seamless / seamless.go

/* A TCP proxy that allow you to deploy new code then switch traffic to it
   without downtime.

   It does "round robin" between the list of current active backends.

   Switching server is done with HTTP interface with the following API:
   /set?backends=host:port,host:port - will set list of backends
   /add?backend=host:port - will add a backend
   /remove?backend=host:port - will remove a backend
   /get - will return host:port,host:port

   Work flow:
	   Start first backend at port 4444
	   Run `./seamless 8080 localhost:4444`

	   Direct all traffic to port 8080 on local machine.

	   When you need to upgrade the backend, start a new one (with new code on
	   a different port, say 4445).
			* `curl http://localhost:6777/add?backend=localhost:4445`
			* `curl http://localhost:6777/remove?backend=localhost:4444`
		`curl http://localhost:6777/set?backends=localhost:4445`

	   New traffic will be directed to new server(s).

Original forward code by Roger Peppe (see
package main

import (

const (
	Version = "0.2.1"

// List of backends
var backends *Backends = &Backends{}

// backend regular expression (<host>:<port>)
var backendRe *regexp.Regexp = regexp.MustCompile("^[^:]+:[0-9]+$")

// isValidBackend returns true if backend is in "host:port" format
func isValidBackend(backend string) bool {
	return backendRe.MatchString(backend)

// parseBackends parses string in format "host:port,host:port" and return list of backends
func parseBackends(str string) ([]string, error) {
	backends := strings.Split(str, ",")
	if len(backends) == 0 {
		return nil, fmt.Errorf("no backends")

	for i, v := range backends {
		backends[i] = strings.TrimSpace(v)
		if !isValidBackend(backends[i]) {
			return nil, fmt.Errorf("'%s' is not valid network address", backends[i])

	return backends, nil

// forward proxies traffic between local socket and remote backend
func forward(local net.Conn, remoteAddr string) {
	remote, err := net.Dial("tcp", remoteAddr)
	if remote == nil {
		log.Printf("remote dial failed: %v\n", err)
	go io.Copy(local, remote)
	go io.Copy(remote, local)

// die prints error message and aborts the program
func die(format string, args ...interface{}) {
	msg := fmt.Sprintf(format, args...)
	fmt.Fprintf(os.Stderr, "error: %s\n", msg)

// startHttpServer start the HTTP server interface in a given port
func startHttpServer(port int) error {
	http.HandleFunc("/set", setHandler)
	http.HandleFunc("/get", getHandler)
	http.HandleFunc("/add", addHandler)
	http.HandleFunc("/remove", removeHandler)

	return http.ListenAndServe(fmt.Sprintf(":%d", port), nil)

// getHandler handles /current and return the current backend
func getHandler(w http.ResponseWriter, req *http.Request) {
	w.Header().Set("Content-Type", "text/plain")
	fmt.Fprintf(w, "%s\n", backends)

// setHandler handler /set and sets backends
func setHandler(w http.ResponseWriter, req *http.Request) {
	newBackends, err := parseBackends(req.FormValue("backends"))
	if err != nil {
		msg := fmt.Sprintf("error: %s", err)
		http.Error(w, msg, http.StatusBadRequest)

	getHandler(w, req)

// addHandler handles /add to add a new backend
func addHandler(w http.ResponseWriter, req *http.Request) {
	backend := req.FormValue("backend")
	if len(backend) == 0 {
		msg := "error: missing 'backend' parameter"
		http.Error(w, msg, http.StatusBadRequest)

	getHandler(w, req)

// removeHandler handles /remove and remove a backend
func removeHandler(w http.ResponseWriter, req *http.Request) {
	err := ""

	defer func() {
		if len(err) != 0 {
			log.Printf("error: %s\n", err)
			http.Error(w, err, http.StatusBadRequest)
		} else {
			getHandler(w, req)

	backend := req.FormValue("backend")
	if len(backend) == 0 {
		err = "missing 'backend' parameter"

	count := backends.Remove(backend)
	if count == 0 {
		err = fmt.Sprintf("backend '%s' not found", backend)

// seamless launches the HTTP API and then start proxying
func seamless(localAddr string, apiPort int, backendList []string, out chan error) {
	local, err := net.Listen("tcp", localAddr)
	if local == nil {
		out <- fmt.Errorf("cannot listen: %v", err)


	go func() {
		if err := startHttpServer(apiPort); err != nil {
			out <- fmt.Errorf("cannot listen on %d: %v", apiPort, err)

	for {
		conn, err := local.Accept()
		if conn == nil {
			die("accept failed: %v", err)
		backend, err := backends.Next()
		if err != nil {
			log.Printf("error: can't get next backend %v\n", err)
		go forward(conn, backend)

func main() {
	flag.Usage = func() {
		fmt.Fprintf(os.Stderr, "usage: seamless LISTEN_PORT BACKENDS\n")
		fmt.Fprintf(os.Stderr, "command line switches:\n")
	port := flag.Int("httpPort", 6777, "http interface port")
	version := flag.Bool("version", false, "show version and exit")

	if *version {
		fmt.Printf("seamless %s\n", Version)

	if flag.NArg() != 2 {
	localAddr := fmt.Sprintf(":%s", flag.Arg(0))

	var err error
	backendList, err := parseBackends(flag.Arg(1))
	if err != nil {
		die(fmt.Sprintf("%s", err))

	out := make(chan error)
	go seamless(localAddr, *port, backendList, out)

	err = <-out
	if err != nil {
		die("%s", err)
Tip: Filter by directory path e.g. /media app.js to search for public/media/app.js.
Tip: Use camelCasing e.g. ProjME to search for
Tip: Filter by extension type e.g. /repo .js to search for all .js files in the /repo directory.
Tip: Separate your search with spaces e.g. /ssh pom.xml to search for src/ssh/pom.xml.
Tip: Use ↑ and ↓ arrow keys to navigate and return to view the file.
Tip: You can also navigate files with Ctrl+j (next) and Ctrl+k (previous) and view the file with Ctrl+o.
Tip: You can also navigate files with Alt+j (next) and Alt+k (previous) and view the file with Alt+o.