AWS SNS/SQS

Amazon SNS (Simple Notification Service) and SQS (Simple Queue Service) provide managed messaging for event sourcing on AWS.

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Why SNS/SQS

Strength	Benefit
Fully managed	No infrastructure to operate
Serverless	Lambda integration
Cost effective	Pay per request
High availability	Multi-AZ by default
Fan-out	SNS → multiple SQS queues

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Trade-offs

Concern	Consideration
AWS lock-in	Not portable to other clouds
256 KB limit	Large events need S3 offloading
Ordering	FIFO queues required for ordering

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Configuration

[bus]
backend = "sns_sqs"

[bus.sns_sqs]
region = "us-east-1"
topic_prefix = "angzarr"

Environment Variables

export AWS_REGION="us-east-1"
export SNS_SQS_TOPIC_PREFIX="angzarr"
export BUS_BACKEND="sns_sqs"
# AWS credentials via standard mechanisms

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Topology

SNS provides fan-out to SQS queues:

SNS Topic: angzarr-events-player
    │
    ├── SQS: player-projector-queue
    ├── SQS: output-projector-queue
    └── SQS: topology-tracker-queue

SNS Topic: angzarr-events-hand
    │
    ├── SQS: hand-saga-queue
    └── SQS: output-projector-queue

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FIFO vs Standard

Standard Queues

• At-least-once delivery
• Best-effort ordering
• Higher throughput

FIFO Queues

• Exactly-once processing
• Strict ordering within message groups
• 300 TPS limit (3000 with batching)

For event sourcing, FIFO queues with message group ID = aggregate root:

sqs.send_message()
    .queue_url(&fifo_queue_url)
    .message_body(&event_json)
    .message_group_id(&format!("{}#{}", domain, root))
    .message_deduplication_id(&event_id)
    .send()
    .await?;

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Large Event Handling

Events exceeding 256 KB use S3 for payload storage:

1. Store payload in S3
2. Publish reference to SNS/SQS
3. Consumer fetches from S3

This is the "claim check" pattern, handled automatically by OffloadingEventBus.

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Dead Letter Queues

Configure DLQs for failed messages:

# CloudFormation/Terraform
PlayerProjectorDLQ:
  Type: AWS::SQS::Queue
  Properties:
    QueueName: player-projector-dlq

PlayerProjectorQueue:
  Type: AWS::SQS::Queue
  Properties:
    QueueName: player-projector-queue
    RedrivePolicy:
      deadLetterTargetArn: !GetAtt PlayerProjectorDLQ.Arn
      maxReceiveCount: 5

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Setup

# Create SNS topics
aws sns create-topic --name angzarr-events-player
aws sns create-topic --name angzarr-events-hand

# Create SQS queues
aws sqs create-queue --queue-name player-projector-queue

# Subscribe queue to topic
aws sns subscribe \
  --topic-arn arn:aws:sns:us-east-1:123456789:angzarr-events-player \
  --protocol sqs \
  --notification-endpoint arn:aws:sqs:us-east-1:123456789:player-projector-queue

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Helm Deployment

# values.yaml
bus:
  backend: sns_sqs

sns_sqs:
  enabled: true
  region: us-east-1
  topicPrefix: angzarr
  # IAM role-based auth recommended

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When to Use SNS/SQS

• AWS native — Already on AWS
• Lambda integration — Serverless event handlers
• Cost sensitive — Pay-per-use model
• Managed — No operational overhead

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Next Steps

• Pub/Sub — GCP equivalent
• AMQP — Self-managed alternative