Early Hints

A 103 Early Hints response (RFC 8297) carries Link headers ahead of the real response. A browser that gets one starts preloading the listed stylesheets and scripts, or connecting to the listed origins, while the server is still working on the page: a slow database query or upstream no longer delays those downloads. Since 18.12.0.

In code#

EarlyHintsLayer sends the same hints for every request it wraps:

use rust_web_server::app::App;
use rust_web_server::core::New;
use rust_web_server::early_hints::EarlyHintsLayer;

let app = App::new().wrap(EarlyHintsLayer::new([
    "</app.css>; rel=preload; as=style",
    "</app.js>; rel=preload; as=script",
    "<https://cdn.example.com>; rel=preconnect",
]));

A handler or middleware that only knows the hints once it sees the request calls early_hints::send before it returns. The 103 goes out at once and the final response follows:

use rust_web_server::early_hints;

// inside a handler, before the slow part
early_hints::send(&["</product.css>; rel=preload; as=style"]);

send returns false and does nothing when there is nowhere to send the hint: under TestClient, for an app called directly, on HTTP/1.1 unless enabled (below), on HTTP/1.0, after early_hints::MAX_PER_REQUEST (4) 103s for the request, or for a value that is empty or contains CR, LF or NUL. early_hints::available() says whether a send would reach the client.

In the config file#

[[route]]
[route.match]
path = "/shop/*"
[route.action]
type = "proxy"
upstream = "shop"
[route.middleware]
early_hints = ["</shop.css>; rel=preload; as=style"]

See Early hints on the config-driven proxy page. A proxy route — and ReverseProxy in code — also passes on a backend's own 103, keeping only its Link values.

Metrics#

rws_early_hints_total{source} counts the 103s sent: configured (EarlyHintsLayer, a route's early_hints), upstream (a backend's 103 passed on) and handler (early_hints::send).

Protocols#

Browsers act on 103 only over HTTP/2 and HTTP/3, and some older HTTP/1.1 clients mishandle a 1xx they did not ask for, so HTTP/1.1 is opt-in. The http1-only build runs the handler on the thread that owns the socket and writes its hints just before the response. HTTP/1.1 requests with a streamed body (a large upload read while the handler runs) get no hints.