PDE Designing Data Processing Systems Practice Question
A company uses Pub/Sub with push subscriptions to deliver events to a Cloud Run service. Recently, the service has been returning HTTP 429 (Too Many Requests), causing messages to be retried and eventually sent to the dead letter topic. What is the MOST likely cause?
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
✓
The Cloud Run service needs more instances to handle the incoming request rate
Push subscriptions can be rate limited by the receiving service. Increasing ackDeadline gives more time but doesn't reduce rate. Using pull subscriptions shifts the rate control to the subscriber. Adjusting max delivery attempts only affects how many retries before dead letter, not rate limiting.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The subscription ackDeadline is set too low, causing messages to be redelivered
Why it's wrong here
Low ackDeadline causes redelivery but not HTTP 429 from the receiver.
- ✗
The push endpoint is not acknowledging messages quickly enough, causing a backlog
Why it's wrong here
Acknowledgment timeout doesn't cause 429; the endpoint itself is returning 429 due to overload.
- ✗
The dead letter topic is misconfigured, causing messages to be sent to it prematurely
Why it's wrong here
Dead letter topic is triggered after max delivery attempts, not directly causing 429.
- ✓
The Cloud Run service needs more instances to handle the incoming request rate
Why this is correct
Cloud Run scales based on requests; if max instances reached, it returns 429. Increasing instances or adjusting concurrency resolves this.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PDE practice question is part of Courseiva's free Google Cloud certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the PDE exam.