Which AWS service allows for running code in response to events?

AWS Lambda is designed specifically for running code in response to events. It operates on a serverless model, meaning you can execute your code without the need to provision or manage servers. When an event occurs—such as an update in an Amazon S3 bucket, a message arriving in an Amazon SNS topic, or an HTTP request that reaches an API Gateway—AWS Lambda automatically triggers the associated function to run the necessary code.

This service is particularly useful for creating responsive applications that react to changes in your environment. As the code execution is based on the occurrence of events, it enables a highly scalable architecture; resources are only utilized when needed, and you only pay for the compute time that you consume.

In contrast, other services listed focus on different use cases: Amazon ECS is primarily a container orchestration service for managing Docker containers; Amazon EFS is a managed file storage service that provides scalable file storage for use with AWS cloud services; and AWS Batch is designed for running batch computing workloads on AWS. While these services can also work in an event-driven architecture, they do not provide the same direct capability for running code in response to individual events in the same automated and scalable manner as AWS Lambda does.