Principle Make Operations Idempotent

---
name: principle-make-operations-idempotent
description: "Apply when an operation must tolerate retries after partial completion."
license: MIT
metadata:
  author: jstack-maintainers
  source: michael-denyer/pstack-claude
  source-version: "0.9.30"
  source-commit: 45f768349a6d7d7e71509fee3f5bccfad54b3bad
  owner: software-factory
  risk: low
  capabilities: jstack,engineering-principle
user-invocable: false
---

# 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.