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Level 2 / Project 11 - Retry Loop Practice

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Estimated time: 40 minutes

Focus

  • retry attempts with clear logging

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-2/11-retry-loop-practice
python project.py --operations 10 --failure-rate 0.6 --attempts 5
python project.py --operations 5 --failure-rate 0.9 --attempts 3
pytest -q

Expected terminal output

Simulating 10 operations (failure_rate=0.6, max_attempts=5)
  Operation 1: OK (attempts: 2)
  Operation 2: FAILED (attempts: 5)
  ...
=== Summary ===
9 passed

Expected artifacts

  • Retry simulation results 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 --jitter flag that adds random variation to delay times.
  2. Add a --fail-fast mode that stops all operations after the first failure.
  3. Log each retry attempt with a timestamp.

Break it (required) — Core

  1. Set --failure-rate 1.0 — every operation should exhaust all attempts.
  2. Set --attempts 0 — what happens with zero allowed attempts?
  3. Set --delay -1 — negative delay makes no sense.

Fix it (required) — Core

  1. Validate that max_attempts is at least 1.
  2. Validate that delay is non-negative.
  3. Add tests for edge-case configurations.

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

Explain it (teach-back)

  1. What is exponential backoff and why is it used?
  2. Why catch specific exceptions instead of bare except?
  3. What is a closure and how does make_countdown_function use one?
  4. Where is retry logic essential in real systems (networks, APIs, databases)?

Mastery check

You can move on when you can: - implement retry with exponential backoff from memory, - explain why backoff prevents "thundering herd" problems, - describe the difference between flaky and deterministic test helpers, - add jitter to a retry delay calculation.

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