Implementing HTTP/2 Server Push in Go

In the previous article we learned how to send HTTP/2 headers from the server while keeping the connection alive; this article shows how to automatically issue server push.

The author from the 360 Search Technology Team provides a Go implementation that adds two handlers: /crt serving a certificate and /push that triggers a push of /crt to the client.

package main

import (
    "html/template"
    "log"
    "net/http"
)

func main() {
    http.HandleFunc("/header", func(w http.ResponseWriter, r *http.Request) {
        w.Header().Add("X-custom-header", "custom header")
        w.WriteHeader(http.StatusNoContent)
        if f, ok := w.(http.Flusher); ok {
            f.Flush()
        }
        select {}
    })

    // handler for /crt, returns certificate content for client push
    http.HandleFunc("/crt", func(w http.ResponseWriter, r *http.Request) {
        tpl := template.Must(template.ParseFiles("server.crt"))
        tpl.Execute(w, nil)
    })

    // request this path to trigger push of /crt
    http.HandleFunc("/push", func(w http.ResponseWriter, r *http.Request) {
        pusher, ok := w.(http.Pusher)
        if !ok {
            log.Println("not support server push")
        } else {
            err := pusher.Push("/crt", nil)
            if err != nil {
                log.Printf("Failed for server push: %v", err)
            }
        }
        w.WriteHeader(http.StatusOK)
    })

    log.Println("start listen on 8080...")
    log.Fatal(http.ListenAndServeTLS(":8080", "server.crt", "server.key", nil))
}

Running the server with TLS and using the h2c client demonstrates the exchange of HEADERS, PUSH_PROMISE, and DATA frames, with the client receiving the promised resource without an explicit request.

An automatic push example uses a timer (time.Tick) inside the handler to repeatedly push /crt every 5 seconds, incrementing the promised stream IDs by 2 each time.

http.HandleFunc("/autoPush", func(w http.ResponseWriter, r *http.Request) {
    w.Header().Add("X-custom-header", "custom")
    w.WriteHeader(http.StatusNoContent)
    if f, ok := w.(http.Flusher); ok {
        f.Flush()
    }
    pusher, ok := w.(http.Pusher)
    if ok {
        for {
            select {
            case <-time.Tick(5 * time.Second):
                err := pusher.Push("/crt", nil)
                if err != nil {
                    log.Printf("Failed for server push: %v", err)
                }
            }
        }
    }
})

The article also shows the internal Go HTTP/2 implementation of the Push method, how a PUSH_PROMISE frame is constructed, and how the server processes push requests via the serve loop.

By following these examples, developers can implement both on‑demand and periodic server‑push functionality in Go.