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Add HTTP/2 103 Early Hints Support to Hyper #3980

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#3991
Open
Add HTTP/2 103 Early Hints Support to Hyper#3980
#3991
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A-http2Area: HTTP/2 specific.C-featureCategory: feature. This is adding a new feature.
@apu031

Description

@apu031
apu031
opened on Nov 19, 2025

Summary

This tracking issue covers the implementation of HTTP/2 103 Early Hints support across the hyper ecosystem. The work has been completed across three repositories with a phased merge strategy.

Implementation Overview

HTTP 103 Early Hints (RFC 8297) allows servers to send resource preloading hints while preparing the final response, enabling browsers to start downloading critical resources early for improved performance.

Completed Work

Repository Changes

HTTP Repository (http crate)

  • Added StatusCode::EARLY_HINTS constant
  • Branch: http-103

H2 Repository (HTTP/2 protocol layer)

  • Implemented HTTP/2 informational responses (1xx) protocol support
  • Added comprehensive unit tests
  • Branch: http-103

Hyper Repository (Application layer)

Implementation Details

Server-Side API

async fn handler(mut req: Request<Body>) -> Result<Response<Body>, Error> {
    if let Some(mut sender) = req.extensions_mut().remove::<InformationalSender>() {
        let hints = Response::builder()
            .status(StatusCode::EARLY_HINTS)
            .header("link", "</css/critical.css>; rel=preload; as=style")
            .header("link", "</js/app.js>; rel=preload; as=script")
            .body(())
            .unwrap();
        
        sender.try_send(hints)?;
    }
    
    Ok(Response::new("Final response"))
}

Client-Side API

let config = InformationalConfig::new().with_callback(|response| {
    if response.status() == StatusCode::EARLY_HINTS {
        for link in response.headers().get_all("link") {
            start_preload(link);
        }
    }
});
let client = Client::builder().http2_only(true).build_with_config(config);

Benefits

  • Improved page load performance through parallel resource loading
  • RFC 8297 compliant implementation
  • Backward compatible with existing applications
  • Comprehensive test coverage

Pull Requests

Phase 1: Foundation Components (Ready Now)

  1. http crate PR: Add StatusCode::EARLY_HINTS constant - Add HTTP 103 Early Hints status code http#794
  2. h2 crate PR: HTTP/2 informational responses protocol support - Implement HTTP/2 informational responses (1xx) support h2#865
  3. Joel Dice's hyper PR prototype support for 1xx informational responses #3815: Foundational 1xx informational responses - prototype support for 1xx informational responses #3815

Phase 2: Complete Implementation (After Phase 1 Merge)

  1. Apu Islam's hyper PR: Complete HTTP/2 103 Early Hints implementation - [Will be submitted after Phase 1 merges]

Merge Strategy

Phase 1: Foundation Components

Phase 2: Complete Implementation

  • hyper: Complete HTTP/2 103 Early Hints with client callbacks and production examples

Action Items

Phase 1 (Ready for Review)

Phase 2 (After Phase 1 Merge)

  • Submit comprehensive hyper PR
  • Review complete implementation
  • Verify production examples and documentation

References

Status

Implementation complete across all three repositories. Phase 1 PRs ready for immediate review and merge. Phase 2 will follow after foundation is established.

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    A-http2Area: HTTP/2 specific.C-featureCategory: feature. This is adding a new feature.

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