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    <p>PUT means what the HTTP spec defines it to mean. Clients and servers cannot change that meaning. If clients or servers use PUT in a way that contradicts its definition, at least the following thing might happen:</p> <p>Put is by definition idempotent. That means a client (or intermediary!) can repeat a PUT any number of times and be sure that the effect will be the same. Suppose an intermediary receives a PUT request from a client. When it forwards the request to the server, there is a network problem. The intermediary knows by definition that it can retry the PUT until it succeeds. If the server uses PUT in a non idempotent way these potential multiple calls will have an undesired effect.</p> <p>If you want to do a partial update, use PATCH or use POST on a sub-resource and return 303 See Other to the 'main' resource, e.g.</p> <pre><code> POST /account/445/owner/address Content-Type: application/x-www-form-urlencoded street=MyWay&zip=22222&city=Manchaster 303 See Other Location: /account/445 </code></pre> <p>EDIT: On the general question why partial updates cannot be idempotent:</p> <p>A partial update cannot be idempotent in general because the idempotency depends on the media type semantics. IOW, you might be able to specify a format that allows for idempotent patches, but PATCH cannot be guaranteed to be idempotent for every case. Since the semantics of a method cannot be a function of the media type (for orthogonality reasons) PATCH needs to be defined as non-idempotent. And PUT (being defined as idempotent) cannot be used for partial updates.</p>
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    1. POWhat's the justification behind disallowing partial PUT?
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    1. USJan Algermissen
    1. USJan Algermissen
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    1. POWhat's the justification behind disallowing partial PUT?
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    1. COHi Jan, as I said already; "I understand that this is the existing definition of PUT - this question is about the reason(s) why it would be defined that way". Your example in the 2nd paragraph implies that the partial update is non-idempotent - obviously this is what PATCH is for. I am interested in *idempotent* partial updates - i.e. a partial PUT.
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    2. COPATCH is being re-added. Also; Jan - idempotency doesn't need to depend on any specific set of media types with regard to its definition in the protocol, the semantics of a method are a function of it's definition as part of a uniform interface.. I'm still struggling to work out what is actually lost by changing the definition of PUT to something as simple as 'an idempotent change of state' (i.e. one that doesn't prevent partials).
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    3. CO@Mike, as I tried to explain above: You cannot define a partial update to be idempotent without taking the particular media type into account. The idempotency depends on the media type used and since you cannot define method semantics to depend on media type semantics, an update can only be idempotent for the general case if it is a full replacement. A method that specifies a partial update MUST be defined non-idempotent. IOW, you simply cannot ever define an "'an idempotent change of state' (i.e. one that doesn't prevent partials)". It is a contradiction in itself.
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