MQTT broker role in IOT Protocols - Mini Project: Build & Apply

Understanding the MQTT Broker Role

📖 Scenario: You are working on a smart home system where multiple devices communicate using MQTT. You need to understand how the MQTT broker manages message delivery between devices.

🎯 Goal: Build a simple simulation of an MQTT broker role using Python dictionaries and functions to show how messages are received from publishers and sent to subscribers.

📋 What You'll Learn

Create a dictionary to hold topics and their subscribers

Add a function to register subscribers to topics

Add a function to publish messages to a topic and deliver to subscribers

Print the messages delivered to each subscriber

💡 Why This Matters

🌍 Real World

MQTT brokers are central in IoT systems to route messages between devices like sensors, lights, and controllers.

💼 Career

Understanding MQTT broker roles helps in configuring and troubleshooting IoT networks and cloud messaging services.

1

Create the topics dictionary

Create a dictionary called topics with these exact keys: 'home/livingroom' and 'home/kitchen'. Each key should have an empty list as its value to hold subscribers.

IOT Protocols

# Create the topics dictionary with empty subscriber lists
# Your code here

Hint

Think of topics as channels where devices subscribe to get messages.

2

Add subscriber registration function

Add a function called subscribe that takes topic and client as parameters and appends client to the list in topics[topic].

IOT Protocols

topics = {'home/livingroom': [], 'home/kitchen': []}

# Define subscribe(topic, client) to add client to topics[topic]
# Your code here

Hint

This function lets a device say it wants messages from a topic.

3

Add publish function to send messages

Add a function called publish that takes topic and message as parameters. It should loop over each client in topics[topic] and print "Delivering '{message}' to {client}".

IOT Protocols

topics = {'home/livingroom': [], 'home/kitchen': []}

def subscribe(topic, client):
    topics[topic].append(client)

# Define publish(topic, message) to deliver message to all clients subscribed to topic
# Your code here

Hint

This simulates the broker sending messages to all subscribers of a topic.

4

Subscribe clients and publish messages

Use subscribe to add 'device1' and 'device2' to 'home/livingroom', and 'device3' to 'home/kitchen'. Then call publish to send 'Lights on' to 'home/livingroom' and 'Temperature 22C' to 'home/kitchen'. This will print the delivery messages.

IOT Protocols

topics = {'home/livingroom': [], 'home/kitchen': []}

def subscribe(topic, client):
    topics[topic].append(client)

def publish(topic, message):
    for client in topics[topic]:
        print(f"Delivering '{message}' to {client}")

# Subscribe devices and publish messages
# Your code here

Hint

Devices get messages only for topics they subscribed to.

Practice

1. What is the main role of an MQTT broker in IoT communication?

easy

A. To receive messages from devices and forward them to subscribers

B. To generate sensor data for devices

C. To act as a device that publishes messages only

D. To encrypt messages between devices

Solution

1. Step 1: Understand MQTT broker function

   The MQTT broker acts as a middleman that receives messages from publishers and forwards them to subscribers.

2. Step 2: Eliminate incorrect roles

   Generating data, acting only as a publisher, or encrypting messages are not the broker's main roles.

3. Final Answer:

   To receive messages from devices and forward them to subscribers -> Option A

4. Quick Check:

   Broker = message receiver and forwarder [OK]

Hint: Broker forwards messages between devices [OK]

Common Mistakes:

• Thinking broker generates data
• Confusing broker with publisher device
• Assuming broker only encrypts messages

2. Which network port does an MQTT broker usually listen on by default?

easy

A. 8080

B. 22

C. 443

D. 1883

Solution

1. Step 1: Recall default MQTT port

   The MQTT protocol commonly uses port 1883 for unencrypted communication.

2. Step 2: Identify other ports

   Port 8080 is for HTTP alternate, 443 for HTTPS, and 22 for SSH, not MQTT.

3. Final Answer:

   1883 -> Option D

4. Quick Check:

   MQTT default port = 1883 [OK]

Hint: MQTT default port is 1883 [OK]

Common Mistakes:

• Choosing common web ports like 80 or 443
• Confusing MQTT port with SSH port 22
• Assuming MQTT uses random ports

3. Given this MQTT broker behavior: A device publishes a message to topic home/livingroom/temperature. What does the broker do next?

medium

A. Sends the message to all devices regardless of subscription

B. Sends the message to all devices subscribed to home/livingroom/temperature

C. Deletes the message immediately after receiving

D. Saves the message locally and does not forward it

Solution

1. Step 1: Understand topic-based forwarding

   The MQTT broker forwards messages only to devices subscribed to the specific topic.

2. Step 2: Eliminate incorrect behaviors

   Broker does not just save or delete messages without forwarding, nor does it broadcast to all devices.

3. Final Answer:

   Sends the message to all devices subscribed to home/livingroom/temperature -> Option B

4. Quick Check:

   Broker forwards by topic subscription [OK]

Hint: Broker forwards messages only to subscribed topics [OK]

Common Mistakes:

• Assuming broker broadcasts to all devices
• Thinking broker stores messages without forwarding
• Believing broker deletes messages immediately

4. You configured an MQTT broker but devices cannot connect. Which of these is a likely cause?

medium

A. Broker is not listening on the correct port

B. Devices are publishing to the correct topic

C. Broker is forwarding messages correctly

D. Devices have correct subscription filters

Solution

1. Step 1: Check broker listening port

   If the broker is not listening on the expected port (usually 1883), devices cannot connect.

2. Step 2: Understand other options

   Publishing topics, forwarding messages, and subscription filters affect message flow but not initial connection.

3. Final Answer:

   Broker is not listening on the correct port -> Option A

4. Quick Check:

   Connection issue = wrong broker port [OK]

Hint: Check broker port if devices can't connect [OK]

Common Mistakes:

• Blaming topic or subscription before connection
• Ignoring broker network settings
• Assuming forwarding affects connection

5. You want to ensure that an MQTT broker only forwards messages to devices subscribed to topics starting with office/. Which broker feature helps achieve this?

hard

A. Device authentication

B. Message encryption

C. Topic filtering with wildcards

D. Retained messages

Solution

1. Step 1: Understand topic filtering

   MQTT brokers use topic filters with wildcards to forward messages only to matching subscriptions.

2. Step 2: Exclude unrelated features

   Encryption secures data, authentication controls access, and retained messages store last message; none filter topics.

3. Final Answer:

   Topic filtering with wildcards -> Option C

4. Quick Check:

   Topic filtering controls message forwarding [OK]

Hint: Use topic filters with wildcards to control forwarding [OK]

Common Mistakes:

• Confusing encryption with filtering
• Thinking authentication filters topics
• Assuming retained messages control forwarding