SNS topics and subscriptions in AWS - Step-by-Step Execution

Process Flow - SNS topics and subscriptions

Create SNS Topic

↓

Create Subscription

↓

Publish Message to Topic

↓

SNS Delivers Message to Subscribers

↓

Subscribers Receive Message

This flow shows how an SNS topic is created, subscriptions are added, messages are published, and then delivered to subscribers.

Execution Sample

AWS

aws sns create-topic --name MyTopic
aws sns subscribe --topic-arn <topic-arn> --protocol email --notification-endpoint [e-mail]
aws sns publish --topic-arn <topic-arn> --message "Hello Subscribers!"

This code creates an SNS topic, subscribes an email endpoint, and publishes a message to the topic.

Process Table

Step	Action	Input	Output	State Change
1	Create SNS Topic	Name=MyTopic	TopicArn=arn:aws:sns:region:account:MyTopic	Topic created with ARN
2	Subscribe to Topic	TopicArn=arn:aws:sns:region:account:MyTopic, Protocol=email, Endpoint=[e-mail]	SubscriptionArn=arn:aws:sns:region:account:MyTopic:sub-id	Subscription added to topic
3	Publish Message	TopicArn=arn:aws:sns:region:account:MyTopic, Message="Hello Subscribers!"	MessageId=msg-id	Message queued for delivery
4	Deliver Message	MessageId=msg-id	Success	Message sent to subscriber endpoint
5	Subscriber Receives Message	Email inbox	Message content: "Hello Subscribers!"	Subscriber notified
6	End	No further actions	N/A	Process complete

💡 All subscribers have received the published message, process ends.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
TopicArn	None	arn:aws:sns:region:account:MyTopic	arn:aws:sns:region:account:MyTopic	arn:aws:sns:region:account:MyTopic	arn:aws:sns:region:account:MyTopic	arn:aws:sns:region:account:MyTopic
SubscriptionArn	None	None	arn:aws:sns:region:account:MyTopic:sub-id	arn:aws:sns:region:account:MyTopic:sub-id	arn:aws:sns:region:account:MyTopic:sub-id	arn:aws:sns:region:account:MyTopic:sub-id
MessageId	None	None	None	msg-id	msg-id	msg-id
MessageDelivered	False	False	False	False	True	True

Key Moments - 3 Insights

Why do we need to create a subscription after creating a topic?

Creating a topic only sets up the channel. Subscriptions tell SNS where to send messages. See execution_table step 2 where subscription is added after topic creation.

What happens if no subscriptions exist when a message is published?

SNS accepts the message but has no endpoints to deliver to, so no delivery occurs. This is implied by the need for subscriptions in step 2 before publishing in step 3.

Does publishing a message guarantee delivery to subscribers immediately?

Publishing queues the message for delivery (step 3), but actual delivery happens next (step 4). Delivery depends on subscriber availability.

Visual Quiz - 3 Questions

Look at the execution table, what is the TopicArn value after step 1?

A arn:aws:sns:region:account:MyTopic

B None

C arn:aws:sns:region:account:MyTopic:sub-id

D msg-id

💡 Hint

Check the 'TopicArn' row in variable_tracker after step 1.

At which step does the message get queued for delivery?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Action' and 'State Change' columns in execution_table for message publishing.

If no subscriptions were created, what would happen at step 4?

A Message would be delivered to subscribers

B SNS would throw an error and stop

C Message would be queued but not delivered

D Message would be deleted immediately

💡 Hint

Refer to key_moments about publishing without subscriptions.

Concept Snapshot

SNS Topics and Subscriptions:
- Create a topic to act as a message channel.
- Add subscriptions to specify where messages go.
- Publish messages to the topic.
- SNS delivers messages to all subscribers.
- Subscribers receive notifications asynchronously.

Full Transcript

This visual execution shows how AWS SNS topics and subscriptions work. First, a topic is created which acts like a message channel. Then, subscriptions are added to tell SNS where to send messages, such as an email address. When a message is published to the topic, SNS queues it and then delivers it to all subscribed endpoints. The subscriber finally receives the message. Variables like TopicArn, SubscriptionArn, and MessageId track the state through each step. Key moments clarify why subscriptions are needed and how delivery works. The quiz tests understanding of these steps and states.

Practice

1. What is the main purpose of an SNS topic in AWS?

easy

A. To store files and documents securely

B. To act as a message hub where messages are sent and distributed

C. To run virtual servers in the cloud

D. To manage user permissions and access

Solution

1. Step 1: Understand SNS topic role

   An SNS topic is like a message board where messages are posted for others to receive.

2. Step 2: Compare options to SNS function

   Only To act as a message hub where messages are sent and distributed describes a message hub, which matches SNS topic's purpose.

