Data Systems Coding Lens

Use this skill to add a practical data systems review pass to coding work. The goal is not to explain theory. The goal is to catch failure modes before they become production bugs.

Two leitwörter anchor every pass and recur in the checks below. Source of truth: name the one place that durably owns each fact, so derived copies (caches, indexes, read models) can always be rebuilt from it. Idempotent: every write that can be retried or replayed must produce the same result the second time — no duplicate money, emails, jobs, or records. State both by name as you review.

First Pass

Start by identifying the state boundary:

1. What data is created, read, updated, deleted, cached, derived, or emitted?
2. What is the source of truth?
3. Which paths are synchronous, asynchronous, retried, replayed, or user visible?
4. Which invariant would make the feature incorrect if it broke, and where does the code assert it?
5. Which failure would be silent, expensive, or hard to repair?

If the task is small, keep this pass brief and fold it into the implementation. If the task touches shared data contracts or production records, write the answer down before editing.

Do not overcomplicate pure UI copy, styling, one off scripts, docs only edits, or local throwaway utilities. In those cases, only apply the lens if the change touches persistent data, background work, or production operational risk.