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Level 6 / Project 01 - SQL Connection Simulator

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— This project — — Flashcards — —

Focus

  • connection config and retry patterns

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-6/01-sql-connection-simulator
python project.py --input data/sample_input.txt --output data/output_summary.json
pytest -q

Expected terminal output

{
  "rows_inserted": 7,
  "rows": [ ... ],
  "health": {"status": "healthy", "sqlite_version": "..."},
  "pool_stats": {"created": 1, "reused": 1, ...}
}

Expected artifacts

  • data/output_summary.json — full run results with pool stats
  • Passing tests (pytest -q → 8+ passed)
  • Updated notes.md

Alter it (required)

  1. Add a --timeout CLI flag that sets ConnectionConfig.timeout (observe what happens with very small values like 0.001).
  2. Track peak concurrent connections (the highest number of connections checked out simultaneously) and include it in pool.stats().
  3. Add a ConnectionPool.shrink() method that closes idle connections down to a target count.
  4. Re-run script and tests after each change.

Break it (required)

  1. Set pool_size=0 and observe what happens when connections cannot be pooled.
  2. Pass an invalid database path (e.g. /nonexistent/dir/db.sqlite) and observe the retry/failure behaviour.
  3. Close a connection manually, then return it to the pool — what happens on the next acquire?

Fix it (required)

  1. Add a guard in release() that pings the connection before returning it to the pool (discard broken ones).
  2. Validate that pool_size >= 1 in ConnectionConfig.__post_init__.
  3. Add tests for each broken case above.

Explain it (teach-back)

  1. Why do context managers (with blocks) matter for database connections?
  2. What is the performance difference between creating a new connection per query vs. pooling?
  3. Why does the retry use exponential backoff instead of a fixed delay?
  4. In a production web server, what problems arise if the pool is too small? Too large?

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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