Whenever a user uploads a photo, a three-second job must produce three thumbnails from it. Uploads are wildly uneven: nothing for an hour, then five thousand in a minute. The team does not want to run, patch or scale servers, and wants to pay only when photos actually arrive. What should they use?
AWS Certified Cloud Practitioner (CLF-C02), objective 3. Cloud technology and services easy
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The options
Correct An AWS Lambda function triggered by the S3 upload event
Correct. Lambda runs the code in response to the event, scales out to run many invocations at once without being told, requires no server management, and is billed only for the time the code runs.
Not correct An EC2 Auto Scaling group whose instances poll the bucket for new objects
Wrong. It works, but you pay for at least one instance running all day, you own its patching, and scaling out takes minutes — longer than the burst you are trying to absorb.
Not correct One large EC2 instance sized for the peak
Wrong. It sits almost entirely idle, costs the peak price around the clock, and is a single point of failure. This is the on-premises habit that elasticity exists to break.
Not correct An Amazon ECS task on EC2 scheduled to run every five minutes
Wrong on two counts. A scheduled task runs on a clock rather than on the upload, so thumbnails are up to five minutes late, and running the tasks on EC2 leaves you managing the instances underneath.
Why
Lambda's fit test is short, event-driven and stateless: you supply the function, AWS supplies everything under it, invocations scale out concurrently, and there is a maximum run time per invocation of fifteen minutes. Its economics are the mirror image of an instance's — you pay nothing while nothing happens, which is what makes it excellent for spiky work and usually a poor choice for a process that is busy all day, where a container or an instance is cheaper.
Where this comes from
- Cited
- AWS exam guide task statement 3.3
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