gotalk

gotalk

Gotalk exists to make it easy for programs to talk with one another over the internet, like a web app coordinating with a web server, or a bunch of programs dividing work amongst each other.

[][godoc] [][godoc] Gotalk...

• is an efficient, easily debuggable multiplexing data transfer protocol
• is transport agnostic: works on any byte stream
• offers a high-level, easy-to-get-started API for WebSockets
• enables arbitrary number of requests & responses over a single persistent connection
• includes a small built-in JavaScript library
• provides a small and focused [Go API][godoc]

[Go API Documentation on godoc.org →][godoc]

Usage

Gotalk is a simple go module - import it into your program and go build:

import "github.com/rsms/gotalk"

To use a specific version, run go get github.com/rsms/[email protected] (substituting the version number for the version you desire.)

Examples can be found in the examples directory. Build them with go build:

$ cd examples/websocket-chat
$ go build
$ ./websocket-chat
Listening on http://localhost:1235/

Here's a minimal but complete example program: (examples/websocket-minimal)

package main
import (
  "net/http"
  "github.com/rsms/gotalk"
)
func main() {
  gotalk.Handle("echo", func(in string) (string, error) {
    return in, nil
  })
  http.Handle("/gotalk/", gotalk.WebSocketHandler())
  http.Handle("/", http.FileServer(http.Dir(".")))
  print("Listening on http://localhost:1234/\n")
  panic(http.ListenAndServe("localhost:1234", nil))
}

Developing Gotalk & contributing

See CONTRIBUTING.md

Other implementations

Introduction

Gotalk takes the natural approach of bidirectional and concurrent communication — any peer have the ability to expose "operations" as well as asking other peers to perform operations. The traditional restrictions of who can request and who can respond usually associated with a client-server model is nowhere to be found in Gotalk.

Gotalk in a nutshell

Bidirectional — There's no discrimination on capabilities depending on who connected or who accepted. Both "servers" and "clients" can expose operations as well as send requests to the other side.

Concurrent — Requests, results, and notifications all share a single connection without blocking each other by means of pipelining. There's no serialization on request-result or even for a single large message, as the Gotalk protocol is frame-based and multiplexes messages over a single connection. This means you can perform several requests at once without having to think about queueing or blocking.

Simple — Gotalk has a simple and opinionated API with very few components. You expose an operation via "handle" and send requests via "request".

Debuggable — The Gotalk protocol's wire format is ASCII-based for easy on-the-wire inspection of data. For example, here's a protocol message representing an operation request: r0001005hello00000005world. The Gotalk protocol can thus be operated over any reliable byte transport.

Practical — Gotalk includes a JavaScript implementation for Web Sockets alongside the full-featured Go implementation, making it easy to build real-time web applications. The Gotalk source code also includes a number of easily-readable examples.

By example

There are a few examples in the examples directory demonstrating Gotalk. But let's explore a simple program right now — here's a little something written in Go which demonstrates the use of an operation named "greet":

func server() {
  gotalk.Handle("greet", func(in GreetIn) (GreetOut, error) {
    return GreetOut{"Hello " + in.Name}, nil
  })
  if err := gotalk.Serve("tcp", "localhost:1234"); err != nil {
    log.Fatalln(err)
  }
}

func client() {
  s, err := gotalk.Connect("tcp", "localhost:1234")
  if err != nil {
    log.Fatalln(err)
  }
  greeting := &GreetOut{}
  if err := s.Request("greet", GreetIn{"Rasmus"}, greeting); err != nil {
    log.Fatalln(err)
  }
  log.Printf("greeting: %+v\n", greeting)
  s.Close()
}

Let's look at the above example in more detail, broken apart to see what's going on.

We begin by importing the gotalk library together with log which we use for printing to the console:

package main
import (
  "log"
  "github.com/rsms/gotalk"
)

We define two types: Expected input (request parameters) and output (result) for our "greet" operation:

type GreetIn struct {
  Name string `json:"name"`
}
type GreetOut struct {
  Greeting string `json:"greeting"`
}

Registers a process-global request handler for an operation called "greet" accepting parameters of type GreetIn, returning results of type GreetOut:

func server() {
  gotalk.Handle("greet", func(in GreetIn) (GreetOut, error) {
    return GreetOut{"Hello " + in.Name}, nil
  })

Finally at the bottom of our server function we call gotalk.Serve, which starts a local TCP server on port 1234:

  if err := gotalk.Serve("tcp", "localhost:1234"); err != nil {
    log.Fatalln(err)
  }
}

In out client function we start by connecting to the server:

func client() {
  s, err := gotalk.Connect("tcp", "localhost:1234")
  if err != nil {
    log.Fatalln(err)
  }

Finally we send a request for "greet" and print the result:

  greeting := GreetOut{}
  if err := s.Request("greet", GreetIn{"Rasmus"}, &greeting); err != nil {
    log.Fatalln(err)
  }
  log.Printf("greeting: %+v\n", greeting)

  s.Close()
}

Output:

greeting: {Greeting:Hello Rasmus}

Gotalk in the web browser

Gotalk is implemented not only in the full-fledged Go package, but also in a JavaScript library. This allows writing web apps talking Gotalk via Web Sockets possible.

