4.3.4 Subscription Filter Policies

Use Subscription Filter Policies to Optimize Messaging

SNS Message Filtering

Scenario thi: Use Message Filter feature của SNS để enable subscribers to SNS topic receive only a subset of topic messages.

Ví dụ: Insurance quote application

• Request for insurance → forwarded to backend systems
• Sales system handles insurance policies
• Analytics system processes all quote requests

Giải pháp: Use fanout message pattern using SNS and SQS to decouple website from backend systems.

{
  "eventType": ["order_placed"],
  "amount": [{"numeric": [">=", 100]}],
  "store": [{"prefix": "us-"}]
}

Lợi ích:

• Filter at subscription level
• Reduce unnecessary Lambda invocations
• Reduce SQS messages processed
• No extra cost for filtering
• Event notifications get to right backend system

Filter Policy Operators

Without vs With Filtering

Without: SNS → SQS → Lambda (processes ALL messages, discards 90%)
With:    SNS → Filter → SQS → Lambda (processes only relevant 10%)

Benefits:

• ✅ Cost savings: Fewer Lambda invocations
• ✅ Performance: Less unnecessary processing
• ✅ Simplicity: No filter logic in consumer code
• ✅ Right messages to right systems

EventBridge Content-Based Filtering

{
  "source": ["my-app"],
  "detail-type": ["OrderCreated"],
  "detail": {
    "amount": [{"numeric": [">", 100]}],
    "region": ["us-east-1", "eu-west-1"]
  }
}

• Richer filtering than SNS
• Pattern matching on event content
• Multiple targets per rule

Real-world Example

Insurance Application:

Quote Request → SNS Topic
                  │
                  ├── Filter: {"type": ["new_policy"]} → SQS → Sales Lambda
                  │
                  └── Filter: {"type": ["quote"]} → SQS → Analytics Lambda