Master MQTT: From Protocol Basics to Powerful Client Tools

MQTT

1. MQTT protocol description

Message Queuing Telemetry Transport (MQTT) is a lightweight message transport protocol based on the publish/subscribe model.

2. MQTT protocol origin

In 1999, Andy Stanford‑Clark and Arlen Nipper needed a protocol for satellite‑connected oil‑pipeline monitoring that minimized battery consumption and bandwidth, leading to the invention of MQTT.

3. MQTT protocol usage scenarios

It is suitable for IoT applications in low‑bandwidth and unstable network environments, providing real‑time reliable messaging with minimal code.

MQTT is widely used in IoT, mobile internet, smart hardware, vehicular networks, smart cities, telemedicine, power, oil and energy, and other fields.

4. MQTT protocol characteristics

Easy to implement

Lightweight and efficient; the smallest packet is only 2 bytes

Uses TCP for stable, reliable connections

Based on a publish‑subscribe model supporting duplex communication

Highly flexible and extensible

5. C/S and pub/sub difference

C/S model: client communicates directly with the server.

pub/sub: the publisher and subscriber are decoupled; communication is mediated by a broker.

6. Message filtering

The broker can filter messages so each subscriber receives only the messages it is interested in.

Filtering options:

1) Topic‑based filtering: each message has a topic; clients subscribe to topics of interest.

2) Content‑based filtering: the broker filters messages based on their payload content.

3) Type‑based filtering: common in object‑oriented languages, filtering by message (event) type.

7. MQTT vs. message queue

Message queue: a message can be consumed by only one client.

MQTT: every subscriber receives the message; all subscribers share the same payload.

8. MQTT client

Both publisher and subscriber are MQTT clients. The concept of publisher/subscriber simply indicates whether the client is sending or receiving messages.

MQTT clients are any devices running an MQTT library and connecting to an MQTT broker, ranging from micro‑controllers to full‑featured servers.

9. MQTT broker

The broker receives all messages, filters them, determines which clients subscribed to each message, and forwards them accordingly.

The broker also stores session data, handles authentication and authorization.

10. MQTT connection

The client initiates a connection by sending a CONNECT message; the broker responds with CONNACK and status codes. Once established, the connection remains open until the client disconnects or the link breaks.

11. Message type description

Publish message

After connecting, a client can publish messages. Each message includes a topic and a payload (data format defined by the publisher).

Subscribe message

The client sends a SUBSCRIBE message to the broker with the desired topics.

ACK messages

After subscribing, the broker replies with SUBACK. Unsubscribing is confirmed with UNSUBACK.

12. Topic related concepts

In MQTT, a topic is a UTF‑8 string used by the broker to filter messages for each client. Topics have a hierarchical structure separated by /.

Compared with traditional message queues, MQTT topics are lightweight; clients do not need to pre‑create topics before publishing or subscribing.

13. Topic wildcards

Clients can subscribe using exact topics or wildcards. Two types of wildcards exist:

Single‑level wildcard ( +) replaces one level of the topic hierarchy.

Multi‑level wildcard ( #) matches multiple levels and must be the last character in a topic, preceded by a /.

When a client subscribes with a multi‑level wildcard, it receives all messages that match the preceding part of the topic; using # alone receives every message.

MQTT client tools

1. Mosquitto

Mosquitto is a popular open‑source MQTT broker and client library that implements MQTT v5.0, v3.1.1, and v3.1.

After installation, Mosquitto provides two command‑line tools: mosquitto_pub and mosquitto_sub.

# Download
    https://github.com/eclipse/mosquitto
# Official site
    https://mosquitto.org/

mosquitto_pub

Publishes a message to an MQTT broker.

Parameters:

-h    # broker address (default localhost)
-p    # broker port
-u    # username for authentication
-P    # password for authentication
-m    # message payload
-t    # topic name
-i    # client identifier

Example:

# Publish "Hello, MQTT" to topic "topic/test" on localhost
mosquitto_pub -h localhost -t "topic/test" -m "Hello, MQTT"
# Publish to a remote broker
mosquitto_pub -h 10.0.0.33 -t "/xyz" -i "client3" -m "How are you?" -u bruce -P lan1532

mosquitto_sub

Subscribes to a topic on an MQTT broker.

# Subscribe to all messages on "topic/test" on localhost
mosquitto_sub -h localhost -t "topic/test"
# Subscribe with client ID and authentication
mosquitto_sub -h localhost -t "/xyz" -i "client1" -u bruce -P lan1532

Configuration file example (excerpt):

pid_file /var/run/mosquitto.pid
persistence true
persistence_location /var/lib/mosquitto/
log_dest file /var/log/mosquitto/mosquitto.log
include_dir /etc/mosquitto/conf.d
allow_anonymous false
password_file /mosquitto/pwdfile
acl_file /mosquitto/aclfile
port 1883
protocol mqtt
max_connections -1

To create user credentials:

# Create a new password file with user "tom"
mosquitto_passwd -c /mosquitto/pwdfile tom
# Edit ACL file to grant permissions
user tom
topic write /#
topic read /#

2. MQTTX

EMQX is a widely used, large‑scale distributed IoT MQTT server.

MQTTX is an open‑source, cross‑platform desktop client that supports full MQTT 5.0 features and runs on macOS, Linux, and Windows.

# EMQX download page: https://www.emqx.io/zh/downloads?os=Windows
# Project repository: https://github.com/emqx/MQTTX
# Release page: https://github.com/emqx/MQTTX/releases
# Official site: https://mqttx.app/zh