2015-11-06 22:55:41 +01:00
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---
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set: 4
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code: 428
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title: Precondition Required
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---
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2015-11-09 02:44:05 +01:00
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The origin server requires the request to be conditional.
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2015-11-07 05:34:40 +01:00
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Its typical use is to avoid the "lost update" problem, where a client GETs a
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resource's state, modifies it, and PUTs it back to the server, when meanwhile a
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third party has modified the state on the server, leading to a conflict. By
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requiring requests to be conditional, the server can assure that clients are
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working with the correct copies.
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Responses using this status code SHOULD explain how to resubmit the request
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successfully. For example:
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```
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HTTP/1.1 428 Precondition Required
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Content-Type: text/html
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<html>
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2015-11-09 20:01:11 +01:00
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<head>
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<title>Precondition Required</title>
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</head>
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<body>
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<h1>Precondition Required</h1>
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<p>This request is required to be conditional; try using "If-Match".</p>
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</body>
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2015-11-07 05:34:40 +01:00
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</html>
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```
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Responses with the 428 status code MUST NOT be stored by a cache.
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2015-11-09 02:44:05 +01:00
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---
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* Source: [RFC6585 Section 3][1]
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2015-11-07 05:34:40 +01:00
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[1]: <http://tools.ietf.org/html/rfc6585#section-3>
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