Quick Answer
The correct answer is the Cosmos DB output binding, along with a Blob input binding and an HTTP trigger. The Blob input binding reads the source data from Azure Storage Blob, while the Cosmos DB output binding writes the processed results to Cosmos DB; the HTTP trigger is essential because it provides the invocation mechanism for this serverless workflow, allowing an HTTP request to start the function that reads, processes, and writes data. On the AZ-204 exam, this scenario tests your understanding of how bindings connect Azure Functions to external services without writing connection code, and a common trap is forgetting the trigger—bindings alone cannot execute a function. Remember the mnemonic "TIB" for Trigger, Input, and Binding: every function needs a trigger to start, an input to read from, and an output to write to, so for a Blob-to-Cosmos DB pipeline, you always need the HTTP trigger, Blob input, and Cosmos DB output together.
This AZ-204 practice question tests your understanding of develop azure compute solutions. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 are designing a serverless application using Azure Functions that needs to read from an Azure Storage Blob, process the data, and write to Azure Cosmos DB. Which THREE bindings are required?
HTTP trigger
Triggers the function execution.
Cosmos DB input binding
Why wrong: Not needed; we write, not read.
Blob input binding
Reads the blob.
Cosmos DB output binding
Writes to Cosmos DB.
Timer trigger
Why wrong: Could be used, but HTTP trigger is more common for on-demand; however, the question asks for bindings required, and a trigger is always needed. Since only three correct, HTTP trigger is acceptable.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
HTTP trigger
The HTTP trigger is correct because the question describes a serverless application that needs to be invoked to read from Blob Storage, process data, and write to Cosmos DB. An HTTP trigger allows the function to be started via an HTTP request, which is the typical entry point for such event-driven processing. Without a trigger, the function cannot execute; the HTTP trigger provides the necessary invocation mechanism.
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.
HTTP trigger
Why this is correct
Triggers the function execution.
Related concept
Read the scenario before looking for a memorised answer.
Cosmos DB input binding
Why it's wrong here
Not needed; we write, not read.
Blob input binding
Why this is correct
Reads the blob.
Related concept
Read the scenario before looking for a memorised answer.
Cosmos DB output binding
Why this is correct
Writes to Cosmos DB.
Related concept
Read the scenario before looking for a memorised answer.
Timer trigger
Why it's wrong here
Could be used, but HTTP trigger is more common for on-demand; however, the question asks for bindings required, and a trigger is always needed. Since only three correct, HTTP trigger is acceptable.
Common exam traps
The trap here is that candidates often confuse input bindings with output bindings, mistakenly thinking a Cosmos DB input binding is needed to read data, when in fact the data source is Blob Storage and only an output binding to Cosmos DB is required.
Detailed technical explanation
Under the hood, the Blob input binding uses the Azure Storage SDK to read blob content as a stream or string, which is then passed to the function. The Cosmos DB output binding uses the Azure Cosmos DB SDK to upsert documents into a specified container, leveraging the function's return value or an IAsyncCollector for batch writes. In a real-world scenario, you might also use a Blob trigger instead of HTTP if the function should automatically fire when a new blob is created, but the question explicitly requires an HTTP trigger for on-demand invocation.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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FAQ
Develop Azure compute solutions — This question tests Develop Azure compute solutions — Read the scenario before looking for a memorised answer..
The correct answer is: HTTP trigger — The HTTP trigger is correct because the question describes a serverless application that needs to be invoked to read from Blob Storage, process data, and write to Cosmos DB. An HTTP trigger allows the function to be started via an HTTP request, which is the typical entry point for such event-driven processing. Without a trigger, the function cannot execute; the HTTP trigger provides the necessary invocation mechanism.
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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Same concept, more angles
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Variation 1. You are developing a serverless application using Azure Functions. The function must process messages from an Azure Storage Queue and write results to Azure Cosmos DB. Which binding should you use for the output?
easyWhy D: Option D is correct because the Azure Cosmos DB output binding allows you to write the results of queue-triggered function execution directly to a Cosmos DB container. The function processes messages from an Azure Storage Queue (input binding) and uses the output binding to insert or upsert documents into Cosmos DB without writing any SDK code.
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Last reviewed: Jun 24, 2026
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