Basic application setup
Let's start by creating a basic Go client application. It will:
- Read a client identifier from command line flags
- Connect to a MQTT broker (we'll use the public
mqtt.eclipse.org:1338) - Subscribe to the MQTT topic
/e4go/demo/messagesand print any incoming messages to stdout - Wait for user input on stdin, so that the user can type in a message and press enter. Messages will then be published on the peer MQTT topic
/e4go/demo/messages.
Let's first move to an empty directory, and create our application file:
$ mkdir -p e4demo && \
cd e4demo && \
go mod init e4demo && \
touch e4demo.go
Now the code:
package main
import (
"bufio"
"flag"
"fmt"
"os"
"time"
mqtt "github.com/eclipse/paho.mqtt.golang"
)
func main() {
// 1 - Read a client identifier from a command line flag
var clientName string
flag.StringVar(&clientName, "client", "", "the client name")
flag.Parse()
if len(clientName) == 0 {
panic("-client is required")
}
// 2 - Connect to a MQTT broker (we'll use the public mqtt.eclipse.org:1338)
brokerEndpoint := "mqtt.eclipse.org:1883"
mqttClient, err := initMQTT(brokerEndpoint, clientName)
if err != nil {
panic(fmt.Sprintf("failed to init mqtt client: %v", err))
}
fmt.Printf("> connected to %s\n", brokerEndpoint)
// 3 - Subscribe to message MQTT topic and print incoming messages to stdout
messageTopic := "/e4go/demo/messages"
token := mqttClient.Subscribe(messageTopic, 1, func(_ mqtt.Client, msg mqtt.Message) {
fmt.Printf("< received raw message on %s: %s\n", msg.Topic(), msg.Payload())
})
timeout := time.Second
if !token.WaitTimeout(timeout) {
panic(fmt.Sprintf("failed to subscribe to MQTT topic: %v\n", token.Error()))
}
fmt.Printf("> subscribed to MQTT topic %s\n", messageTopic)
// 4 - Wait for user input on stdin and publish messages
// on the peer MQTT topic `/e4go/demo/messages` once user press the enter key.
fmt.Printf("> type anything and press enter to send the message to %s:\n", messageTopic)
scanner := bufio.NewScanner(os.Stdin)
for scanner.Scan() {
message := scanner.Text()
if len(message) == 0 { // Skip empty messages
continue
}
if token := mqttClient.Publish(messageTopic, 1, true, message); token.Error() != nil {
fmt.Printf("> failed to publish message: %v\n", token.Error())
continue
}
fmt.Println("> message published successfully")
}
}
We can now add our initMQTT function:
func initMQTT(brokerEndpoint, clientID string) (mqtt.Client, error) {
opts := mqtt.NewClientOptions()
opts.AddBroker(brokerEndpoint)
opts.SetClientID(clientID)
opts.SetCleanSession(true)
mqttClient := mqtt.NewClient(opts)
timeout := time.Second
if token := mqttClient.Connect(); token.WaitTimeout(timeout) && token.Error() != nil {
return nil, token.Error()
}
return mqttClient, nil
}
And we're done. We now have a basic application able to send and receive message over MQTT.
Click here to download the full source at this point
We can now try it and exchange messages between alice and bob.
Run in a first terminal and start an instance for alice, and type in the message for bob:
$ go run e4demo.go -client alice
> connected to mqtt.eclipse.org:1883
> subscribed to MQTT topic /e4go/demo/messages
> type anything and press enter to send the message to /e4go/demo/messages:
Hello, I'm alice, and this is a secret message for bob!
> message published successfully
And start a second one in another terminal for bob:
$ go run e4demo.go -client bob
> connected to mqtt.eclipse.org:1883
> subscribed to MQTT topic /e4go/demo/messages
< received raw message on /e4go/demo/messages: Hello, I'm alice, and this is a secret message for bob!
> type anything and press enter to send the message to /e4go/demo/messages:
alice and bob can now exchange messages between each others.
But the evil eve can sneak in, subscribe to the topic, and read / write messages:
$ go run e4demo.go -client eve
> connected to mqtt.eclipse.org:1883
< received raw message on /e4go/demo/messages: Hello, I'm alice, and this is a secret message for bob!
> subscribed to MQTT topic /e4go/demo/messages
> type anything and press enter to send the message to /e4go/demo/messages:
Let's jump to the next section and see how E4 can keep those message secret!