Quick Answer
The answer is the Cosmos DB trigger. This is the correct choice because the Azure Cosmos DB trigger is specifically designed to listen to the change feed of a container, which captures all inserts and updates to documents. When audit documents are inserted or updated, the change feed automatically invokes the associated Azure Function, making it the ideal serverless reaction mechanism without needing custom polling logic. On the Microsoft Azure Developer Associate AZ-204 exam, this tests your understanding of how to integrate Azure Functions with Cosmos DB for event-driven architectures, often appearing in scenarios involving real-time data processing or audit logging. A common trap is confusing the Cosmos DB trigger with the output binding—remember, the trigger reads the change feed, while the output binding writes data. For a quick memory tip: think of the Cosmos DB trigger as a "change listener" that only fires on inserts and updates, never deletes, just like a security camera that only records when something new appears or moves.
This AZ-204 practice question tests your understanding of develop for azure storage. 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.
A serverless app must react whenever audit documents are inserted or updated in Cosmos DB. Which trigger should the Azure Function use?
Queue trigger
Why wrong: Queue triggers respond to queue messages, not Cosmos DB changes directly.
Timer trigger
Why wrong: A timer trigger runs on a schedule and does not automatically follow the change feed.
HTTP trigger
Why wrong: HTTP triggers require incoming HTTP requests.
Cosmos DB trigger
The Cosmos DB trigger reads the change feed and invokes the function for inserts and updates.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
Cosmos DB trigger
The Azure Cosmos DB trigger listens to the change feed of a Cosmos DB container, which captures inserts and updates to documents. This makes it the ideal choice for reacting to audit document changes in a serverless app, as it automatically invokes the function when new or modified documents appear in the feed.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Queue trigger
Why it's wrong here
Queue triggers respond to queue messages, not Cosmos DB changes directly.
Timer trigger
Why it's wrong here
A timer trigger runs on a schedule and does not automatically follow the change feed.
HTTP trigger
Why it's wrong here
HTTP triggers require incoming HTTP requests.
Cosmos DB trigger
Why this is correct
The Cosmos DB trigger reads the change feed and invokes the function for inserts and updates.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates may confuse the Cosmos DB trigger with a generic database trigger, forgetting that it specifically relies on the change feed and not on direct database events like stored procedures or triggers in SQL Server.
Detailed technical explanation
Under the hood, the Cosmos DB trigger polls the change feed using the `_ts` and `_lsn` properties to track processed changes, and it supports leases via a separate container for checkpointing and scaling. In a real-world scenario, if the audit container has high throughput, the trigger can scale out across multiple function instances using the lease container to partition the change feed. A subtle behavior is that the trigger only captures inserts and updates, not deletes, unless you use a soft-delete pattern.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
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FAQ
Develop for Azure storage — This question tests Develop for Azure storage — Read the scenario before looking for a memorised answer..
The correct answer is: Cosmos DB trigger — The Azure Cosmos DB trigger listens to the change feed of a Cosmos DB container, which captures inserts and updates to documents. This makes it the ideal choice for reacting to audit document changes in a serverless app, as it automatically invokes the function when new or modified documents appear in the feed.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Variation 1. A serverless app must react whenever audit documents are inserted or updated in Cosmos DB. Which trigger should the Azure Function use? The design must avoid adding custom operational scripts.
mediumWhy D: The Azure Cosmos DB trigger is the correct choice because it natively listens to the Cosmos DB change feed, which captures inserts and updates to documents. This allows the Azure Function to react automatically without any custom scripts or polling logic, aligning with the serverless and operational simplicity requirements.
Last reviewed: Jun 11, 2026
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