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Your First Load Test

A walkthrough from zero to a threshold-gated result — and how to interpret what you see.

What we're testing

We'll use httpbin.org as a safe public target. It echoes requests back so you can inspect what lmn sends.

Step 1: Run a baseline

lmn run -H https://httpbin.org/get -R 200 -C 20

This fires 200 GET requests at 20 concurrent workers. You'll see a results table when it completes.

Reading the output:

  • p50 (median) — half your requests were faster than this. This is your typical experience.
  • p99 — 99% of requests completed faster than this. This is your worst-case tail latency.
  • Error Rate — percentage of non-2xx responses. Anything above 0% is worth investigating.
  • Throughput — requests per second sustained over the run.

Tip

p99 is the number to watch for SLA purposes. A p50 of 100ms with a p99 of 2000ms means occasional users are hitting a very slow path.

Step 2: Set a threshold

Now make the run fail if performance degrades. Create lmn.yaml:

run:
  host: https://httpbin.org/get
  method: get

execution:
  request_count: 200
  concurrency: 20

thresholds:
  - metric: latency_p99
    operator: lt
    value: 2000.0   # p99 must be under 2 seconds
  - metric: error_rate
    operator: lt
    value: 0.0      # zero errors

lmn run -f lmn.yaml
echo "Exit: $?"

If thresholds pass, you'll see a green summary and exit code 0. If they fail, exit code is 2 and the table marks the failing metrics.

Step 3: Understand what causes failures

High p99 is usually caused by: - Server-side slow paths (database queries, external calls) - Resource contention at high concurrency — try reducing -C to isolate

High error rate is usually caused by: - Rate limiting (429s) — reduce concurrency or add a lower request count - Server overload (503s) — your concurrency is too high for the server - Auth failures (401s) — check your headers

Step 4: Wire it into CI

# .github/workflows/load-test.yml
- name: Run load test
  run: lmn run -f lmn.yaml
  env:
    API_TOKEN: ${{ secrets.API_TOKEN }}

Exit code 2 will fail the workflow step. See Thresholds & CI Gating for a complete example.