3. Final Answer:

   To act as a message hub where messages are sent and distributed -> Option B

4. Quick Check:

   SNS topic = message hub [OK]

Hint: SNS topics distribute messages to subscribers [OK]

Common Mistakes:

• Confusing SNS topics with storage services
• Thinking SNS manages servers
• Mixing SNS with user access control

2. Which of the following is the correct way to subscribe an email endpoint to an SNS topic using AWS CLI?

easy

A. aws sns subscribe --subscription-arn arn:aws:sns:us-east-1:123456789012:MyTopic --protocol email --notification-endpoint [e-mail]

B. aws sns subscribe --topic-arn arn:aws:sns:us-east-1:123456789012:MyTopic --notification-endpoint [e-mail]

C. aws sns subscribe --topic-arn arn:aws:sns:us-east-1:123456789012:MyTopic --protocol email --notification-endpoint [e-mail]

D. aws sns subscribe --topic-arn arn:aws:sns:us-east-1:123456789012:MyTopic --protocol email --endpoint [e-mail]

Solution

1. Step 1: Review AWS CLI subscribe syntax

   The correct command uses 'aws sns subscribe' with '--topic-arn', '--protocol', and '--endpoint' parameters.

2. Step 2: Check option correctness

   The correct command is 'aws sns subscribe --topic-arn arn:aws:sns:us-east-1:123456789012:MyTopic --protocol email --endpoint [e-mail]'. Distractors use incorrect parameters like --notification-endpoint, miss --protocol, or use --subscription-arn.

3. Final Answer:

   aws sns subscribe --topic-arn arn:aws:sns:us-east-1:123456789012:MyTopic --protocol email --endpoint [e-mail] -> Option D

4. Quick Check:

   Correct CLI syntax = aws sns subscribe --topic-arn arn:aws:sns:us-east-1:123456789012:MyTopic --protocol email --endpoint [e-mail] [OK]

Hint: Use --protocol and --endpoint with aws sns subscribe [OK]

Common Mistakes:

• Using wrong parameter names like --notification-endpoint instead of --endpoint
• Missing the --protocol flag
• Confusing topic ARN with subscription ARN

3. Given this Python code using boto3, what will be the output if the subscription is successful?

import boto3
sns = boto3.client('sns')
response = sns.subscribe(
    TopicArn='arn:aws:sns:us-east-1:123456789012:MyTopic',
    Protocol='email',
    Endpoint='[e-mail]'
)
print(response['SubscriptionArn'])

medium

A. 'pending confirmation' string indicating subscription needs confirmation

B. An error message about missing permissions

C. The ARN of the SNS topic

D. The ARN of the new subscription if confirmed

Solution

1. Step 1: Understand SNS email subscription behavior

   Email subscriptions require the user to confirm by clicking a link. Until then, the SubscriptionArn is 'pending confirmation'.

2. Step 2: Analyze the printed output

   The code prints response['SubscriptionArn'], which will be 'pending confirmation' until user confirms.

3. Final Answer:

   'pending confirmation' string indicating subscription needs confirmation -> Option A

4. Quick Check:

   Email subscription = pending confirmation [OK]

Hint: Email subscriptions show 'pending confirmation' until user confirms [OK]

Common Mistakes:

• Expecting immediate subscription ARN without confirmation
• Confusing topic ARN with subscription ARN
• Assuming no confirmation needed for email protocol

4. You wrote this AWS CLI command to subscribe an HTTP endpoint but get an error:

aws sns subscribe --topic-arn arn:aws:sns:us-east-1:123456789012:MyTopic --protocol http --notification-endpoint http://example.com/notify

What is the most likely cause of the error?

medium

A. The HTTP endpoint URL must use HTTPS, not HTTP

B. The topic ARN is invalid

C. The protocol should be 'https' instead of 'http'

D. The notification endpoint URL is missing a trailing slash

Solution

1. Step 1: Check protocol requirements for HTTP endpoints

   AWS SNS requires HTTP endpoints to use HTTPS for security; HTTP URLs cause errors.

2. Step 2: Validate the given URL and protocol

   The URL uses 'http://' which is not allowed; it must be 'https://'.

3. Final Answer:

   The HTTP endpoint URL must use HTTPS, not HTTP -> Option A

4. Quick Check:

   HTTP endpoints require HTTPS URL [OK]

Hint: Use HTTPS URLs for HTTP/HTTPS SNS subscriptions [OK]

Common Mistakes:

• Assuming HTTP URLs are accepted
• Thinking trailing slash matters
• Confusing protocol parameter values

5. You want to send a notification to multiple endpoints: an email, an SMS number, and a Lambda function. How should you configure SNS to achieve this?

hard

A. Send messages directly to each endpoint without using SNS topics

B. Create separate SNS topics for each endpoint type and publish messages to all

C. Create one SNS topic and subscribe all three endpoints to it

D. Use only email and SMS subscriptions; Lambda cannot subscribe to SNS

Solution

1. Step 1: Understand SNS subscription flexibility

   An SNS topic can have multiple subscriptions of different protocols (email, SMS, Lambda).

2. Step 2: Choose the simplest and best practice approach

   Using one topic with multiple subscriptions is efficient and standard practice.

3. Final Answer:

   Create one SNS topic and subscribe all three endpoints to it -> Option C

4. Quick Check:

   One topic, multiple subscriptions = Create one SNS topic and subscribe all three endpoints to it [OK]

Hint: One SNS topic can have many different subscriptions [OK]

Common Mistakes:

• Thinking Lambda cannot subscribe to SNS
• Creating multiple topics unnecessarily
• Sending messages directly without SNS