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.