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Level 3 / Project 14 - Service Simulator

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— This project — — Flashcards — Browser

Estimated time: 60 minutes

Focus

  • emulate service responses and failures

Why this project exists

This project gives you level-appropriate practice in a realistic operations context. Goal: run the baseline, alter behavior, break one assumption, recover safely, and explain the fix.

Run (copy/paste)

Use <repo-root> as the folder containing this repository's README.md.

cd <repo-root>/projects/level-3/14-service-simulator
python project.py request --seed 42
python project.py load --count 100 --json
python project.py retry --max-retries 5 --seed 42
pytest -q

Expected terminal output

{"status_code": 200, "body": {"message": "OK"}, ...}
Load test: 100 requests
  200: 82
  429: 5
  500: 8
  504: 5
10 passed

Expected artifacts

  • Simulated responses on stdout
  • Passing tests
  • Updated notes.md

Checkpoint: Baseline code runs and all tests pass. Commit your work before continuing.

Alter it (required) — Extension

  1. Add a --failure-rate flag to override the default error rate.
  2. Add exponential backoff to the retry_request function (wait 2^attempt ms).
  3. Add a health subcommand that checks if the service is "healthy" (3 consecutive 200s).

Break it (required) — Core

  1. Set success_rate to 0.0 — does retry_request exhaust all retries?
  2. Run load with --count 0 — what happens with zero requests?
  3. Create a service with --seed and verify results are identical each run.

Fix it (required) — Core

  1. Add validation for rate parameters (must sum to <= 1.0).
  2. Handle the zero-requests edge case in run_load_test.
  3. Add a --timeout flag that limits total retry duration.

Checkpoint: All modifications done, tests still pass. Good time to review your changes.

Explain it (teach-back)

  1. What HTTP status codes mean 200, 429, 500, 504?
  2. How does random.Random(seed) make tests deterministic?
  3. What is retry logic and when should you retry vs. fail fast?
  4. Why use a class (SimulatedService) instead of standalone functions?

Mastery check

You can move on when you can: - simulate service responses with configurable probabilities, - implement retry logic with max attempts, - use seed-based randomness for reproducible tests, - understand HTTP status code categories.

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