# Connect to Deployment via Paho Go

This article guides you to use the paho.mqtt.golang Library in go project to realize the functions of connecting, subscribing, sending, and receiving messages between a MQTT client and a MQTT broker.

Go (opens new window) it's Google Developed a programming language with strong static type, compilation type, parallel style, and garbage collection function. paho.mqtt.golang is an mqtt library, which provides a simple API for connecting to the mqtt server and sending and receiving messages in the go project.

# Prerequisites

# Deploy MQTT Broker

You can use the free public MQTT broker (opens new window) provided by EMQX. This service was created based on the EMQX Cloud (opens new window). The information about broker access is as follows:
- Broker: broker.emqx.io
- TCP Port: 1883
- TLS/SSL Port: 8883
You can create a deployment (opens new window) as well. Find connection information in the deployment overview. Make sure the deployment is running. At the same time, you can use WebSocket to test the connection to the MQTT server. If you are creating your own deployment, check Authentication (opens new window) and set the username and password in Authentication & ACL > Authentication for verification.

# Installation Dependency

The project uses go version 1.15.13 for development and testing. The following commands can be used to confirm the go version.

➜ ~ go version
go version go1.15.13 darwin/amd64

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Create a new folder go-mqtt, enter the folder and run the following command
```
➜ ~ go mod init go-mqtt
go: creating new go.mod: module go-mqtt
```
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Use the go get <Library> command to install paho.mqtt.golang

➜ ~ go get github.com/eclipse/paho.mqtt.golang
go: downloading github.com/eclipse/paho.mqtt.golang v1.3.5
go: github.com/eclipse/paho.mqtt.golang upgrade => v1.3.5

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Create the main.go file and import the paho.mqtt.golang client

package main

import (
    mqtt "github.com/eclipse/paho.mqtt.golang"
)

func main(){
}

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# Connect over TCP Protocol

This section introduces how to connect to MQTT Broker over TCP Protocol.

Connection settings
The example code will use a public MQTT server for connection. If you have created your own deployment, please find the corresponding connection address in the deployment console and set the username and password accordingly.
```
const protocol = "tcp"
const broker = "broker.emqx.io" // MQTT Broker address
const port = 1883
const topic = "t/1"
const username = "emqx"
const password = "******"
```
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The Key Code of Connection

We write a function to create and return a mqtt client.

func createMqttClient() mqtt.Client {
    connectAddress := fmt.Sprintf("%s://%s:%d", protocol,broker, port)
    client_id := fmt.Sprintf("go-client-%d", rand.Int())

    fmt.Println("connect address: ", connectAddress)
    opts := mqtt.NewClientOptions()
    opts.AddBroker(connectAddress)
    opts.SetUsername(username)
    opts.SetPassword(password)
    opts.SetClientID(client_id)
    opts.SetKeepAlive(time.Second * 60)
    client := mqtt.NewClient(opts)
    token := client.Connect()
    // if connection failed, exit
    if token.WaitTimeout(3*time.Second) && token.Error() != nil {
        log.Fatal(token.Error())
    }
    return client
}

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# Connect over SSL/TLS Protocol

This section introduces how to connect to MQTT Broker over SSL/TLS Protocol.

Connection settings
Set port to 8883 and protocol type to "ssl".
```
 const protocol = "ssl"
 const port = 8883
```
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The Key Code of Connection

If you need to configure the server certificate, you can load the certificate using function loadTLSConfig(). If not, the connection code is the same as TCP protocol.

func createMqttClient() mqtt.Client {
    connectAddress := fmt.Sprintf("%s://%s:%d", protocol, broker, port)
    client_id := fmt.Sprintf("go-client-%d", rand.Int())

    fmt.Println("connect address: ", connectAddress)
    opts := mqtt.NewClientOptions()
    opts.AddBroker(connectAddress)
    opts.SetUsername(username)
    opts.SetPassword(password)
    opts.SetClientID(client_id)
    opts.SetKeepAlive(time.Second * 60)

    // Optional: set server CA
    // opts.SetTLSConfig(loadTLSConfig("caFilePath"))

    client := mqtt.NewClient(opts)
    token := client.Connect()
    if token.WaitTimeout(3*time.Second) && token.Error() != nil {
        log.Fatal(token.Error())
    }
    return client
}

func loadTLSConfig(caFile string) *tls.Config {
    // load tls config
    var tlsConfig tls.Config
    tlsConfig.InsecureSkipVerify = false
    if caFile != "" {
        certpool := x509.NewCertPool()
        ca, err := ioutil.ReadFile(caFile)
        if err != nil {
            log.Fatal(err.Error())
        }
        certpool.AppendCertsFromPEM(ca)
        tlsConfig.RootCAs = certpool
    }
    return &tlsConfig
}

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This section introduces how to subscribe to topics and publish messages after you successfully connect to the MQTT broker.

# Publish Message

Defines a publish function for publishing messages.

