Google PCA Manage and provision cloud infrastructure Practice Question
A company runs a web application on Compute Engine with an HTTP Load Balancer. Users report intermittent 502 Bad Gateway errors. What is the most likely cause?
⚠ Common exam trap
Many exam-takers confuse 502 errors with SSL or quota issues, but the PCA exam specifically tests that 502 errors from an HTTP Load Balancer are almost always due to backend unavailability or overload, not frontend configuration problems.
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
✓
Backend instances are unhealthy or overloaded.
The 502 Bad Gateway error from an HTTP Load Balancer typically indicates that the backend instances are failing to respond to the load balancer's health checks or are overwhelmed, causing the load balancer to consider them unhealthy and return a 502 error. This is the most common cause because the load balancer relies on healthy backends to forward traffic, and overloaded or failing instances cannot handle requests.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Load balancer quota exceeded.
Why it's wrong here
Quota exhaustion blocks new load balancer configuration or resources, producing provisioning failures rather than intermittent 502s from healthy backends. It is tempting because quotas can disrupt traffic, but the correct cause is a backend failing health checks or returning invalid responses intermittently.
- ✗
Firewall rules block health checks.
Why it's wrong here
Firewall rules blocking health checks mark backends unhealthy, which removes them from rotation rather than producing intermittent 502s. It is tempting because firewall misconfiguration disrupts load balancing, but the correct cause is a backend intermittently failing health checks or returning errors.
- ✗
SSL certificate expired.
Why it's wrong here
An expired certificate causes browser TLS handshake failures and warnings, not 502 Bad Gateway responses from the load balancer. It is tempting because certificate problems break HTTPS, but the 502 originates from the proxy failing to obtain a valid response from a healthy backend.
- ✓
Backend instances are unhealthy or overloaded.
Why this is correct
An HTTP(S) load balancer returns 502 when it cannot obtain a valid response from a backend. Unhealthy instances removed by health checks, or overloaded instances timing out, both produce this, matching the intermittent pattern reported.
Go deeper
Related to this question
Learn chapter
Load Balancing and Autoscaling
Key term
Load balancer
A load balancer is a device or software that distributes incoming network traffic across multiple servers so no single server gets overwhelmed.
Key term
HTTP
HTTP stands for Hypertext Transfer Protocol, the set of rules web browsers and servers use to communicate and transfer web pages over the internet.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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