Amazon SNS (Notifications & Fan-out)

{
  "Type": "Notification",
  "MessageId": "da567617-4c3a-4a36-a9f8-abc123",
  "TopicArn": "arn:aws:sns:us-east-1:123456789012:MyTopic",
  "Subject": "Order Processed",
  "Message": "Order #12345 has been processed",
  "Timestamp": "2025-01-18T12:00:00.000Z",
  "SignatureVersion": "1",
  "Signature": "...",
  "MessageAttributes": {
    "orderType": {
      "Type": "String",
      "Value": "priority"
    }
  }
}

// Filter by message attribute
{
  "store": ["wholesale"],
  "price_usd": [{"numeric": [">=", 100]}]
}

// Subscriber only receives messages where:
// - store attribute equals "wholesale" AND
// - price_usd is greater than or equal to 100

// Filter by message body (payload-based)
// Set FilterPolicyScope to "MessageBody"
{
  "customer": {
    "tier": ["premium", "enterprise"]
  }
}

import boto3

sns = boto3.client('sns')

response = sns.publish(
    TopicArn='arn:aws:sns:us-east-1:123456789012:MyTopic.fifo',
    Message='Process order #12345',
    MessageGroupId='customer-123',      # Required for FIFO
    MessageDeduplicationId='order-12345' # Or use content-based
)

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Sid": "AllowCrossAccountPublish",
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::111122223333:root"
      },
      "Action": "sns:Publish",
      "Resource": "arn:aws:sns:us-east-1:123456789012:MyTopic"
    }
  ]
}

aws sns set-topic-attributes \
  --topic-arn arn:aws:sns:us-east-1:123456789012:MyTopic \
  --attribute-name KmsMasterKeyId \
  --attribute-value alias/aws/sns

# Or use custom CMK for cross-account access
aws sns set-topic-attributes \
  --topic-arn arn:aws:sns:us-east-1:123456789012:MyTopic \
  --attribute-name KmsMasterKeyId \
  --attribute-value arn:aws:kms:us-east-1:123456789012:key/12345678-1234

1. Create Platform Application
   └── Register with push service (APNs, FCM, etc.)
       └── Get platform credentials

2. Register Device Token
   └── App sends device token to your backend
       └── Backend creates SNS Platform Endpoint
           └── Returns endpoint ARN

3. Send Notification
   └── Publish to endpoint ARN (direct)
   └── OR subscribe endpoint to topic (broadcast)

Publisher (us-east-1)
    │
    ▼
SNS Topic (us-east-1)
    │
    ├──► SQS Queue (us-east-1)
    │
    └──► SNS Topic (eu-west-1)  ← Cross-region subscription
            │
            ├──► SQS Queue (eu-west-1)
            │
            └──► Lambda (eu-west-1)

Best Practice: Topic-Queue-Chaining

Publisher
    │
    ▼
SNS Topic (fan-out)
    │
    ├──► SQS Queue 1 ──► Service A (can be offline)
    │
    ├──► SQS Queue 2 ──► Service B (can be offline)
    │
    └──► SQS Queue 3 ──► Service C (can be offline)

Benefits:
├── Messages persist even if consumers are down
├── Each service processes at its own pace
├── Failed messages go to DLQ for retry
└── Decouples all services completely

1. Publisher sends message to topic
   └── Message validated (size, attributes)

2. SNS processes message
   ├── Applies filter policies
   └── Determines eligible subscribers

3. Delivery attempts
   ├── Immediate for most endpoints
   ├── Retry policy for HTTP/S failures
   └── Success or failure logged

4. After all deliveries
   └── Message deleted (no persistence in SNS)

Note: SNS does NOT retain messages!
Use SQS if you need persistence.

# Create topic
aws sns create-topic --name MyTopic

# Create FIFO topic
aws sns create-topic --name MyTopic.fifo \
  --attributes FifoTopic=true,ContentBasedDeduplication=true

# Subscribe SQS queue
aws sns subscribe \
  --topic-arn arn:aws:sns:us-east-1:123456789012:MyTopic \
  --protocol sqs \
  --notification-endpoint arn:aws:sqs:us-east-1:123456789012:MyQueue

# Publish message
aws sns publish \
  --topic-arn arn:aws:sns:us-east-1:123456789012:MyTopic \
  --message "Hello World" \
  --message-attributes '{"store":{"DataType":"String","StringValue":"retail"}}'

# Set filter policy
aws sns set-subscription-attributes \
  --subscription-arn arn:aws:sns:us-east-1:123456789012:MyTopic:abc123 \
  --attribute-name FilterPolicy \
  --attribute-value '{"store":["retail"]}'