Lambda runs your code in response to events without you provisioning servers. You upload a function, pick memory/timeout, and AWS handles scaling — running as many concurrent copies as needed. You pay per request and duration, nothing when idle.

Functions are triggered by events: an S3 upload, an API Gateway request, a schedule, a queue message, a DynamoDB change. Common patterns: serverless APIs, event processing, glue/automation. Watch out for cold starts (first invocation latency), execution time limits (15 min max), and statelessness.

# A Lambda handler triggered by an event
def handler(event, context):
    name = event.get("name", "world")
    return {"statusCode": 200, "body": f"Hello, {name}!"}

Event-driven patterns:
  S3 upload        → Lambda resizes the image
  API Gateway      → Lambda serves a REST endpoint
  EventBridge cron → Lambda runs a nightly cleanup
  SQS message      → Lambda processes the job

1. Give three event sources that can trigger a Lambda function.
2. Explain when Lambda is a better fit than an EC2 instance, and when it isn’t.
3. Describe what a cold start is and one way to reduce it.
4. Why must Lambda functions be stateless?

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