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Level 7 / Project 14 - Cache Backfill Runner

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Focus

  • historical backfill workflow

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-7/14-cache-backfill-runner
python project.py --input data/sample_input.txt --output data/output_summary.json
pytest -q

Expected terminal output

... output_summary.json written ...
2 passed

Expected artifacts

  • data/output_summary.json
  • Passing tests
  • Updated notes.md

Alter it (required)

  1. Add a max_backfills limit so the cache only auto-backfills N times before requiring manual intervention.
  2. Add an evict_oldest() method that removes the least-recently-added cache entry when at capacity.
  3. Re-run script and tests — verify backfill limiting and eviction work.

Break it (required)

  1. Set miss_threshold to 0.0 so every single miss triggers a backfill.
  2. Provide an empty source dict and request keys that do not exist.
  3. Observe excessive backfills (thrashing) or None results with misleading stats.

Fix it (required)

  1. Validate that miss_threshold is between 0.01 and 1.0 to prevent thrashing.
  2. Skip backfill when the source is empty (nothing to load).
  3. Add tests for zero threshold, empty source, and backfill count limits.

Explain it (teach-back)

  1. Why does threshold-based backfill prevent unnecessary source reads?
  2. What happened when miss_threshold was set to zero?
  3. How did the minimum threshold validation prevent cache thrashing?
  4. Where would you use cache backfill in a real high-traffic web application?

Mastery check

You can move on when you can: - run baseline without docs, - explain one core function line-by-line, - break and recover in one session, - keep tests passing after your change.

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