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200-901 Practice Question: A developer is working on a Python application…
A developer is working on a Python application that automates the configuration of multiple Cisco IOS-XE devices using RESTCONF. The application uses the requests library. The developer notices that sometimes the PUT request to update the interface description returns a 409 Conflict error. Upon investigation, the developer finds that the issue occurs when two instances of the application are running concurrently and attempt to update the same interface. The developer wants to implement a strategy to avoid conflicts. Which approach is most effective?
⚠ Common exam trap
Cisco often tests the misconception that retries or changing HTTP methods (PUT to PATCH) can resolve concurrency conflicts, when in fact they do not address the root cause of simultaneous writes to the same resource.
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
✓
Use a distributed lock mechanism to ensure exclusive access
A distributed lock mechanism (Option B) is the most effective approach because it ensures exclusive access to the shared resource (the interface configuration) across multiple application instances. In a concurrent environment, retries (Option A) cannot prevent the fundamental race condition—both instances may still attempt conflicting writes. A distributed lock, such as one based on Redis or ZooKeeper, serializes access, guaranteeing that only one instance modifies the interface at a time, which directly resolves the 409 Conflict error from RESTCONF.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Implement a retry mechanism with exponential backoff and random jitter
Why it's wrong here
Retries can reduce collisions but do not prevent them, and may still fail under heavy concurrency.
- ✓
Use a distributed lock mechanism to ensure exclusive access
Why this is correct
A lock guarantees that only one instance modifies the resource at a time, eliminating conflicts.
- ✗
Change the PUT to PATCH and hope for partial updates
Why it's wrong here
PATCH can also conflict; the method change does not address concurrency.
- ✗
Use a timestamp in the request to force overwrite
Why it's wrong here
RESTCONF does not typically support last-write-wins; forcing overwrite may cause data inconsistency.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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