Server-Sent Events in Go

main.go

package main

import (
	"context"
	"fmt"
	"log"
	"net/http"
	"time"
)

func send(ctx context.Context, rw http.ResponseWriter, ivl time.Duration) {
	flusher, _ := rw.(http.Flusher)
	tck := time.NewTicker(ivl)
	defer tck.Stop()

	for {
		select {
		case <-ctx.Done():
			return
		case date := <-tck.C:
			fmt.Fprintln(rw, date.Format(time.StampNano))
			flusher.Flush()
		}
	}
}

func parse(data string) time.Duration {
	dur, err := time.ParseDuration(data)
	if err != nil {
		return 5 * time.Second
	}
	return dur
}

func ticker(rw http.ResponseWriter, req *http.Request) {
	rw.Header().Set("Cache-Control", "no-cache")
	rw.Header().Set("Connection", "keep-alive")
	rw.Header().Set("Content-Type", "text/event-stream")

	if _, ok := rw.(http.Flusher); !ok {
		http.Error(rw, "client doesn't support streaming", http.StatusBadRequest)
		return
	}

	ivl := req.URL.Query().Get("interval")
	send(req.Context(), rw, parse(ivl))
}

func main() {
	log.Printf("info: starting server on :5000")
	http.HandleFunc("/tick", ticker)
	http.ListenAndServe(":5000", nil)
}

tick.sh

#!/usr/bin/env bash

if [ "$#" -ne  "1" ]; then
  echo "error: interval is missing"
  echo "usage: ${0} <interval>"
  exit 1
fi

curl -N "localhost:5000/tick?interval=${1}"