Support Http Response 103 (Early Hints)

[joakime]

Target Jetty version(s)
10+

Enhancement Description

What would it take to have Jetty support response code 103 (Early Hints)?

• https://datatracker.ietf.org/doc/html/rfc8297
• https://www.fastly.com/blog/beyond-server-push-experimenting-with-the-103-early-hints-status-code
• https://developers.cloudflare.com/cache/about/early-hints/

[sbordet]

I wonder how a 103 is any different from a HTTP/2 PUSH_PROMISE, which would carry exactly the same information.

The only difference I can see is that PUSH_PROMISE may send the response content before the client has a chance to cancel it, but typically bandwidth is not a problem.

Server push has been removed from HTTP/3, and a 103 may possibly break clients that are unaware of it.

Furthermore, there is no API on the server to send a 103, which is similar to 100 Continue as in the server sends 2 (or more) responses for one request (yes you can have multiple 103 followed by a 200).
We send the 100 implicitly when the application calls getInputStream(), but in the Servlet realm there is no API.

We need to consider whether to add a "send informational response" API in Jetty 12.

[gregw]

I think a 103 is very similar to our current support of 102 PROCESSING. We currently allow a sendError(102) to be done that sends a partial response, but without any of the current header fields from the response. I think all that is needed is that we duplicate the 102 handling, but use a temporary MetaData.Response to contain the headers as they currently are.

However, I'm unsure if 102 or 103 are handled completely by http>=2

We also need to consider if we want optional or mandatory Expect headers.

[joakime]

Chrome 95 was the first version to support 103 Early Hints.

https://chromium.googlesource.com/chromium/src/+/master/docs/early-hints.md

Fastly opened support in Nov 2020.

Cloudfare opened support in Sep 2021

[gregw]

I've opened a branch for this and started by adding a better test for 100 and 102 responses. These work for all protocols.
I'll implement 103 support later today.

[gregw]

@joakime can you pass this on to the user