Just went through the senior SWE / L5-equivalent system design round at PANW and wanted to leave some notes while it's fresh. The system design round at cybersecurity companies is a bit different from the standard "design Twitter" style questions you practice for, and I was caught slightly off guard.
What they asked: Design a real-time threat detection pipeline that ingests endpoint telemetry at scale. Variants I've heard from others: design a distributed firewall policy evaluation system, design a log aggregation service with anomaly flagging.
The common thread: security context. They want to see you think about adversarial conditions, not just traffic spikes. Who might be sending malformed data? What if an attacker floods your ingestion layer?
What mattered most: Fault tolerance design. What happens when a downstream consumer crashes mid-stream? Latency vs. consistency tradeoffs, framed for security (a missed alert is worse than a duplicate alert) Horizontal scaling of stateful components Storage tiering: hot path for live alerting vs. cold storage for forensic queries
What they cared less about than I expected: UI/API design, cost optimization, the usual generic stuff. This is infra-heavy territory.
Level calibration note: At senior / L5 they want you to drive the conversation and anticipate the follow-up questions before they ask. If you wait to be led, that's a flag. I proposed two approaches to the ingestion layer and asked which constraint mattered more to them. They liked that.
Prep: Reviewing Kafka internals, Elasticsearch indexing behavior, and any incident post-mortems from security vendors (Cloudflare, Fastly) is genuinely useful. The PANW engineering blog has a few pieces on Cortex XSIAM internals worth a skim.
Total round length was 55 minutes. I had 10 left at the end but ran out of design surface, which I think is fine.