既然在PATCH请求中传入资源全部字段即可实现PUT请求的全量更新效果，为何仍需PUT方法？

Great question—this is one of those REST API design nuances that trips up a lot of developers, so let’s break it down clearly.

First: The Core Semantic Difference Between PUT and PATCH #

Before diving into your questions, it’s critical to anchor on the intent behind each method, not just what they can technically do:

• PUT: Designed for full resource replacement. When you send a PUT request to /users/123, you’re saying: "Replace the entire user resource at this URI with exactly what’s in my request body."
• PATCH: Designed for partial resource updates. When you send a PATCH request to the same URI, you’re saying: "Only modify the specific fields I’ve included in my request body; leave the rest as they are."

Question 1: Why does PUT exist if PATCH can handle full updates? #

Even though you can stuff a full resource into a PATCH request, here’s why PUT is still non-negotiable:

• Semantic clarity is king: REST APIs rely on shared conventions to be intuitive. A developer seeing a PUT request immediately knows the intent is to replace the entire resource—they don’t have to parse the request body to figure that out. This reduces confusion and makes your API easier to use and maintain.
• Idempotency guarantees (with strict intent): Both PUT and PATCH can be idempotent, but PUT’s idempotency is baked into its semantic definition. No matter how many times you send the same PUT request, the end state of the resource will be identical. With PATCH, idempotency depends entirely on what’s in the request body (e.g., a PATCH that increments a counter isn’t idempotent, but a full-resource PATCH is). Using PUT removes ambiguity about whether the operation is safe to retry.
• Resource creation behavior: By convention, PUT will create a resource at the target URI if it doesn’t already exist (assuming the server allows it). PATCH, on the other hand, almost always requires the resource to exist first—sending a PATCH to a non-existent URI typically returns a 404. This is a key behavioral difference that aligns with their intended uses.

Question 2: If a PATCH with all fields acts like a PUT, why not just use PATCH for everything? #

Again, this boils down to semantics and convention—and those matter more than technical capability:

• Consistent API design: If you allow PATCH to act as both partial and full updates, you’re breaking the shared convention that developers expect. This forces them to guess when to use which method, leading to bugs (e.g., a developer sends a partial PATCH expecting only certain fields to update, but the server treats it as a full replacement by mistake).
• Server-side validation logic: Most APIs implement different validation rules for PUT vs. PATCH. For PUT, the server will likely require all mandatory fields to be present (since you’re replacing the entire resource). For PATCH, it only validates the fields included in the request. If you use a full-resource PATCH, the server might not run those full validation checks, leading to invalid resources slipping through.
• Future-proofing: If your API evolves later (e.g., adding new fields), a PUT request will naturally require the new fields (or handle defaults) as part of full replacement, while a PATCH would only touch the fields explicitly sent. Using the correct method now prevents headaches down the line when the API changes.

内容的提问来源于stack exchange，提问作者Aayush Taneja