principle-make-operations-idempotent
Apply when designing commands, lifecycle steps, or processing loops that run amid crashes, restarts, and retries. Converge to the same end state regardless of partial prior runs.
# 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.