idempotent-script-design

name	idempotent-script-design
description	Use this skill when writing scripts, cron jobs, data pipelines, or any automated process that may be run multiple times. Design every operation to be safely re-runnable without side effects.
category	automation

Idempotent Script Design

An idempotent operation produces the same result whether run once or ten times.

Techniques:

• Check before create: CREATE TABLE IF NOT EXISTS, mkdir -p, check file existence before writing.
• Upsert instead of insert: Use INSERT ... ON CONFLICT DO UPDATE in SQL.
• Atomic writes: Write to a temp file, then rename — prevents corrupt partial output.
• Track progress: Write a completion marker or checkpoint so reruns skip done work.

Testing idempotency: Run your script twice on the same input and verify the output is identical.

Anti-patterns:

• Appending to a file without checking if the data is already there.
• Inserting rows without checking for duplicates.