layer 7 balancers read HTTP and can route by path or header.–
Agent service (5% sampled)App serviceLoad balancer forwards to it.Calls Trajectory stream (publishes).Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s.3 replicas × 500 slots · timeout 1.0 s
Trajectory streamEvent streamAgent service (5% sampled) publishes to it.Calls LLM-judge consumers.Kafka keeps order only within a partition, so the partition key decides what stays in sequence.12 partitions · retention 20 s
LLM-judge consumersWorkerTrajectory stream consumes from it.Queues deliver at least once, so a worker can see the same message twice.4 concurrent jobs

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