michael-denyer/pstack-claude

principle-make-operations-idempotent

Apply when designing commands, lifecycle steps, or processing loops that run amid crashes, restarts, and retries.

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Make Operations Idempotent

Design operations so they converge to the correct state regardless of how many times they run or where they start from. Every state-mutating operation should answer: "What happens if this runs twice? What happens if the previous run crashed halfway?"

Why: Commands, lifecycle operations, and processing loops run where crashes, restarts, and retries are normal. If partial state changes the next run's outcome, every restart becomes a debugging session.

The pattern:

  • Convergent startup: scan for existing state, clean stale artifacts, adopt live sessions
  • Content-based cleanup: compare by content equivalence, not creation order
  • Self-healing locks: use PID-based stale lock detection
  • Idempotent scheduling: failed work respawns cleanly, fresh input regenerated after each cycle

The test:

  1. What happens if this runs twice in a row?
  2. What happens if the previous run crashed at every possible point?
  3. Does re-execution converge to the same end state?

If any answer is "it depends on what state was left behind," the operation needs a reconciliation step.

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