Amazon SNS (Simple Notification Service)

Core concept: SNS is a pub/sub service — one message published to a Topic is pushed to all subscribers simultaneously (fan-out).

---

🔰 What Is SNS?

Think of SNS as a radio broadcast — one speaker, many listeners. The publisher sends one message to a topic, and SNS delivers it to all subscribers in parallel. Compare with SQS where each message goes to exactly one consumer.

---

SNS vs SQS

Feature	SNS (Pub/Sub)	SQS (Queue)
Model	Push to all subscribers	Pull by one consumer
Persistence	❌ No retention	✅ Up to 14 days
Consumers	All subscribers get every message	One consumer per message
Delivery	Fire-and-forget	Guaranteed (with retries)
Use case	Fan-out, alerts, notifications	Decoupling, buffering, rate control

When to Use Both Together (Fan-Out)

S3 Event / Application Event
        │
        ▼
   [SNS Topic]  ← single publish
   /    │    \
  ▼     ▼     ▼
[SQS] [SQS] [Lambda]  ← multiple independent consumers
Queue  Queue  Function
(ETL) (Audit) (Alert)

Why fan-out with SNS + SQS?

1. Atomic delivery — one publish, all subscribers receive
2. Consumer independence — each queue processes at its own pace
3. Failure isolation — if audit service fails, ETL is unaffected
4. Easy extension — add new consumers without changing producer

---

Subscription Protocols

Protocol	Use Case	Retry
SQS	Async processing (fan-out)	SQS retains message
Lambda	Serverless processing	SNS retry policy
HTTP/HTTPS	Webhook delivery	3 retries, then DLQ
Email	Human notifications	No retry
SMS	Mobile alerts	No retry
Kinesis Data Firehose	Stream to S3/Redshift	Managed
Mobile Push	APNs, FCM, ADM	Platform-specific

---

Message Filtering

Without filtering, ALL subscribers receive ALL messages. Filter policies let subscribers receive only relevant messages:

// Publisher sends message with attributes:
{
  "MessageBody": "{\"orderId\": \"ORD-123\", \"amount\": 150}",
  "MessageAttributes": {
    "category": { "DataType": "String", "StringValue": "electronics" },
    "price": { "DataType": "Number", "StringValue": "150" },
    "region": { "DataType": "String", "StringValue": "us-east-1" }
  }
}

// Subscription 1 filter: Only electronics over $100
{
  "category": ["electronics"],
  "price": [{ "numeric": [">=", 100] }]
}

// Subscription 2 filter: Only US regions
{
  "region": [{ "prefix": "us-" }]
}

// Subscription 3: No filter → receives ALL messages

Filter Policy Scope

Scope	Description
MessageAttributes (default)	Filter on message attributes
MessageBody	Filter on JSON body content (newer feature)

---

SNS FIFO

Feature	Standard SNS	FIFO SNS
Ordering	Best-effort	Strict FIFO
Deduplication	❌	✅ (5-min window)
Throughput	Nearly unlimited	300 TPS (3,000 with batching)
Subscribers	All protocols	SQS FIFO only
Naming	Any	Must end in .fifo

---

SNS + Lambda DLQ

If Lambda subscription fails after retries:

SNSTopic:
  Type: AWS::SNS::Subscription
  Properties:
    Protocol: lambda
    Endpoint: !GetAtt MyFunction.Arn
    TopicArn: !Ref MyTopic
    RedrivePolicy:
      deadLetterTargetArn: !GetAtt SNSFailureDLQ.Arn

---

Java SDK — Publishing

SnsClient snsClient = SnsClient.create();

// Publish with message attributes (for filtering)
snsClient.publish(PublishRequest.builder()
    .topicArn("arn:aws:sns:us-east-1:123:order-events")
    .message("{\"orderId\": \"ORD-123\", \"amount\": 150}")
    .messageAttributes(Map.of(
        "category", MessageAttributeValue.builder()
            .dataType("String").stringValue("electronics").build(),
        "price", MessageAttributeValue.builder()
            .dataType("Number").stringValue("150").build()))
    .build());

// FIFO topic
snsClient.publish(PublishRequest.builder()
    .topicArn("arn:aws:sns:us-east-1:123:order-events.fifo")
    .message("{\"orderId\": \"ORD-456\"}")
    .messageGroupId("customer-A")
    .messageDeduplicationId("ORD-456-event")
    .build());

---

🎯 DVA-C02 Exam Tips

SNS Exam Cheat Sheet

1. Fan-out = SNS → multiple SQS queues (classic exam pattern)
2. Message filtering = subscribers only receive relevant messages (saves processing)
3. SNS FIFO = only SQS FIFO subscribers supported
4. No message persistence — if subscriber is down, message is lost (use SQS for persistence)
5. SNS + SQS = reliable fan-out (SQS retains messages even if consumer is slow)
6. Up to 12.5M subscriptions per topic
7. Message size: 256 KB max
8. Cross-account: Use SNS access policy to allow other accounts to publish/subscribe

---

🧪 Practice Questions

Q1. OrderPlaced event needs to go to inventory, billing, and shipping services. Best architecture?

A) App publishes to 3 SQS queues
B) SNS Topic → 3 SQS subscriptions
C) Kinesis Data Stream
D) Direct Lambda calls

✅ Answer & Explanation

B — SNS fan-out: one publish, all subscribers receive in parallel. Adding new consumers doesn't change the producer.

---

Q2. Only the billing service needs "electronics" orders above $100. Other services need all orders. How?

A) Separate SNS topics per category
B) SNS message filtering on the billing subscription
C) Lambda pre-processor
D) SQS message groups

✅ Answer & Explanation

B — Filter policies on individual subscriptions. Billing gets {"category": ["electronics"], "price": [{"numeric": [">=", 100]}]}. Others have no filter.

---

Q3. SNS FIFO topic needs guaranteed ordering. Which subscriber types are supported?

A) Lambda, SQS, HTTP
B) SQS FIFO only
C) All subscriber types
D) Lambda and SQS FIFO

✅ Answer & Explanation

B — SNS FIFO only supports SQS FIFO queue subscribers.

---

🔗 Resources

• SNS Developer Guide
• SNS Message Filtering
• Fan-Out Pattern