A company is migrating a monolithic web application on Amazon EC2 to a serverless microservices architecture and wants an event-driven, loosely coupled design using the publish/subscribe pattern. Which solution is the most cost-effective?

The most cost-effective solution uses Amazon API Gateway HTTP API, which is cheaper than REST API, and Amazon SNS for the publish/subscribe pattern. HTTP API offers lower latency and cost compared to REST API for simple proxy integrations. AWS Lambda processes the API request and publishes messages to an SNS topic. SNS is a fully managed pub/sub messaging service, ideal for fan-out to multiple subscribers, and is generally more cost-effective for this pattern than SQS for direct fan-out or Kinesis Data Streams for event delivery. SQS is a queueing service, not a pub/sub service, and would require additional logic for fan-out. Kinesis Data Streams is designed for real-time data processing and analytics, making it overkill and more expensive for a simple event-driven pub/sub pattern.