Google PCA Manage and provision cloud infrastructure Practice Question
A global e-commerce site uses an external HTTPS load balancer with a backend service pointing to a managed instance group. Some users report 503 errors during peak traffic. The backend instances are healthy and not overloaded. What is the most likely cause?
⚠ Common exam trap
Google Cloud often tests the misconception that 503 errors always indicate backend overload or health check failures, when in fact the load balancer's rate limiting configuration can cause 503s with perfectly healthy instances.
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 load balancer's max rate per backend is configured too low
A 503 error from an external HTTPS load balancer with healthy backends typically indicates that the load balancer is throttling requests. The 'max rate per backend' setting limits the number of requests per second that the load balancer forwards to each backend instance. When this limit is exceeded, the load balancer returns 503 errors even though the instances themselves are not overloaded, which matches the scenario of peak traffic.
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 CDN cache is not warming up properly
Why it's wrong here
CDN cache warming affects hit ratio and origin latency, not load balancer 503 responses, and the stem describes a backend service rather than CDN behaviour. It is tempting because cache misses during traffic spikes can degrade performance, making warming the right answer when origin load from cold caches is the actual problem.
- ✗
The backend service's health check interval is too short
Why it's wrong here
A short health check interval probes backends more frequently, which does not produce 503s when instances are healthy; capacity exhaustion or connection draining does. It is tempting because tuning health checks is a common remedy when backends are wrongly marked unhealthy, which is the correct fix for that different symptom.
- ✗
The SSL certificate is expired
Why it's wrong here
An expired SSL certificate causes TLS handshake failures and browser certificate warnings, not intermittent 503s under peak load. It is tempting because certificate expiry is a frequent cause of HTTPS outages, and it would be the correct diagnosis if all users failed consistently rather than only some during peaks.
- ✓
The load balancer's max rate per backend is configured too low
Why this is correct
The max rate per backend setting caps requests per second sent to each instance; when set too low, the load balancer sheds excess traffic and returns 503s even though instances are healthy. Raising this limit resolves the throttling-induced errors during peak load.
Go deeper
Related to this question
Learn chapter
IAM Policies, Service Accounts, and Auditing
Key term
Service
A service is a software component or system that performs a specific function and is available to be used by other programs or users over a network.
Key term
HTTPS
HTTPS (Hypertext Transfer Protocol Secure) is the secure version of HTTP that encrypts data between a web browser and a web server using SSL/TLS protocols.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This PCA 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 PCA exam.