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Level 6 / Project 05 - Transaction Rollback Drill

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Focus

  • rollback-first failure handling

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-6/05-transaction-rollback-drill
python project.py --input data/sample_input.txt --output data/output_summary.json
pytest -q

Expected terminal output

{
  "committed": 3,
  "rolled_back": 1,
  "final_accounts": [
    {"id": 1, "name": "Alice", "balance": 425.0},
    ...
  ]
}

Expected artifacts

  • data/output_summary.json — transfer results with account balances
  • Passing tests (pytest -q → 6+ passed)
  • Updated notes.md

Alter it (required)

  1. Add a transfer fee: deduct an extra 1% from the source account on each successful transfer.
  2. Add a --max-amount flag that rejects any single transfer above a threshold.
  3. Add a rollback_all mode: if any transfer in the batch fails, roll back the entire batch.
  4. Re-run script and tests after each change.

Break it (required)

  1. Transfer the exact full balance (e.g. 500.00 from an account with 500.00) — does the CHECK constraint allow balance = 0?
  2. Transfer between the same account (from=1, to=1) — what happens?
  3. Remove the SAVEPOINT logic and observe what happens when a mid-batch transfer fails.

Fix it (required)

  1. Add a guard that rejects self-transfers (from_id == to_id).
  2. Ensure the CHECK constraint allows zero balance but not negative.
  3. Add a test that proves savepoints isolate failures from successful transfers.

Explain it (teach-back)

  1. What is a SAVEPOINT, and how does it differ from a regular transaction?
  2. Why can we roll back a single transfer without losing the others?
  3. What would happen without explicit transaction control (autocommit mode)?
  4. How do real banks handle the "insufficient funds" scenario at scale?

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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