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103 Early Hints

syndicated from planet-php.net on June 30, 2018

103 Early Hints is an experimental status code. It can be used by a server to preemptively send headers to a client or intermediary. A client might be able to use these headers to make certain optimizations as early as possible in the process.



The specific use-case in RFC8297 is preloading. A server might tell a browser that it’s very likely to require certain css stylesheets or scripts to render the document using the Link header:



HTTP/1.1 200 OK
Date: Fri, 26 May 2017 10:02:11 GMT
Content-Length: 1234
Content-Type: text/html; charset=utf-8
Link: </main.css>; rel=preload; as=style
Link: </newstyle.css>; rel=preload; as=style
Link: </script.js>; rel=preload; as=script


When a client receives these headers, it might immediately start fetching those resource in anticipation of the html document using them.



The 103 Early Hints status code can move those headers up even earlier.



A response to a HTTP GET request might then look like this:



HTTP/1.1 103 Early Hints
Link: </style.css>; rel=preload; as=style
Link: </script.js>; rel=preload; as=script



wait a few hundred milliseconds



HTTP/1.1 200 OK
Date: Fri, 26 May 2017 10:02:11 GMT
Content-Length: 1234
Content-Type: text/html; charset=utf-8
Link: </style.css>; rel=preload; as=style
Link: </script.js>; rel=preload; as=script


This GET request got 2 responses The 103 Early Hints status codes were in this case sent as soon as the server could, while the real response (with 200 OK) took a bit longer to produce. This could allow a client to quickly fetch these additional resources.



Another use-case for this might be a HTTP/2-enabled proxy or CDN. This CDN might be able to read these 103 responses and turn the preload links into HTTP/2 pushes.



A caveat



Any header sent with the 103 status codes should be treated just as hints, and might not be ‘correct’. Only once you get the final status code, a client can find out what the real headers should have been.



For that reason any headers supplied in the 103 should not cause any changes to how the real responses were interpreted.



Using this status



The 2 languages I looked at for using this status were PHP and Node.js. PHP does not support 103 Early Hints, because it doesn’t have a native ability to send more than 1 status code back to a client.



Node.js does support it. The HTTP/1 version looks like this (and looks a bit like a hack, as it uses a private function):



Truncated by Planet PHP, read more at the original (another 2684 bytes)