You could write directly to reply.raw.socket an HTTP response with 103 status code.

@mcollina thanks I was not aware of that!

like so?

fastify.get("/", async (request, reply) => {
  const links = [
    "Link: </style.css>; rel=preload; as=style",
    "Link: </script.js>; rel=preload; as=script",
  ];
  const rawHttpRequest = `HTTP/1.1 103 Early Hints\r\n${links.join(
    "\r\n"
  )}\r\n\r\n`;
  reply.raw.socket.write(rawHttpRequest);

  return { hello: "world" };
});

returns

curl -D - http://localhost:3000    

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

HTTP/1.1 200 OK
content-type: application/json; charset=utf-8
content-length: 17
Date: Thu, 12 Nov 2020 14:18:21 GMT
Connection: keep-alive

{"hello":"world"}

That'd be the idea, yes

it's an interesting feature and we can make a plugin for that. I'll make a draft repo for it.

Here is the draft repo i've written: https://github.com/zekth/fastify-early-hints
Feel free to comment / contribute.
Also for testing, it seems that i can't acces the full HTTP message throught the reply.raw, i might need to spawn a curl process to check the message. Any other idea?

I love so much the Fastify plugin system ❤️

Amazing work @zekth! Is it on npm yet? PR to the ecosystem page?

Any other idea?

I think 103 status is not supported yet by the node core (cause it is still in draft IMHO).
For example there the 100-Continue is supported by the http client but 103 doesn't emit any events and searching on the nodejs repo there isn't such test.

So spawn+curl seems the only way to check it right now.

NB: with http2 the plugin trigger an error, should be a warning be emitted that tell the user to switch to server push in this scenario?

@mcollina currently i'm facing some issues with the write operation to the response or to the socket. For example we don't wait the end of the write operation to close the response with fastify, leading to errors. I'm fixing this. Once everything is fixed with the first draft and tests are implemented a first release on npm is thinkable yes.

@Eomm will have to dig on http2 yes.

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

Just a heads-up, this is now supported as an experimental feature in Chromium 95: https://chromium.googlesource.com/chromium/src/+/refs/heads/main/docs/early-hints.md

FYI: On 2022-06-21, Chrome 103 became stable, which supports 103.

Support for early hints landed in Node.js core: nodejs/node#44180.
This will likely be in Node.js v18.8.0 and likely backported to Node.js v16.
I'm not sure we should do anything to support it (possibly we should forward the call to Node core).

Something that would be interesting is to build a plugin that automatically inspect responses and automatically collect metadata and send them through on next call. Take a look at https://github.com/google/node-h2-auto-push for inspiration.

There is this plugin https://github.com/zekth/fastify-early-hints from @zekth we could start from.

The first step, migrating to fastify v4?

@Eomm I'm +1. I can transfer ownership

Go for it, then we will update it 👍🏽
Easy GitHub activity 🟩🟩🟩🟩🟩🟩 😄

Done

@zekth can you add me to npm so I can add it to the org?

Can we close this one?

@mcollina
We never released the plugin.

For reference https://github.com/fastify/fastify-early-hints

@mcollina

We never released the plugin.

why?

I dont know. Maybe because @climba03003 suggested to implement node native api call when it is released. It is released with node 18.11.0.

It can be released, but it can improve to use the native one when possible.
Not sure if it should be released as 1.0.0?

I choose to release in a major to imply it is migrated to organization and a stable release.
https://www.npmjs.com/package/@fastify/early-hints