MQTT🤖

About

MQTT is a protocol for message queueing over a network. This project provides a client library for the Go programming language. Message-delivery guarantees are maintained at all costs, even on (protocol, network or persistence) errors. The client recovers from errors atomatically. Message transfers in both directions do zero-copy.

The development was kindly sponsored by Northvolt, as a gift to the open-source community.

This is free and unencumbered software released into the public domain.

Introduction

The client supports confirmed message delivery with full progress disclosure. Message transfers without an confirmation can be as simple as the following.

err := client.Publish(ctx.Done(), []byte("20.8℃"), "bedroom")
if err != nil {
	log.Print("thermostat update lost: ", err)
	return
}

A read routine sees inbound messages from any of the subscribed topics.

for {
	message, topic, err := client.ReadSlices()
	switch {
	case err == nil:
		r, _ := utf8.DecodeLastRune(message)
		switch r {
		case 'K', '℃', '℉':
			log.Printf("%q at %q", message, topic)
		}

	case errors.Is(err, mqtt.ErrClosed):
		return // client terminated

	default:
		log.Print("broker unavailable: ", err)
		time.Sleep(time.Second) // backoff
	}
}

The examples from the package documentation provide more detail on error reporting and the delivery alternatives.

Command-Line Client

Run go install github.com/pascaldekloe/mqtt/cmd/mqttc@latest to build the binary.

NAME
	mqttc — MQTT broker access

SYNOPSIS
	mqttc [options] address

DESCRIPTION
	The command connects to the address argument, with an option to
	publish a message and/or subscribe with topic filters.

	When the address does not specify a port, then the defaults are
	applied, which is 1883 for plain connections and 8883 for TLS.

OPTIONS
  -ca file
    	Amend the trusted certificate authorities with a PEM file.
  -cert file
    	Use a client certificate from a PEM file (with a corresponding
    	-key option).
  -client identifier
    	Use a specific client identifier. (default "generated")
  -key file
    	Use a private key (matching the client certificate) from a PEM
    	file.
  -net name
    	Select the network by name. Valid alternatives include tcp4,
    	tcp6 and unix. (default "tcp")
  -pass file
    	The file content is used as a password.
  -prefix string
    	Print a string before each inbound message.
  -publish topic
    	Send a message to a topic. The payload is read from standard
    	input.
  -quiet
    	Suppress all output to standard error. Error reporting is
    	deduced to the exit code only.
  -quote
    	Print inbound topics and messages as quoted strings.
  -server name
    	Use a specific server name with TLS
  -subscribe filter
    	Listen with a topic filter. Inbound messages are printed to
    	standard output until interrupted by a signal(3). Multiple
    	-subscribe options may be applied together.
  -suffix string
    	Print a string after each inbound message. (default "\n")
  -timeout duration
    	Network operation expiry. (default 4s)
  -tls
    	Secure the connection with TLS.
  -topic
    	Print the respective topic of each inbound message.
  -user name
    	The user name may be used by the broker for authentication
    	and/or authorization purposes.
  -verbose
    	Produces more output to standard error for debug purposes.

EXIT STATUS
	(0) no error
	(1) MQTT operational error
	(2) illegal command invocation
	(5) connection refused: unacceptable protocol version
	(6) connection refused: identifier rejected
	(7) connection refused: server unavailable
	(8) connection refused: bad username or password
	(9) connection refused: not authorized
	(130) close on SIGINT
	(143) disconnect on SIGTERM

EXAMPLES
	Send a message:

		echo "hello" | mqttc -publish chat/misc localhost

	Print messages:

		mqttc -subscribe "news/#" -prefix "📥 " :1883

	Health check:

		mqttc -tls q1.example.com:8883 || echo "exit $?"

BUGS
	Report bugs at <https://github.com/pascaldekloe/mqtt/issues>.

SEE ALSO
	mosquitto_pub(1)

Standard Compliance

The implementation follows version 3.1.1 of the OASIS specification in a strict manner. Support for the originating IBM specification may be added at some point in time.

There are no plans to support protocol version 5. Version 3 is lean and well suited for IOT. The additions in version 5 may be more of a fit for backend computing.

See the Broker wiki for implementation specifics.