What is an Idempotency?

The concept comes from mathematics (a function where f(f(x)) = f(x)) but is load-bearing in distributed systems engineering, where network failures mean a caller often can't tell whether a request succeeded or simply lost its response. Setting a user's email address to “[email protected]” is idempotent — running it five times leaves the exact same end state as running it once. Incrementing a counter by one, or charging a credit card, is not — each execution changes the outcome further. Idempotency is what makes safe retries possible. If a client isn't sure a request landed, it can simply resend an idempotent request without fear of duplicating an effect — a non-negotiable property for anything involving money, inventory, or irreversible external actions, where a duplicate execution (charging a customer twice, sending the same alert three times) is a real-world bug, not just a data inconsistency.