Which AWS service allows you to run code in response to events?

AWS Lambda is the service that enables you to run code in response to events without the need to provision or manage servers. It operates on a serverless architecture, meaning that you can execute your code in reaction to various triggers, such as changes in data within Amazon S3, updates in a DynamoDB table, or HTTP requests via Amazon API Gateway. This capability allows developers to build applications that automatically respond to events in real time, making it more efficient and cost-effective since you pay only for the compute time consumed when your code is executed.

In contrast, Amazon S3 is primarily a storage service that provides object storage for data. It can trigger Lambda functions based on events like file uploads, but it doesn't directly run code itself. Amazon EC2 is a service that allows you to run virtual servers in the cloud, but it requires you to manage the infrastructure, which does not align with a purely event-driven execution model like Lambda provides. AWS CloudTrail is a service for logging and monitoring API calls within your AWS account, which is useful for auditing but does not execute code in response to events.