An online shop's checkout writes the order and then calls the fulfilment service directly, waiting for it to answer. When fulfilment is slow or restarting, checkouts fail and orders are lost. The team wants checkout to accept the order regardless, and fulfilment to work through the backlog at its own pace. What should they introduce?
AWS Certified Cloud Practitioner (CLF-C02), objective 3. Cloud technology and services medium
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The options
Correct An Amazon SQS queue that checkout writes to and fulfilment polls
Correct. The queue holds each order until a consumer has processed and deleted it, so a spike or an outage in fulfilment becomes a growing backlog rather than failed checkouts.
Not correct An Amazon SNS topic that checkout publishes to
Wrong for this requirement. SNS pushes each message out to its subscribers as it arrives; it is the fan-out tool for telling several parties that something happened, not a buffer that one worker drains at whatever rate it can manage.
Not correct Retry logic in checkout with a longer timeout
Wrong. Checkout is still coupled to fulfilment's health, and the customer now waits longer before being told it failed. Retries help with a blip; they do not survive a restart.
Not correct A larger instance for the fulfilment service
Wrong. It raises the ceiling without changing the shape of the problem: the next spike above the new ceiling, or the next restart, fails in exactly the same way.
Why
Amazon SQS is a pull-based queue: producers write, one consumer group reads, messages are held until deleted, a visibility timeout stops two workers taking the same message, and a dead-letter queue catches what repeatedly fails. Its value is loose coupling — either side can be down, slow or scaling and the other carries on. Keep the contrast sharp: a queue holds work until somebody does it, and a topic tells everybody that it happened.
Where this comes from
- Cited
- AWS exam guide task statement 3.8
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