AZ-204 Develop for Azure storage Practice Question
Which TWO Azure services can be used to trigger an Azure Function when a new blob is created in a storage account? (Choose two.)
⚠ Common exam trap
It's easy for candidates to assume the Blob Storage trigger is the only native option, forgetting that Event Grid is also a first-class trigger for blob creation events and is actually the recommended approach for low-latency scenarios.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Azure Event Grid trigger
Option C (Azure Event Grid trigger) is correct because Azure Storage integrates with Event Grid to publish blob-created events (Microsoft.Storage.BlobCreated), and an Azure Function can subscribe to that event via an Event Grid trigger to react to new blobs. Option D (Azure Blob Storage trigger) is correct because it is the native Azure Functions binding that polls/uses the Blob storage extension to invoke a function when a new or updated blob is detected in a container. Option A (Azure Cosmos DB trigger) is incorrect because it responds to changes in a Cosmos DB container's change feed, not to blob creation. Option B (Azure Queue Storage trigger) is incorrect because it fires on messages placed in a queue, not on blob creation events. Option E (Azure Service Bus trigger) is incorrect because it responds to messages on Service Bus queues or topics, which are unrelated to blob creation.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Azure Cosmos DB trigger
Why it's wrong here
The Azure Cosmos DB trigger is specifically designed to process changes from a Cosmos DB container's change feed. It monitors for document insertions, updates, and deletions within the NoSQL database itself, enabling real-time data processing. This trigger type is not integrated with Azure Blob Storage and therefore cannot detect or respond to the creation of new blobs.
- ✗
Azure Queue Storage trigger
Why it's wrong here
An Azure Queue Storage trigger activates an Azure Function when a new message is added to a specified storage queue. Its primary purpose is to enable message-driven processing workflows, where functions react to queued tasks or data. Since it operates solely on queue messages and has no inherent capability to monitor Blob Storage events, it cannot be used to trigger a function upon new blob creation.
- ✓
Azure Event Grid trigger
Why this is correct
Azure Event Grid is a highly scalable event routing service capable of subscribing to various event sources, including Azure Blob Storage. It can detect specific events like "Microsoft.Storage.BlobCreated" and reliably deliver them to an Azure Function endpoint. This makes Event Grid an excellent choice for triggering functions in response to new blob creations, offering robust event delivery and filtering capabilities.
- ✓
Azure Blob Storage trigger
Why this is correct
The Azure Blob Storage trigger is purpose-built for reacting to changes within Azure Blob Storage containers. It directly monitors a specified container for new or modified blobs, automatically invoking the associated function when such an event occurs. This native trigger provides a straightforward and efficient mechanism to execute code immediately upon the creation of a new blob.
- ✗
Azure Service Bus trigger
Why it's wrong here
The Azure Service Bus trigger is designed to process messages from Azure Service Bus Queues or Topics, facilitating enterprise messaging patterns. It activates an Azure Function when a new message arrives in the configured Service Bus entity, enabling robust, asynchronous communication. This trigger type is entirely distinct from Blob Storage events and lacks the functionality to monitor or respond to the creation of new blobs.
Quick reference
Azure Blob Storage Tier Comparison
| Tier | Storage Cost | Retrieval Cost | Latency | Use Case |
|---|---|---|---|---|
| Hot | Highest | Lowest | Immediate | Active data, frequent reads |
| Cool | Lower | Higher | Immediate | Data accessed < once / month |
| Cold | Lower still | Higher | Immediate | Data accessed < once / quarter |
| Archive | Lowest | Highest + rehydration delay | Hours | Long-term compliance retention |
Go deeper
Related to this question
Learn chapter
Azure Container Apps and KEDA
Key term
Azure Event Grid
Azure Event Grid is a fully managed event routing service that allows applications and services to react to events in real time using a publish-subscribe model.
Key term
Azure Functions Bindings
Azure Functions Bindings are declarative connections that link your serverless function code to Azure services or external resources, handling input and output data automatically without writing extra networking or authentication code.
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