AZ-204 Develop Azure compute solutions Practice Question
You are implementing an Azure Durable Functions application that processes orders. The function must call three external APIs (payment gateway, inventory system, and shipping calculator) in parallel, then aggregate the results once all three have completed. Which Durable Functions pattern should you use?
⚠ Common exam trap
Candidates often confuse 'parallel execution' with 'chaining' or 'monitoring', but the key differentiator is the need to wait for all parallel tasks to finish before aggregating results, which is the hallmark of the Fan-out/Fan-in pattern.
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
✓
Fan-out/Fan-in
The Fan-out/Fan-in pattern is designed exactly for this scenario: it triggers multiple function tasks in parallel (fan-out) and then aggregates their results once all complete (fan-in). In Durable Functions, this is implemented using `CallActivityAsync` in a loop with `Task.WhenAll` to wait for all parallel activities to finish, allowing the orchestrator to collect and process the combined results.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Function chaining
Why it's wrong here
This Durable Functions pattern orchestrates a sequence of activity functions, where the output of one activity becomes the input for the next. While it ensures a defined workflow, its fundamental design is sequential, meaning activities execute one after another. Therefore, it cannot be used to execute multiple API calls concurrently or to aggregate results from parallel operations, making it unsuitable for fan-out/fan-in scenarios.
- ✓
Fan-out/Fan-in
Why this is correct
This pattern is specifically designed for scenarios requiring parallel execution of multiple tasks followed by aggregation of their results. An orchestrator function initiates multiple activity functions concurrently (fan-out), often using Task.WhenAll in C# to asynchronously wait for all of them to complete. Once all parallel activities have finished, the orchestrator then collects and processes their individual outputs (fan-in) to produce a single, consolidated result. This perfectly matches the requirement for parallel API calls and subsequent data aggregation.
- ✗
Monitor
Why it's wrong here
The Monitor pattern in Durable Functions is used for long-running processes that periodically check the status of an external system or resource until a specific condition is met. It leverages durable timers to schedule future checks and does not inherently support the concurrent execution of multiple distinct tasks. Its primary purpose is state observation and conditional continuation, not parallel task execution and result aggregation.
- ✗
Human interaction
Why it's wrong here
This Durable Functions pattern enables an orchestrator function to pause its execution indefinitely until an external event is received, typically triggered by a human user or an external system requiring manual intervention. The orchestrator waits for an external signal to resume its workflow. This pattern is entirely focused on incorporating manual steps into an automated process and is not designed for, nor does it facilitate, the parallel execution of API calls or the aggregation of their results.
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