SNS subscription filter policies are a powerful feature in Amazon Simple Notification Service that allow subscribers to receive only a subset of messages published to a topic. Instead of receiving every message, subscribers can define filter criteria to selectively process relevant messages.
Filte…SNS subscription filter policies are a powerful feature in Amazon Simple Notification Service that allow subscribers to receive only a subset of messages published to a topic. Instead of receiving every message, subscribers can define filter criteria to selectively process relevant messages.
Filter policies are JSON objects attached to SNS subscriptions. They evaluate message attributes against defined conditions, and only matching messages are delivered to the subscriber. This reduces unnecessary processing and costs by filtering at the SNS level rather than at the application level.
Filter policies support two scopes: MessageAttributes (default) and MessageBody. With MessageAttributes scope, filtering occurs on message attribute key-value pairs. With MessageBody scope, filtering evaluates the JSON message body content.
Key filter operators include:
- Exact matching: Strings or numbers must match precisely
- Prefix matching: Using the "prefix" keyword for string beginnings
- Anything-but matching: Excludes specified values
- Numeric matching: Supports ranges with operators like greater-than, less-than, and between
- Exists matching: Checks if an attribute is present
- IP address matching: Filters based on CIDR blocks
For troubleshooting filter policies:
1. Verify JSON syntax is valid
2. Ensure attribute names match exactly (case-sensitive)
3. Confirm attribute types align with filter conditions
4. Check that message attributes are included when publishing
5. Use CloudWatch metrics to monitor filtered vs delivered messages
Optimization best practices:
- Design attribute schemas that support efficient filtering
- Use specific filters to minimize unnecessary deliveries
- Combine multiple conditions using AND logic within policies
- Leverage OR logic by specifying arrays of acceptable values
- Consider MessageBody filtering for complex JSON payloads
Filter policies can contain up to 150 keys with up to 150 values each. The total combination of values cannot exceed 10,000. Understanding these limits helps avoid configuration errors and ensures scalable message filtering in distributed applications.
Why SNS Subscription Filter Policies Are Important
SNS subscription filter policies are crucial for building efficient, cost-effective, and scalable messaging architectures. They allow subscribers to receive only the messages they actually need, reducing unnecessary processing, lowering costs, and improving application performance. For the AWS Developer Associate exam, understanding filter policies demonstrates your ability to design optimized event-driven architectures.
What Are SNS Subscription Filter Policies?
A subscription filter policy is a JSON document attached to an SNS subscription that defines which messages the subscriber receives. Instead of receiving all messages published to a topic, subscribers can filter messages based on message attributes. This enables a publish once, consume selectively pattern where different subscribers can process different subsets of messages from the same topic.
How Filter Policies Work
When a message is published to an SNS topic with message attributes, SNS evaluates each subscription's filter policy against those attributes. The message is delivered only if the attributes match the filter policy criteria.
Key Components: - Message Attributes: Key-value pairs attached to messages when published - Filter Policy: JSON document defining matching criteria on the subscription - Filter Policy Scope: Can be set to MessageAttributes (default) or MessageBody
Supported Operators: - Exact matching: String values must match exactly - Prefix matching: Using the prefix keyword for partial string matches - Numeric matching: Supports exact, range, and comparison operators - Anything-but matching: Excludes specific values - Exists matching: Checks if an attribute exists - IP address matching: For CIDR block matching
Example Filter Policy: { "event_type": ["order_placed", "order_shipped"], "customer_tier": [{"prefix": "premium"}], "order_value": [{"numeric": [">=", 100]}] } This policy accepts messages where event_type is either order_placed or order_shipped, AND customer_tier starts with premium, AND order_value is 100 or greater.
Filter Policy Scope Options
- MessageAttributes: Filters based on message attributes (metadata) - MessageBody: Filters based on the actual message content (must be valid JSON)
Exam Tips: Answering Questions on SNS Subscription Filter Policies
Tip 1: Remember that filter policies use AND logic between different attribute keys and OR logic between values within the same key.
Tip 2: When a question mentions reducing message processing costs or preventing unnecessary Lambda invocations, filter policies are often the answer.
Tip 3: Filter policies are applied at the subscription level, not the topic level. Each subscription can have its own unique filter policy.
Tip 4: If no filter policy is set, the subscription receives ALL messages published to the topic.
Tip 5: Questions about routing different message types to different endpoints from a single topic typically involve filter policies.
Tip 6: Know the difference between MessageAttributes scope (for metadata filtering) and MessageBody scope (for content filtering). MessageBody filtering requires JSON-formatted message content.
Tip 7: Filter policies can be up to 256 KB in size and support complex nested conditions.
Tip 8: When comparing SNS filter policies to SQS message filtering or EventBridge rules, understand that SNS filter policies are specifically for controlling message delivery to individual subscribers.
Common Exam Scenarios
- Scenario asking how to send order notifications to different processing systems based on order type: Use filter policies - Scenario about reducing Lambda invocation costs when only certain messages need processing: Implement filter policies - Scenario requiring fanout with selective message delivery: Combine SNS topics with subscription filter policies