prometheus

The observability failure that scales with your success. Cardinality is the cost driver in every metrics system, and the fix has to be structural or it comes back next quarter.

A question with a right answer and a subtle reason. Histogram versus summary is the part that catches people, and it turns on whether you can aggregate across instances.

Most candidates recite the pull model and stop. The interviewer asking this wants the storage engine story, because that is where Prometheus hurts in production: crash recovery, cold queries, disk death.

Anyone can divide bad requests by total. This answer shows the aggregation mistakes that quietly corrupt SLO dashboards, the rate-versus-increase traps that make budgets lie, and when subqueries cost too much.

Forty healthy islands equal one blind estate. Compare the consolidation patterns honestly, keep alerting local, curate what crosses the WAN, and make the global view read-only and cheap.