proxy middleware should use httputil.ReverseProxy for SSE requests #2624

aldas · 2024-04-15T17:38:16Z

Relates to #1172

Use httputil.ReverseProxy to proxy SSE requests as it has support for streaming responses. See:
https://github.com/golang/go/blob/b107d95b9a66bfe7150fd4f2915e9bb876a6999a/src/net/http/httputil/reverseproxy.go#L601

can be tested with

create separate package and execute this code to start serving proxy application at port 8080 that proxies requests to localhost:8081

package main

import (
	"errors"
	"github.com/labstack/echo/v4"
	"github.com/labstack/echo/v4/middleware"
	"log"
	"net/http"
	"net/url"
)

func main() {
	e := echo.New()
	e.Use(middleware.Logger())
	e.Use(middleware.Recover())

	tmpURL, err := url.Parse("http://localhost:8081")
	if err != nil {
		log.Fatal(err)
	}
	e.Use(middleware.Proxy(middleware.NewRoundRobinBalancer([]*middleware.ProxyTarget{{URL: tmpURL}})))

	if err := e.Start(":8080"); err != nil && !errors.Is(err, http.ErrServerClosed) {
		log.Fatal(err)
	}
}

Create application for serving SSE

Go file for application

package main

import (
	"bytes"
	"errors"
	"fmt"
	"github.com/labstack/echo/v4"
	"github.com/labstack/echo/v4/middleware"
	"log"
	"net/http"
	"time"
)

func main() {
	e := echo.New()

	e.Use(middleware.Logger())
	e.Use(middleware.Recover())
	e.File("/", "./index.html")

	e.GET("/sse", func(c echo.Context) error {
		log.Printf("SSE client connected, ip: %v", c.RealIP())

		w := c.Response()
		w.Header().Set("Content-Type", "text/event-stream")
		w.Header().Set("Cache-Control", "no-cache")
		w.Header().Set("Connection", "keep-alive")

		ticker := time.NewTicker(1 * time.Second)
		defer ticker.Stop()
		for {
			select {
			case <-c.Request().Context().Done():
				log.Printf("SSE client disconnected, ip: %v", c.RealIP())
				return nil
			case <-ticker.C:
				event := Event{
					Data: []byte("ping: " + time.Now().Format(time.RFC3339Nano)),
				}
				if err := event.WriteTo(w); err != nil {
					return err
				}
				w.Flush()
			}
		}
	})

	if err := e.Start(":8081"); err != nil && !errors.Is(err, http.ErrServerClosed) {
		log.Fatal(err)
	}
}

// Event structure is defined here: https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events#event_stream_format
type Event struct {
	ID      []byte
	Data    []byte
	Event   []byte
	Retry   []byte
	Comment []byte
}

// WriteTo writes Event to given ResponseWriter
func (ev *Event) WriteTo(w http.ResponseWriter) error {
	// Marshalling part is taken from: https://github.com/r3labs/sse/blob/c6d5381ee3ca63828b321c16baa008fd6c0b4564/http.go#L16
	if len(ev.Data) == 0 && len(ev.Comment) == 0 {
		return nil
	}

	if len(ev.Data) > 0 {
		if _, err := fmt.Fprintf(w, "id: %s\n", ev.ID); err != nil {
			return err
		}

		sd := bytes.Split(ev.Data, []byte("\n"))
		for i := range sd {
			if _, err := fmt.Fprintf(w, "data: %s\n", sd[i]); err != nil {
				return err
			}
		}

		if len(ev.Event) > 0 {
			if _, err := fmt.Fprintf(w, "event: %s\n", ev.Event); err != nil {
				return err
			}
		}

		if len(ev.Retry) > 0 {
			if _, err := fmt.Fprintf(w, "retry: %s\n", ev.Retry); err != nil {
				return err
			}
		}
	}

	if len(ev.Comment) > 0 {
		if _, err := fmt.Fprintf(w, ": %s\n", ev.Comment); err != nil {
			return err
		}
	}

	if _, err := fmt.Fprint(w, "\n"); err != nil {
		return err
	}

	return nil
}

in the same folder as app create index.html

<!DOCTYPE html>
<html>
<body>

<h1>Getting server updates</h1>
<div id="result"></div>

<script>
  // Example taken from: https://www.w3schools.com/html/html5_serversentevents.asp
  if (typeof (EventSource) !== "undefined") {
    const source = new EventSource("/sse");
    source.onmessage = function (event) {
      document.getElementById("result").innerHTML += event.data + "<br>";
    };
  } else {
    document.getElementById("result").innerHTML = "Sorry, your browser does not support server-sent events...";
  }
</script>

</body>
</html>

Open http://localhost:8080 in your browser. You should see Ping messages streamed, assuming proxy middleware handles SSE requests as raw proxy

aldas changed the title ~~proxy middleware should use raw http proxy for SSE requests~~ proxy middleware should use raw proxy for SSE requests Apr 15, 2024

aldas force-pushed the proxy_sse branch from f62a7ee to c134df4 Compare April 15, 2024 17:47

proxy middleware should use http proxy for SSE requests

ebf0003

aldas force-pushed the proxy_sse branch from c134df4 to ebf0003 Compare April 15, 2024 18:05

aldas changed the title ~~proxy middleware should use raw proxy for SSE requests~~ proxy middleware should use httputil.ReverseProxy for SSE requests Apr 15, 2024

aldas merged commit e0f2a02 into labstack:master Apr 15, 2024
14 checks passed

aldas deleted the proxy_sse branch April 15, 2024 18:12

aldas mentioned this pull request Apr 15, 2024

Reverse proxy for Server-Sent Events #1172

Closed

renovate bot mentioned this pull request Nov 1, 2024

fix(deps): update all minor, patch, or digest dependencies on go cloudnativedaysjp/emtec-ecu#82

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proxy middleware should use httputil.ReverseProxy for SSE requests #2624

proxy middleware should use httputil.ReverseProxy for SSE requests #2624

aldas commented Apr 15, 2024 •

edited

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proxy middleware should use httputil.ReverseProxy for SSE requests #2624

proxy middleware should use httputil.ReverseProxy for SSE requests #2624

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aldas commented Apr 15, 2024 • edited Loading

aldas commented Apr 15, 2024 •

edited

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