Diagnosing slow auto-scaled PaaS workloads

WHAT THIS TESTS The interviewer wants disciplined, layered diagnosis rather than reflexively throwing more instances at the problem. Auto-scaling can mask or even worsen a bottleneck that lives outside the app tier.

A GOOD ANSWER COVERS Name two distinct categories. Application metrics measure what the code is doing: per-endpoint request latency, throughput in requests per second, error rate, time spent in database queries, and time blocked on external dependencies. Infrastructure metrics measure the platform: CPU and memory utilization per instance, current instance count versus desired, request queue depth, network saturation, and how long the platform takes to react to a scaling trigger. The skill is correlating them.

COMMON WRONG ANSWERS Listing only CPU. Assuming auto-scaling makes performance problems impossible. Adding capacity without first confirming the app tier is the constraint, when the real limit is a single shared database or a third-party API.

LIKELY FOLLOW-UPS What does high latency combined with low CPU tell you. How would scaling lag itself cause poor performance during a traffic spike. How do you find a slow database query.

ONE CONCRETE EXAMPLE Suppose response times triple under load. You check infrastructure metrics and see CPU sitting at thirty percent across all instances even though more were added, so capacity is not the issue. You then look at application metrics and find that database query time dominates each request, and the connection pool is exhausted because every new instance opened more connections to a database that cannot keep up. The bottleneck is the shared data tier, and auto-scaling actually made it worse by multiplying connection pressure. The fix is at the data layer, through query optimization, caching, read replicas, or a connection proxy, not adding more application instances. A second pattern: high latency with healthy CPU and database but slow external dependency calls reveals a downstream service as the limiter. Separating the two metric families is what lets you point to the true cause quickly.