HTTP 103 – An HTTP Status Code for Indicating Hints
To explain the use case for this (as I understand it):<p>http&#x2F;2 push sounds great at first glance for doing things like sending your CSS&#x2F;JS bundles downstream with the HTML. The problem with this is that only the system that builds the HTML knows what needs to be pushed and only the system terminating the http&#x2F;2 connection can send the assets. That means your app servers would need to terminate the http&#x2F;2 connection and would be serving static assets. Serving static assets from your app server sucks for a lot of reasons. Instead what you want to do is tell some upstream proxy that is good at serving static assets what it should push. This is done by having the upstream proxy parse the Link HTTP headers in the response, which many services already do. But, today they can&#x27;t start pushing assets until the main response is being sent since they need the headers from it to instruct them. Ideally you&#x27;d start sending assets ASAP, while the app server is building the page. This is exactly what the 103 response is most useful for.<p>e.g. You have a CDN front for your app servers, the app server sends an early 103 as soon as it knows what assets the page needs, the CDN intercepts this response and uses it to start pushing the assets to the client from the local edge node using http&#x2F;2 push. This lets you benefit from http&#x2F;2 push of your assets while processing the main response without your app server needing to serve static assets or even know about http&#x2F;2 itself.