// server.go:
package main
import (
  "net/http"
  "github.com/rsms/gotalk"
)
func main() {
  gotalk.Handle("echo", func(in string) (string, error) {
    return in, nil
  })
  http.Handle("/gotalk/", gotalk.WebSocketHandler())
  http.Handle("/", http.FileServer(http.Dir(".")))
  err := http.ListenAndServe("localhost:1234", nil)
  if err != nil {
    panic(err)
  }
}

In our html document, we begin by registering any operations we can handle:

Notice how we load a JavaScript from "/gotalk/gotalk.js" — a gotalk web socket server embeds a matching web browser JS library which it returns from {path where gotalk web socket is mounted}/gotalk.js. It uses Etag cache validation, so you shouldn't need to think about "cache busting" the URL.

We can't "listen & accept" connections in a web browser, but we can "connect" so we do just that:

This is enough for enabling the server to do things in the browser ...

But you probably want to have the browser send requests to the server, so let's send a "echo" request just as our connection opens:

var s = gotalk.connection().on('open', function () {
  s.request("echo", "Hello world", function (err, result) {
    if (err) return console.error('echo failed:', err);
    console.log('echo result:', result);
  });
});

We could rewrite our code like this to allow some UI component to send a request:

var s = gotalk.connection();

button.addEventListener('click', function () {
  s.request("echo", "Hello world", function (err, result) {
    if (err) return console.error('echo failed:', err);
    console.log('echo result:', result);
  });
});

The request will fail with an error "socket is closed" if the user clicks our button while the connection isn't open.

There are two ways to open a connection on a socket: Sock.prototype.open which simply opens a connection, and Sock.prototype.openKeepAlive which keeps the connection open, reconnecting as needed with exponential back-off and internet reachability knowledge. gotalk.connection() is a short-hand for creating a new Sock with gotalk.defaultHandlers and then calling openKeepAlive on it.

Protocol and wire format

The wire format is designed to be human-readable and flexible; it's byte-based and can be efficiently implemented in a number of environments ranging from HTTP and WebSocket in a web browser to raw TCP in Go or C. The protocol provides only a small set of operations on which more elaborate operations can be modeled by the user.

This document describes protocol version 1

Here's a complete description of the protocol:

conversation    = ProtocolVersion Message*
message         = SingleRequest | StreamRequest
                | SingleResult | StreamResult
                | ErrorResult | RetryResult
                | Notification | ProtocolError

ProtocolVersion =  

SingleRequest   = "r" requestID operation payload
StreamRequest   = "s" requestID operation payload StreamReqPart*
StreamReqPart   = "p" requestID payload
SingleResult    = "R" requestID payload
StreamResult    = "S" requestID payload StreamResult*
ErrorResult     = "E" requestID payload
RetryResult     = "e" requestID wait payload
Notification    = "n" name payload
Heartbeat       = "h" load time
ProtocolError   = "f" code

requestID       =    

operation       = text3
name            = text3
wait            = hexUInt8
code            = hexUInt8
time            = hexUInt8
load            = hexUInt4

text3           = text3Size text3Value
text3Size       = hexUInt3
text3Value      = 

payload         = payloadSize payloadData
payloadSize     = hexUInt8
payloadData     = {payloadSize}

hexUInt3        =   
hexUInt4        =    
hexUInt8        =    

Handshake

A conversation begins with the protocol version:

01  -- ProtocolVersion 1

If the version of the protocol spoken by the other end is not supported by the reader, a ProtocolError message is sent with code 1 and the connection is terminated. Otherwise, any messages are read and/or written.

Single-payload requests and results

This is a "single-payload" request ...

+------------------ SingleRequest
|   +---------------- requestID   "0001"
|   |      +--------- operation   "echo" (text3Size 4, text3Value "echo")
|   |      |       +- payloadSize 25
|   |      |       |
r0001004echo00000019{"message":"Hello World"}

... and a corresponding "single-payload" result:

+------------------ SingleResult
|   +---------------- requestID   "0001"
|   |       +-------- payloadSize 25
|   |       |
R000100000019{"message":"Hello World"}

Each request is identified by exactly three bytes—the requestID—which is requestor-specific and has no purpose beyond identity, meaning the value is never interpreted. 4 bytes can express 4 294 967 296 different values, meaning we can send up to 4 294 967 295 requests while another request is still being served. Should be enough.

These "single" requests & results are the most common protocol messages, and as their names indicates, their payloads follow immediately after the header. For large payloads this can become an issue when dealing with many concurrent requests over a single connection, for which there's a more complicated "streaming" request & result type which we will explore later on.

Faults

There are two types of replies indicating a fault: ErrorResult for requestor faults and RetryResult for responder faults.

If a request is faulty, like missing some required input data or sent over an unauthorized connection, an "error" is send as the reply instead of a regular result:

+------------------ ErrorResult
|   +---------------- requestID   "0001"
|   |       +-------- payloadSize 38
|   |       |
E000100000026{"error":"Unknown operation \"echo\""}

A request that produces an error should not be retried as-is,