Lambda runs your code without a server to babysit. You write a function, tell it what should set it off (a file landing in S3, an API call, a scheduled event), and AWS runs it, scales it, and only charges for the milliseconds it works. When nothing’s happening, you pay nothing.
Why the exams love it
Any question describing a small job that fires in response to something, with “no servers to manage,” points to Lambda. Facts to keep: max 15-minute runtime; triggers include S3, API Gateway, EventBridge, SQS, DynamoDB Streams; and cold starts (first-invocation lag) are fixed with provisioned concurrency.
Test yourself
Every time an image is uploaded to S3, a thumbnail must be generated automatically, with no servers to manage. What is the best design?
- Poll the bucket from an EC2 instance
- An S3 event notification that triggers a Lambda function
- A cron job running on EC2
- Process the image in Amazon EMR
👉 Click to reveal the answer & explanation
Correct answer: B. An S3 event triggering Lambda is the classic serverless, event-driven pattern — it scales automatically and costs nothing when idle. EC2 polling or cron (A, C) means managing servers; EMR (D) is for big-data processing, not single-image events.
Related topics
Amazon S3 · Amazon EC2 · Amazon DynamoDB
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