In this function, we define an unconditional loop to publish a message every 1s. Count the number of published messages through msgCount.

func publish(client mqtt.Client) {
    qos := 0
    msgCount := 0
    for {
        payload := fmt.Sprintf("message: %d!", msgCount)
        if token := client.Publish(topic, byte(qos), false, payload); token.Wait() && token.Error() != nil {
            fmt.Printf("publish failed, topic: %s, payload: %s\n", topic, payload)
        } else {
            fmt.Printf("publish success, topic: %s, payload: %s\n", topic, payload)
        }
        msgCount++
        time.Sleep(time.Second * 1)
    }
}

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Define subscription function to subscribe to messages. This function prints the topic and payload of the message on the console.

func subscribe(client mqtt.Client) {
    qos := 0
    client.Subscribe(topic, byte(qos), func(client mqtt.Client, msg mqtt.Message) {
        fmt.Printf("Received `%s` from `%s` topic", msg.Payload(), msg.Topic())
    })
}

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# UnSubscribe Topic

Unsubscribe by the following code. You should specify the topic for unsubscription.

client.Unsubscribe(topic)

# Disconnection

If the client wishes to actively disconnect, it can be done with the following code.

// the parameter specify the waiting time before disconnection
client.Disconnect(250)

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# Complete code

package main

import (
	"crypto/tls"
	"crypto/x509"
	"fmt"
	"io/ioutil"
	"log"
	"math/rand"
	"time"

	mqtt "github.com/eclipse/paho.mqtt.golang"
)

const protocol = "tcp"
const broker = ""
const port = 1883
const topic = "t/1"
const username = ""
const password = ""

func main() {
	client := createMqttClient()
	go subscribe(client)        // we use goroutine to run the subscription function
	time.Sleep(time.Second * 1) // pause 1s to wait for the subscription function to be ready
	publish(client)
}

func createMqttClient() mqtt.Client {
	connectAddress := fmt.Sprintf("%s://%s:%d", protocol, broker, port)
	client_id := fmt.Sprintf("go-client-%d", rand.Int())

	fmt.Println("connect address: ", connectAddress)
	opts := mqtt.NewClientOptions()
	opts.AddBroker(connectAddress)
	opts.SetUsername(username)
	opts.SetPassword(password)
	opts.SetClientID(client_id)
	opts.SetKeepAlive(time.Second * 60)

	// Optional: set server CA
	// opts.SetTLSConfig(loadTLSConfig("caFilePath"))

	client := mqtt.NewClient(opts)
	token := client.Connect()
	if token.WaitTimeout(3*time.Second) && token.Error() != nil {
		log.Fatal(token.Error())
	}
	return client
}

func publish(client mqtt.Client) {
	qos := 0
	msgCount := 0
	for {
		payload := fmt.Sprintf("message: %d!", msgCount)
		if token := client.Publish(topic, byte(qos), false, payload); token.Wait() && token.Error() != nil {
			fmt.Printf("publish failed, topic: %s, payload: %s\n", topic, payload)
		} else {
			fmt.Printf("publish success, topic: %s, payload: %s\n", topic, payload)
		}
		msgCount++
		time.Sleep(time.Second * 1)
	}
}

func subscribe(client mqtt.Client) {
	qos := 0
	client.Subscribe(topic, byte(qos), func(client mqtt.Client, msg mqtt.Message) {
		fmt.Printf("Received `%s` from `%s` topic\n", msg.Payload(), msg.Topic())
	})
}

func loadTLSConfig(caFile string) *tls.Config {
	// load tls config
	var tlsConfig tls.Config
	tlsConfig.InsecureSkipVerify = false
	if caFile != "" {
		certpool := x509.NewCertPool()
		ca, err := ioutil.ReadFile(caFile)
		if err != nil {
			log.Fatal(err.Error())
		}
		certpool.AppendCertsFromPEM(ca)
		tlsConfig.RootCAs = certpool
	}
	return &tlsConfig
}

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# Test

Execution procedure

➜ ~ go run main.go
connect address:  tcp://***.***.***.***:1883
publish success, topic: t/1, payload: message: 0!
Received `message: 0!` from `t/1` topic
publish success, topic: t/1, payload: message: 1!
Received `message: 1!` from `t/1` topic
publish success, topic: t/1, payload: message: 2!
Received `message: 2!` from `t/1` topic
publish success, topic: t/1, payload: message: 3!
Received `message: 3!` from `t/1` topic
publish success, topic: t/1, payload: message: 4!
Received `message: 4!` from `t/1` topic
publish success, topic: t/1, payload: message: 5!
Received `message: 5!` from `t/1` topic
publish success, topic: t/1, payload: message: 6!

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go pubsub

# More Information

In summary, we have implemented the creation of an MQTT connection in a Go project, simulated subscribing, sending and receiving messages, unsubscribing, and disconnecting between the client and MQTT broker. You can download the source code of the example here (opens new window), and you can also find more demo examples in other languages on GitHub (opens new window).

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