An operation is idempotent when applying it more than once produces the same state as applying it once, which is the property that makes retries safe.
Idempotency ensures retrying an operation doesn't change the final state beyond the first successful application. The author contrasts setting a fixed value (idempotent) with incrementing a value (not idempotent), and notes most business-critical operations resemble the latter.
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the author: Idempotency is the property that makes the retry safe. An operation is idempotent when applying it more than once produces the same state as applying it once. For example, setting an account balance to 500 is idempotent, because even the tenth time to execute this operation, the outcome will be the same. In contrast, adding 500 to a balance is not idempotent, since every time it is executed, the balance amount changes. Most operations that matter in a business system resemble the second one.
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