When a file is uploaded, an event triggers a second function. How can we design this event‑driven flow in AWS?

Use S3 event notifications to trigger the first Lambda when an object is created. Inside that Lambda, publish a message to an SNS topic or send the event to an SQS queue. The second Lambda subscribes to that SNS topic or polls the SQS queue, ensuring decoupling and retry logic. Alternatively, use EventBridge to route the S3 event directly to the second Lambda, which simplifies the architecture if you need filtering or cross‑account routing. For more complex workflows, chain the two Lambdas via Step Functions, where the first Lambda passes its output as input to the next state. This pattern provides visibility, error handling, and the ability to add additional steps (e.g., data transformation, database writes) without tightly coupling the functions.