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by JCharante
2114 days ago
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For anybody wondering how: The HTTP HEAD method requests the headers that would be returned if the HEAD request's URL was instead requested with the HTTP GET method. For example, if a URL might produce a large download, a HEAD request could read its Content-Length header to check the filesize without actually downloading the file. https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/HE... |
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When you request a page, the response includes metadata, usually including a Last-Modified timestamp and often including an ETag (entity tag). Then when you make subsequent requests, you can include these in If-Modified-Since and If-None-Match request headers.
If the resource hasn’t changed, then the server responds with 304 Not Modified instead of sending the resource all over again. If the resource has changed, then the server sends it straight away.
Doing it this way means that in the case where the resource has changed, you make one request instead of two, and it also avoids a race condition where the resource changes between the HEAD and the GET requests.