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principle-make-operations-idempotent

Skill17 starsChanged 17 days agoMITView on GitHub

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.

We split this skill into the rules it gives an agent

1 rule. 1 is common, 0 are distinctive, and 0 are very specific to this project.

  • 1

    Common

    Most of the 41 edited versions we read keep these.

  • 0

    Distinctive

    This author's own choices. Worth a look.

  • 0

    Very specific

    Tied to their project's names, files or tools. Probably skip.

  • Common
  • Distinctive
  • Very specific
  1. Common:

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

    line 21

“Common” is measured: kept by at least 6 in 10 of the 41 edited versions of this file we could read. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

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Files
SKILL.md

Make Operations Idempotent

Design operations so they converge to the correct state regardless of how many times they run or where the previous run stopped. 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 idea: derive each step from the state that exists, not from what the previous run meant to do. Read the current state, compare it to the goal, and apply only the difference. A step written this way is safe to repeat, because a repeat finds nothing left to do.

Anything a run leaves behind, such as a lock, a temporary file, a half-written record, or a claimed job, must be recognizable as stale by the next run. Judge staleness by evidence that stays true wherever the previous run could have stopped, not only in the common case. A marker that the next run cannot judge turns every crash into manual cleanup. For example, write to a temporary path and rename it into place, or compare artifacts by content instead of by creation order.

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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