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When it comes to designing RESTful APIs, one of the most common confusions developers face is the choice between PUT and PATCH. Both are used to update resources — but understanding their idempotent nature is key to designing reliable and predictable APIs.

In simple terms, idempotent means that making the same API call multiple times should produce the same result as making it once. In the case of PUT, this is exactly how it behaves. A PUT request replaces the entire resource with the data provided. So, whether you call it once or ten times, the resource remains consistent. This makes PUT ideal for full updates, where you’re confident about the entire state of the resource you’re sending.

PATCH, on the other hand, is designed for partial updates — changing only certain fields of a resource. However, depending on how it’s implemented, PATCH may or may not be idempotent. For instance, a PATCH request that increments a counter by one on every call is not idempotent because the resource changes with each call.

Choosing between the two comes down to intent. If your API design requires complete replacements, PUT ensures clarity and predictability. If you’re updating specific fields, PATCH offers flexibility, but you must take care to maintain idempotent behavior when needed.

Tools like Keploy can play an important role in validating these design decisions. Keploy automatically records and replays API calls, helping developers verify if their endpoints behave consistently across multiple executions — a crucial step in confirming idempotent behavior.