The answer is to enable duplicate detection on the Service Bus queue. This is correct because Azure Service Bus duplicate detection ensures exactly-once processing by having the broker itself track the MessageId of each message within a user-defined time window; any subsequent message with the same MessageId is automatically discarded, preventing the Azure Function from processing duplicates even under heavy load or after restarts. On the AZ-204 exam, this scenario tests your understanding of how to guarantee idempotent message handling without relying on the consumer—a common trap is assuming the function’s retry policy or checkpointing alone solves duplicates, but only the broker-level feature eliminates re-deliveries. Remember: the broker is the bouncer—it checks the ID at the door, so the function never sees the same message twice.
AZ-204 Develop Azure compute solutions Practice Question
This AZ-204 practice question tests your understanding of develop azure compute solutions. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
You deploy an Azure Function app that uses the Premium plan. The function processes messages from an Azure Service Bus queue. Under heavy load, some messages are processed multiple times. You need to ensure exactly-once processing without losing messages. What should you do?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Enable duplicate detection on the Service Bus queue.
Option A is correct because enabling duplicate detection on the Service Bus queue ensures that the Service Bus broker itself discards duplicate messages based on a user-defined time window. This prevents the function from processing the same message multiple times, even if the function host restarts or the message is re-delivered due to transient failures. Duplicate detection works by tracking the MessageId of each message and ignoring any subsequent message with the same MessageId within the detection window.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Enable duplicate detection on the Service Bus queue.
Why this is correct
Duplicate detection ensures the queue removes duplicates based on the MessageId, enabling exactly-once processing.
Related concept
Read the scenario before looking for a memorised answer.
✗
Use Peek-Lock mode instead of Receive and Delete.
Why it's wrong here
Peek-Lock is the default and doesn't guarantee exactly-once if the function crashes after processing.
✗
Set the maxDeliveryCount to 1 on the queue.
Why it's wrong here
This would dead-letter messages on failure, but doesn't prevent duplicates from multiple deliveries.
✗
Reduce the batch size in the function host.json.
Why it's wrong here
Batch size affects throughput, not duplicate processing.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often confuse client-side idempotency (e.g., using a database unique constraint) with broker-level duplicate detection, or they mistakenly believe that Peek-Lock mode alone guarantees exactly-once processing, ignoring the risk of crashes after processing but before completion.
Detailed technical explanation
How to think about this question
Under the hood, Service Bus duplicate detection uses a sliding window of up to 7 days (default 30 seconds) where it stores MessageId hashes in memory. When a message arrives, the broker checks if its MessageId already exists in the window; if so, the duplicate is silently dropped. This is critical in scenarios where the function host scales out under load, because multiple instances might receive the same message from the queue, but only one will be accepted by the broker. A real-world example is a payment processing function where even a single duplicate charge would be catastrophic.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A cloud solutions architect for a retail company is evaluating services for a new workload. The correct answer here reflects best practice for the specific scenario described — not a general cloud recommendation. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Cloud exam questions reward reading the constraint carefully: the same technology can be right or wrong depending on the use case.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this AZ-204 question in full detail.
Develop Azure compute solutions — This question tests Develop Azure compute solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Enable duplicate detection on the Service Bus queue. — Option A is correct because enabling duplicate detection on the Service Bus queue ensures that the Service Bus broker itself discards duplicate messages based on a user-defined time window. This prevents the function from processing the same message multiple times, even if the function host restarts or the message is re-delivered due to transient failures. Duplicate detection works by tracking the MessageId of each message and ignoring any subsequent message with the same MessageId within the detection window.
What should I do if I get this AZ-204 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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