Amazon SNS | Message Filtering.

 

Amazon SNS Message Filtering lets twtech to control which subscribed endpoints receive messages based on message attributes — so that not every subscriber receives every message.

This is especially useful in fan-out patterns with SQS, Lambda, or HTTP subscribers.

 How Message Filtering Works

1. Publishers send messages to an SNS topic with message attributes.
2. Subscribers define filter policies that determine which messages they want.
3. SNS evaluates these filters and delivers only the matching messages.

 Example Scenario

twtech is publishing order events:

• Type: order_created, order_cancelled, order_shipped

twtech has 3 SQS queues:

• One only needs order_created
• One only wants order_shipped
• One wants everything

1.  Message With Attributes

# json

{

  "Message": "Order #12345 was created",

  "MessageAttributes": {

    "eventType": {

      "DataType": "String",

      "StringValue": "order_created"

    }

  }

}

2.  Filter Policy Example

• For "order_created" only:

# json

{

  "eventType": ["order_created"]

}

• For "order_created" or "order_shipped":

# json

{

  "eventType": ["order_created", "order_shipped"]

}

• To receive all messages, use no filter or an empty policy {}.

 Setting Filter Policy (via AWS CLI)

# bash

aws sns set-subscription-attributes \

  --subscription-arn <sub-arn> \

  --attribute-name FilterPolicy \

  --attribute-value '{"eventType": ["order_created"]}'

Supported Filter Types

• String match
• String prefix match ("anything": [{"prefix": "error"}])
• Numeric conditions ("amount": [{"numeric": [">=", 100]}])
• Existence tests ("status": [{"exists": true}])
• Anything-but filters ("eventType": [{"anything-but": "order_cancelled"}])

twtech Notes and Tips

• Filter policies are evaluated server-side, no cost for dropped messages.
• twtech can filter on multiple attributes.
• Filters are case-sensitive.
• twtech can combine SNS filters with SQS message filtering or Lambda routing logic.