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

↓

Each Subscription stores message

↓

Subscribers receive messages from their Subscription

↓

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.

Practice

1. What is the main purpose of an Azure Service Bus topic?

easy

A. To send messages to multiple receivers using subscriptions

B. To store data permanently like a database

C. To host web applications

D. To manage virtual machines

Solution

1. Step 1: Understand the role of a Service Bus topic

   A Service Bus topic acts as a message distributor that sends messages to multiple receivers.

2. Step 2: Compare other options

   Options B, C, and D describe unrelated Azure services or functions.

3. Final Answer:

   To send messages to multiple receivers using subscriptions -> Option A

4. Quick Check:

   Service Bus topic = message distribution [OK]

Hint: Topics distribute messages to many receivers via subscriptions [OK]

Common Mistakes:

• Confusing topics with queues
• Thinking topics store data permanently
• Mixing topics with compute services

2. Which of the following is the correct way to create a subscription under a topic using Azure CLI?

easy

A. az servicebus topic create --resource-group MyGroup --namespace-name MyNamespace --subscription-name MySubscription --name MyTopic

B. az servicebus subscription create --resource-group MyGroup --namespace-name MyNamespace --topic-name MyTopic --name MySubscription

C. az servicebus topic subscription create --resource-group MyGroup --namespace-name MyNamespace --topic-name MyTopic --name MySubscription

D. az servicebus create subscription --resource-group MyGroup --namespace-name MyNamespace --topic MyTopic --subscription MySubscription

Solution

1. Step 1: Identify the correct Azure CLI command structure

   The correct command to create a subscription under a topic is 'az servicebus subscription create' with required parameters.

2. Step 2: Check parameters and order

   az servicebus subscription create --resource-group MyGroup --namespace-name MyNamespace --topic-name MyTopic --name MySubscription uses the correct command and parameters: resource group, namespace, topic name, and subscription name.

3. Final Answer:

   az servicebus subscription create --resource-group MyGroup --namespace-name MyNamespace --topic-name MyTopic --name MySubscription -> Option B

4. Quick Check:

   Correct CLI syntax = az servicebus subscription create --resource-group MyGroup --namespace-name MyNamespace --topic-name MyTopic --name MySubscription [OK]

Hint: Use 'az servicebus subscription create' to add subscriptions [OK]

Common Mistakes:

• Using 'az servicebus topic create' for subscriptions
• Using invalid 'az servicebus topic subscription create'
• Incorrect command verbs or missing flags

3. Given the following code snippet, what will be the output?

from azure.servicebus import ServiceBusClient
connection_str = "Endpoint=sb://example.servicebus.windows.net/;SharedAccessKeyName=RootManageSharedAccessKey;SharedAccessKey=key"
topic_name = "mytopic"
subscription_name = "mysubscription"

with ServiceBusClient.from_connection_string(connection_str) as client:
    receiver = client.get_subscription_receiver(topic_name, subscription_name)
    with receiver:
        messages = receiver.receive_messages(max_message_count=2, max_wait_time=5)
        print(len(list(messages)))

medium

A. 2

B. The code will not compile

C. An exception is raised due to missing message processing

D. 0

Solution

1. Step 1: Understand message receiving behavior

   The code tries to receive up to 2 messages with a 5-second wait. If no messages are available, it returns an empty list.

2. Step 2: Analyze message processing

   Since no messages are explicitly sent before receiving, the list will be empty, so len(messages) is 0.

3. Final Answer:

   0 -> Option D

4. Quick Check:

   Receive without sent messages = 0 [OK]

Hint: Receiving messages without sending returns empty list [OK]

Common Mistakes:

• Assuming messages exist without sending
• Expecting exceptions for no messages
• Confusing receive_messages with peek_messages

4. You have created a subscription but messages are not being received. Which of the following is the most likely cause?

medium

A. The subscription filter is set to exclude all messages

B. The topic does not exist

C. The subscription name is too long

D. The Service Bus namespace is in a different region

Solution

1. Step 1: Check subscription filters

   If a subscription filter excludes all messages, no messages will be received even if the topic and subscription exist.

2. Step 2: Evaluate other options

   The topic does not exist would prevent subscription creation; The subscription name is too long would cause an error; The Service Bus namespace is in a different region does not block message reception.

3. Final Answer:

   The subscription filter is set to exclude all messages -> Option A

4. Quick Check:

   Filter excluding messages = no messages received [OK]

Hint: Check subscription filters if no messages arrive [OK]

Common Mistakes:

• Ignoring filters when troubleshooting
• Assuming region mismatch blocks messages
• Overlooking subscription existence

5. You want to ensure high availability and ordered message processing in your Service Bus topic subscriptions. Which configuration should you apply?

hard

A. Use multiple topics instead of subscriptions

B. Disable partitioning and use duplicate detection only

C. Enable partitioning on the topic and use sessions on subscriptions

D. Enable auto-delete on subscriptions after 1 hour

Solution

1. Step 1: Understand partitioning and sessions

   Partitioning improves availability by spreading messages across nodes. Sessions enable ordered processing by grouping related messages.

2. Step 2: Evaluate other options

   Disabling partitioning reduces availability; multiple topics don't guarantee order; auto-delete removes subscriptions prematurely.

3. Final Answer:

   Enable partitioning on the topic and use sessions on subscriptions -> Option C

4. Quick Check:

   Partitioning + sessions = high availability + order [OK]

Hint: Use partitioning for availability and sessions for order [OK]

Common Mistakes:

• Confusing duplicate detection with ordering
• Thinking multiple topics replace subscriptions
• Setting auto-delete too aggressively