MQTT

MQTT is a lightweight publish/subscribe messaging protocol designed for IoT and M2M communication.

Overview

Connecting to a Broker

1. Select MQTT from the protocol dropdown
2. Configure the connection:
   • Broker: localhost or mqtt.example.com
   • Port: 1883 (default) or 8883 (TLS)
   • Client ID: Auto-generated or custom
3. Click Connect

Connection Settings

Field	Description	Default
Broker	Hostname or IP	-
Port	Broker port	1883
Client ID	Unique identifier	Auto-generated
Username	Auth username	(optional)
Password	Auth password	(optional)
Use TLS	Enable encryption	false

Subscribing to Topics

Once connected:

1. Enter a topic pattern in the Topic field
2. Click Subscribe

Topic Patterns

Pattern	Matches
sensors/temperature	Exact topic
sensors/+	Single-level wildcard
sensors/#	Multi-level wildcard

Examples:

• home/+/temperature matches home/living/temperature, home/bedroom/temperature
• home/# matches all topics under home/

Publishing Messages

1. Enter the target Topic
2. Type your message
3. Select QoS level
4. Toggle Retain if needed
5. Click Publish

QoS Levels

Level	Name	Guarantee
0	At most once	Fire and forget
1	At least once	Acknowledged delivery
2	Exactly once	Four-part handshake

Retained Messages

When Retain is enabled, the broker stores the last message for the topic. New subscribers immediately receive this message.

Message Log

The message log shows:

Column	Description
Direction	↑ Published / ↓ Received
Topic	Message topic
QoS	Quality of Service level
Payload	Message content
Timestamp	When sent/received

Playground Example

The playground includes an MQTT broker:

Broker: localhost
Port: 19511

Publish Temperature

Topic: sensors/temperature
Message: {"value": 22.5, "unit": "celsius"}
QoS: 1
Retain: false

Subscribe to Sensors

Topic: sensors/#

This will receive all messages published to any sensors/ subtopic.

Example: IoT Sensor

Publisher (Sensor)

Topic: home/living-room/temperature
QoS: 1
Message:
{
  "device": "sensor-001",
  "temperature": 23.5,
  "humidity": 45,
  "timestamp": "2024-01-15T10:30:00Z"
}

Subscriber (Dashboard)

Topic: home/+/temperature

Receives temperature from all rooms.

Authentication

For secured brokers:

1. Enter Username and Password
2. Enable Use TLS for encrypted connections
3. Connect

Troubleshooting

Connection Failed

• Verify broker hostname and port
• Check if the broker is running
• Ensure firewall allows the connection

No Messages Received

• Verify topic subscription matches published topic
• Check if wildcard patterns are correct
• Ensure QoS levels are compatible

Authentication Error

• Verify username and password
• Check if the user has subscribe/publish permissions