Topics and topic hierarchy in IOT Protocols - Step-by-Step Execution

Process Flow - Topics and topic hierarchy

Start: Client wants to send message

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Choose Topic: e.g. home/kitchen/temperature

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Publish message to topic

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Broker receives message

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Broker forwards message to subscribers

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Subscribers receive message if subscribed to topic or matching wildcard

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End

This flow shows how a client picks a topic, publishes a message, and how the broker routes it to subscribers based on topic hierarchy.

Execution Sample

IOT Protocols

Publish to topic: home/kitchen/temperature
Subscriber A subscribes to: home/kitchen/#
Subscriber B subscribes to: home/+/temperature
Publish message: 22°C

A message with temperature data is published to a specific topic; subscribers receive it if their subscriptions match the topic hierarchy.

Process Table

Step	Action	Topic Used	Subscriber Subscribed To	Message Received
1	Client publishes message	home/kitchen/temperature	-	-
2	Broker checks subscribers	home/kitchen/temperature	Subscriber A: home/kitchen/#	Yes
3	Broker checks subscribers	home/kitchen/temperature	Subscriber B: home/+/temperature	Yes
4	Broker delivers message to Subscriber A	home/kitchen/temperature	home/kitchen/#	22°C
5	Broker delivers message to Subscriber B	home/kitchen/temperature	home/+/temperature	22°C
6	No other subscribers match	-	-	-

💡 All matching subscribers received the message; no more matches found.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
Published Topic	-	home/kitchen/temperature	home/kitchen/temperature	home/kitchen/temperature	home/kitchen/temperature
Message	-	22°C	22°C	22°C	22°C
Subscriber A Subscription	home/kitchen/#	home/kitchen/#	home/kitchen/#	home/kitchen/#	home/kitchen/#
Subscriber B Subscription	home/+/temperature	home/+/temperature	home/+/temperature	home/+/temperature	home/+/temperature
Message Delivered to A	No	No	Yes	Yes	Yes
Message Delivered to B	No	No	No	Yes	Yes

Key Moments - 3 Insights

Why does Subscriber A receive the message when subscribed to home/kitchen/#?

Because the '#' wildcard matches any subtopic after home/kitchen/, so home/kitchen/temperature fits this pattern (see execution_table step 2 and 4).

Why does Subscriber B receive the message when subscribed to home/+/temperature?

The '+' wildcard matches exactly one level, so home/+/temperature matches home/kitchen/temperature (see execution_table step 3 and 5).

What happens if a subscriber subscribes to home/kitchen only?

They will only receive messages published exactly to home/kitchen, not to subtopics like home/kitchen/temperature (not shown in current table but implied by topic hierarchy).

Visual Quiz - 3 Questions

Look at the execution table, which subscriber receives the message at step 5?

A Subscriber A

B Both Subscriber A and B

C Subscriber B

D No subscriber

💡 Hint

Check the 'Message Received' column at step 5 in the execution_table.

At which step does the broker check Subscriber A's subscription?

A Step 1

B Step 2

C Step 4

D Step 6

💡 Hint

Look at the 'Subscriber Subscribed To' column in the execution_table for Subscriber A.

If Subscriber B changed subscription to home/kitchen/temperature, what would happen?

A Subscriber B would receive the message only if published to home/kitchen/temperature

B Subscriber B would not receive the message

C Subscriber B would receive all messages under home/#

D Subscriber B would receive no messages

💡 Hint

Refer to how exact topic matches work in the variable_tracker and execution_table.

Concept Snapshot

Topics in IoT protocols are like addresses for messages.
They use slashes '/' to separate levels, like folders.
Wildcards '+' (one level) and '#' (multiple levels) help subscribe to many topics.
Messages published to a topic go to all subscribers matching that topic or wildcard.
Understanding topic hierarchy helps control message flow efficiently.

Full Transcript

This visual execution shows how messages are sent and received using topics and topic hierarchy in IoT protocols. A client publishes a message to a topic like home/kitchen/temperature. The broker checks all subscribers to see whose subscriptions match this topic. Subscribers can use wildcards like '#' to match multiple subtopics or '+' to match exactly one level. Subscribers with matching subscriptions receive the message. The execution table tracks each step, showing which subscribers get the message and why. The variable tracker shows how topics, messages, and subscriptions change during the process. Key moments clarify why wildcards work and how exact matches differ. The quiz tests understanding of subscriber message delivery and topic matching. The snapshot summarizes the main ideas about topic hierarchy and wildcards for quick recall.

