I went straight to a blocklist cache sitting in front of the request path, which felt right, but I underestimated how much the interviewer wanted to dig into the sync mechanism.
Start by clarifying requirements and scale, then propose a layered architecture that ingests the external threat feed, caches it locally, and enforces denials at the edge with a fail-open strategy. Emphasize trade-offs between security, availability, and latency, and discuss how to handle feed staleness and false positives.
Pro tip: Proactively discuss fail-open vs. fail-closed behavior and the risk of false positives from the external authority; showing awareness of these trade-offs demonstrates production maturity.
Ask about request volume, latency SLAs, feed update frequency, and whether denial should be global or per-service. Confirm the expected size of the blocklist and the impact of false positives.
Propose a pipeline to fetch the external feed periodically, validate and normalize IPs, and store them in a low-latency store like an in-memory cache or distributed cache (e.g., Redis) with TTL. Consider using a CDN or edge service for global distribution.
Place the check at the outermost layer (load balancer, API gateway, or CDN) to reject requests early. Use efficient data structures (e.g., radix trees, bloom filters) for fast lookups and ensure the check adds minimal latency.
Decide on fail-open vs. fail-closed behavior if the feed or cache is unavailable. Implement TTLs and fallback to a stale copy to avoid outages. Monitor feed health and alert on anomalies.
Log denied requests with reasons, track false positives, and provide a way to override or allowlist. Continuously evaluate the effectiveness and adjust the architecture as needed.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.