Service Bus topics and subscriptions in Azure - Step-by-Step Execution

Process Flow - Service Bus topics and subscriptions

Send message to Topic

↓

Topic receives message

↓

Distribute message to Subscriptions

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Each Subscription stores message

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Subscribers receive messages from their Subscription

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Message processed and removed from Subscription

Messages are sent to a topic, which then distributes copies to each subscription. Subscribers read messages from their subscriptions independently.

Execution Sample

Azure

Create Topic 'orders'
Create Subscription 'priority'
Send message 'Order1' to Topic
Subscription 'priority' receives 'Order1'
Subscriber reads 'Order1' from 'priority'

This sequence shows sending a message to a topic and a subscription receiving and processing it.

Process Table

Step	Action	Topic State	Subscription 'priority' State	Subscriber Output
1	Create Topic 'orders'	Topic 'orders' created, empty	Subscription 'priority' does not exist	No output
2	Create Subscription 'priority'	Topic 'orders' exists	Subscription 'priority' created, empty	No output
3	Send message 'Order1' to Topic	Topic 'orders' has message 'Order1'	Subscription 'priority' empty	No output
4	Topic distributes 'Order1' to subscriptions	Topic 'orders' has message 'Order1'	Subscription 'priority' has message 'Order1'	No output
5	Subscriber reads message from 'priority'	Topic 'orders' has message 'Order1'	Subscription 'priority' empty after read	Subscriber received 'Order1'
6	Message processed and removed	Topic 'orders' has message 'Order1'	Subscription 'priority' empty	Subscriber processed 'Order1'

💡 Message processed and removed from subscription; subscriber output complete.

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
Topic 'orders' messages	none	none	'Order1'	'Order1'	'Order1'	'Order1'
Subscription 'priority' messages	none	none	none	'Order1'	none	none
Subscriber output	none	none	none	none	'Order1'	'Order1 processed'

Key Moments - 2 Insights

Why does the topic still have the message after the subscriber reads it from the subscription?

The topic keeps the original message until it expires or is removed; subscriptions hold copies. See execution_table step 5 where subscription is empty but topic still has the message.

Can multiple subscriptions receive the same message from a topic?

Yes, each subscription gets its own copy of the message from the topic. This is shown in the concept flow where the topic distributes messages to all subscriptions.

Visual Quiz - 3 Questions

Look at the execution table at step 4. What is the state of the subscription 'priority'?

A Has message 'Order1'

B Subscription does not exist

C Empty, no messages

D Subscriber has read the message

💡 Hint

Check the 'Subscription 'priority' State' column at step 4 in the execution_table.

At which step does the subscriber receive the message 'Order1'?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Look at the 'Subscriber Output' column in the execution_table to find when the message appears.

If a second subscription is added, how would the topic distribute messages?

A Only to the first subscription

B To both subscriptions independently

C Messages are split between subscriptions

D Messages are sent only once to the topic

💡 Hint

Refer to the concept_flow where the topic distributes messages to all subscriptions.

Concept Snapshot

Service Bus topics receive messages from senders.
Each subscription gets a copy of every message.
Subscribers read messages independently from their subscriptions.
Messages remain in topic until expired or removed.
Subscriptions act like message inboxes for subscribers.

Full Transcript

Service Bus topics act like message hubs. When a message is sent to a topic, it keeps the message and copies it to each subscription. Each subscription stores its own copy. Subscribers read messages from their subscriptions, not directly from the topic. This allows multiple subscribers to get the same message independently. Messages stay in the topic until they expire or are deleted. Subscriptions act like inboxes holding messages for subscribers to process.