🔹 Will Message

  • A special message defined by the client when it connects.
  • Published by the broker if the client disconnects unexpectedly (without sending a proper DISCONNECT).
  • Example: "Device123 offline"

🔹 Will Topic

  • The MQTT topic where the broker will publish the will message.
  • Other clients subscribed to this topic will receive the notification.

MQTT QoS (Quality of Service)

In MQTT, QoS (Quality of Service) defines the guarantee level of message delivery between a publisher and a subscriber.
There are 3 levels:

🔹 QoS 0 — "At most once"

  • Meaning: The message is delivered at most once, best effort, no acknowledgment.
  • If the client or broker goes offline during transmission, the message may be lost.
  • Use case: Non-critical data, such as sensor readings that update frequently.

🔹 QoS 1 — "At least once"

  • Meaning: The message is delivered at least once.
  • Broker requires acknowledgment (PUBACK).
  • Duplicates may occur (subscriber could get the same message more than once).
  • Use case: Important data that must arrive, but duplicates are acceptable (e.g., logging, notifications).

🔹 QoS 2 — "Exactly once"

  • Meaning: The message is delivered exactly once.
  • Uses a 4-step handshake (PUBREC, PUBREL, PUBCOMP).
  • No duplicates, no missing messages.
  • Highest reliability but more overhead (slower and heavier).
  • Use case: Billing systems, financial transactions, or critical commands.

AT MQTT workflow

pub

purpose AT command note
Start MQTT service AT+CMQTTSTART returns +CMQTTSTART: 0 on success
Acquire a client AT+CMQTTACCQ=0,"a12mmmm",0 0 is the client_index, "a12mmmm" is the client ID, 0 == TCP
Set Will Topic AT+CMQTTWILLTOPIC=0,10 0 is the client_index, 10 is the length of the topic string
Set Will Message AT+CMQTTWILLMSG=0,6,1 0 is the client_index, 6 is the length of the message string, 1 is the QoS level
Connect to Broker AT+CMQTTCONNECT=0,"tcp://broker.hivemq.com:1883",60,1 0 is the client_index, 60 is the keep-alive time in seconds, 1 indicates clean session
Input the topic of publish message AT+CMQTTTOPIC=0,9
Input the publish message AT+CMQTTPAYLOAD=0,4 0 is the client_index, 4 is the length of the message string
Publish the message AT+CMQTTPUB=0,1,60 0 is the client_index, 1 is the QoS level, 60 means pub_timeout

sub | purpose | AT command | note | | ------------------------------------ | ------------------- | ----------------------------------------------------------------- | | Input the topic of subscribe message | AT+CMQTTTOPIC=0,4 | /* subscribe one or more topics which input by AT+CMQTTSUBTOPIC/ | | Subscribe a message to server | AT+CMQTTSUB=0,4,0 | / subcribe one topic*/ |

arduino library

MQTT broker on ubuntu

install service and check status 

sudo apt install -y mosquitto

sudo systemctl status mosquitto

more systemctl check

  • Stop the mosquitto service:
    • $ sudo systemctl stop mosquitto
  • Start the mosquitto service:
    • $ sudo systemctl start mosquitto
  • Restart the mosquitto service:
    • $ sudo systemctl restart mosquitto

Log file 

cat /var/log/mosquitto/mosquitto.log

Conf Setup list

Secure the Mosquitto Server

Create a default.conf under the directory.

nano /etc/mosquitto/conf.d/default.conf

allow_anonymous false  // not allow anonymous
password_file /etc/mosquitto/passwd // set password

optionally 

listener 1883  // set port 1883 public, or listener 1883 localhost for localhost only
listener 1884 // set 1884 for wss protocol websockets

set users 

nano /etc/mosquitto/passwd

electrodragon:electrodragon

Restart the mosquitto service to load the new changes.

sudo systemctl restart mosquitto

MQTT client

sudo apt install -y mosquitto-clients

sub:

mosquitto_sub -t "test"
mosquitto_sub -u electrodragon -P electrodragon -t "test"

pub:

mosquitto_pub -m "ON" -t "test"

A number of my IoT students make use of (the FREE plan with) BeeBotte for their remote MQTT broker.

https://beebotte.com/

It's very easy to set up and works really well with Nod-RED.

execute

From this point forward, you should execute any pub/sub command using the syntax below. Remember to replace electrodragon and EXAMPLE_PASSWORD with the credentials that you defined in the password file.

mosquitto_sub -u electrodragon -P electrodragon -t "home/lights/sitting_room"
mosquitto_pub -u electrodragon -P electrodragon -t "home/lights/sitting_room" -m "ON"

free electrodragon MQTT broker

  • 206.237.31.27
  • user == electrodragon
  • password == electrodragon
  • SSL not enabled

ref

Table of Contents

https://www.electrodragon.com