idempotent-redundancy（幂等冗余）

Idempotent Redundancy

When adding redundant paths (fallbacks, belt-and-suspenders), make them idempotent.

Pattern

Redundancy without idempotency causes loops, churn, or data corruption.

DO

• Use _is_merge: true for Braintrust updates
• Check if value exists before writing (fallback only if missing)
• Use atomic write/rename for file operations
• Make reconciliation steps safe to run repeatedly

DON'T

• Write unconditionally in fallback paths
• Allow multiple writers to overwrite each other
• Fire "repair" actions that can trigger more repairs

Source Sessions

• a541f08a: "Redundancy is good only if idempotent"
• 1c21e6c8: "Belt-and-suspenders, but make it idempotent"
• 6a9f2d7a: "Idempotent repair hooks"