Grokking Microservices for System Design Interviews

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Idempotency

1. The Definition, and Why It Is Not Optional

An operation is idempotent when performing it twice has the same effect as performing it once. Setting a user's email to a value is idempotent. Do it five times, and the email is that value.

Three common operations are not idempotent: appending to a list, incrementing a counter, and charging a card. Each repetition changes the world again.

This is a load-bearing lesson, not a vocabulary item. Here is why: everything else in the architecture manufactures duplicates.

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