Practice

1. What is the main purpose of topics in IoT protocols like MQTT?

easy

A. To encrypt messages for security

B. To store device firmware

C. To organize messages in a clear, folder-like structure

D. To control device power settings

Solution

1. Step 1: Understand the role of topics

   Topics act like folders to organize messages so devices know where to send or receive data.

2. Step 2: Compare options with topic function

   Only organizing messages matches the purpose of topics; encryption and firmware storage are unrelated.

3. Final Answer:

   To organize messages in a clear, folder-like structure -> Option C

4. Quick Check:

   Topics organize messages = B [OK]

Hint: Topics are like folders for messages [OK]

Common Mistakes:

• Confusing topics with security features
• Thinking topics store device data
• Mixing topics with device settings

2. Which of the following is the correct way to write a topic with three levels?

easy

A. home-kitchen-temperature

B. home.kitchen.temperature

C. home\kitchen\temperature

D. home/kitchen/temperature

Solution

1. Step 1: Recall topic level separator

   Topics use slashes (/) to separate levels, like folders in a path.

2. Step 2: Check each option's separator

   Only home/kitchen/temperature uses slashes correctly; others use dots, backslashes, or dashes which are incorrect.

3. Final Answer:

   home/kitchen/temperature -> Option D

4. Quick Check:

   Topic levels separated by '/' = A [OK]

Hint: Use slashes (/) to separate topic levels [OK]

Common Mistakes:

• Using dots instead of slashes
• Using backslashes which are invalid
• Using dashes instead of separators

3. Given the topic building1/floor2/room3/sensor, which part represents the third level?

medium

A. room3

B. floor2

C. building1

D. sensor

Solution

1. Step 1: Split the topic by slashes

   Splitting 'building1/floor2/room3/sensor' by '/' gives ['building1', 'floor2', 'room3', 'sensor'].

2. Step 2: Identify the third element

   The third element (index 2) is 'room3'.

3. Final Answer:

   room3 -> Option A

4. Quick Check:

   Third level = room3 = C [OK]

Hint: Count levels separated by '/' to find position [OK]

Common Mistakes:

• Counting from zero incorrectly
• Choosing first or second level by mistake
• Confusing last level with third

4. Identify the error in this topic string: home//livingroom/temperature

medium

A. Double slash creates an empty level

B. Missing slash between 'home' and 'livingroom'

C. Topic levels must use dots, not slashes

D. Topic cannot contain the word 'temperature'

Solution

1. Step 1: Analyze the topic string

   The string 'home//livingroom/temperature' has two slashes together, meaning an empty level between them.

2. Step 2: Understand topic level rules

   Empty levels are invalid because each level should have a name; double slashes cause this error.

3. Final Answer:

   Double slash creates an empty level -> Option A

4. Quick Check:

   Double slash = empty level error = A [OK]

Hint: Avoid double slashes to prevent empty topic levels [OK]

Common Mistakes:

• Ignoring empty levels caused by double slashes
• Thinking dots are required separators
• Believing certain words are forbidden in topics

5. You want to subscribe to all sensors in any room on floor 1 of building 2 using MQTT topics. Which subscription topic should you use?

hard

A. building2/floor1/*/sensor

B. building2/floor1/+/sensor

C. building2/floor1/room1/sensor

D. building2/floor1/#/sensor

Solution

1. Step 1: Understand MQTT wildcards

   The '+' wildcard matches exactly one level, while '#' matches all remaining levels.

2. Step 2: Analyze the subscription needs

   We want any room on floor1, so '+' replaces the room level. '#' would match too many levels, and '*' is invalid in MQTT.

3. Final Answer:

   building2/floor1/+/sensor -> Option B

4. Quick Check:

   '+' wildcard for one level = D [OK]

Hint: Use '+' for single-level wildcard in MQTT topics [OK]

Common Mistakes:

• Using '#' when only one level is needed
• Using '*' which is not a valid MQTT wildcard
• Specifying exact room instead of wildcard