What is Idempotency?
Idempotency means that you can do something over and over again, and after the first time, nothing new changes. In other words, repeating the same action has the same result as doing it just once.
Let's break it down
- Idempotent: a word that describes an operation that can be repeated without changing the outcome after the first execution.
- Do something over and over again: performing the same action many times.
- Nothing new changes: the system’s state stays the same after the first successful run.
- Same result as doing it just once: the effect you see after the first try is exactly what you get on every later try.
Why does it matter?
Because it makes software and systems more reliable and easier to recover from errors. If a request fails halfway, you can safely retry it without worrying about creating duplicate data or unintended side effects.
Where is it used?
- Web APIs: endpoints like “create order” are designed to be idempotent so a client can resend a request if the network drops.
- Database operations: “INSERT … ON CONFLICT DO NOTHING” ensures the same row isn’t added twice.
- Payment processing: charging a credit card once even if the payment request is sent multiple times.
- Infrastructure as Code: tools like Terraform apply the same configuration repeatedly without altering resources that are already correct.
Good things about it
- Increases fault tolerance by allowing safe retries.
- Prevents duplicate actions, protecting data integrity.
- Simplifies debugging and testing because outcomes are predictable.
- Improves user experience; users don’t see unexpected double actions.
Not-so-good things
- Designing truly idempotent operations can be complex, especially for actions that naturally have side effects.
- May require extra bookkeeping (e.g., unique request IDs) which adds overhead.
- Not all real-world tasks can be made idempotent, limiting its applicability.
- Over-reliance on idempotency can mask deeper issues like unstable networks or poor error handling.