Takeaways
- The origin server requires the request to be conditional.
- Its typical use is to avoid the "lost update" problem, where a client
GETs a resource's state, modifies it, andPUTs it back to the server, when meanwhile a third party has modified the state on the server, leading to a conflict. - By requiring requests to be conditional, the server can assure that clients are working with the correct copies.
Responses using this status code should explain how to resubmit the request successfully.
Example
HTTP/1.1 428 Precondition Required
Content-Type: text/html<html>
<head>
<title>Precondition Required</title>
</head>
<body>
<h1>Precondition Required</h1>
<p>This request is required to be conditional; try using "If-Match".</p>
</body>
</html>Information
Responses with the 428 status code must not be stored by a cache.
When a precondition header is not matching the server side state, the response should be 412 Precondition Failed.