A service processes customer payments from a message queue. Because the queue provides at-least-once delivery, the same payment message can be delivered more than once if the consumer times out before committing its state. Currently, the service sometimes charges the customer twice.
Which design change most directly prevents duplicate charges while still allowing safe retries?
Answer choices
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
Delete the message from the queue immediately after receive to prevent redelivery.
Deleting immediately can cause lost work when processing fails after receipt. It doesn’t ensure that retries won’t occur or that state commits are consistent.
Make the payment processing idempotent by recording an idempotency key for each payment and ensuring repeated deliveries do not apply the charge twice.
Idempotency ensures that reprocessing the same payment message has no additional side effects. Recording an idempotency key and using conditional logic prevents duplicate charges.
Increase the queue visibility timeout to a very large value so messages rarely reappear.
A larger visibility timeout reduces redelivery frequency but doesn’t remove duplicates. It can also delay recovery when a consumer crashes or fails permanently.
Switch to a single-threaded consumer with one worker so messages are processed in order.
Single-threaded ordering reduces concurrency issues but doesn’t prevent at-least-once redelivery duplicates. If a timeout occurs, the same message can still be processed again.
Common exam trap
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Technical deep dive
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
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FAQ
Static NAT maps one inside address to one outside address.
The correct answer is: Make the payment processing idempotent by recording an idempotency key for each payment and ensuring repeated deliveries do not apply the charge twice. — At-least-once delivery means duplicates are a normal, expected outcome—especially when a consumer times out before updating durable state. The most reliable resilience pattern is to make the operation idempotent. Record an idempotency key per payment (for example, in a database) and ensure the charge is only applied if the key has not been processed before. With conditional writes or equivalent mechanisms, retries can safely occur without duplicate side effects. Why others are wrong: Deleting immediately after receive can cause lost payments if processing fails before committing. Increasing visibility timeout reduces redeliveries but does not guarantee uniqueness and can worsen failure recovery times. Using a single-threaded consumer impacts ordering, not delivery semantics; duplicates can still occur when the consumer does not commit state before timeouts.
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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