Just cleared the MongoDB senior SWE loop two weeks ago, got an offer for an Atlas team role. Sharing notes on the system design round because it's different enough from typical FAANG prep that I wish I'd had this info going in.
The system design session was 60 minutes, one interviewer. No coding. They give you a vague prompt like "design a change data capture pipeline" or "design a multi-region document store sync mechanism" and they want you to drive. They're not just looking for the right answer, they want to see how you navigate ambiguity, ask clarifying questions, and make explicit trade-offs.
Few things I noticed:
MongoDB-specific framing matters. They really want you to ground your design in document model concepts. Mentioning sharding strategies, replica sets, oplog-based sync, or Atlas Search relevance won't hurt. I mentioned WiredTiger storage engine characteristics when justifying a write optimization. The interviewer visibly nodded. Know your audience.
No whiteboarding in the old sense. We used a shared Excalidraw. Make sure you're comfortable drawing box-and-arrow diagrams fast, because silence while you draw is awkward over video.
They push back. At least once mid-design they said "what if your message queue is unavailable?" It felt adversarial but it's really just probing depth. Stay calm, say you'd add a retry with exponential backoff, explain why. They want to see you adapt not crumble.
Depth over breadth. I tried to cover too many subsystems early and the interviewer slowed me down. They wanted me to pick one component and go deep. I think 60 minutes is genuinely not enough to design a full distributed system, so picking your battles is the skill.
Prep suggestions: read about change streams in MongoDB docs, understand how Atlas handles cross-region replication, and practice talking through CAP theorem trade-offs in document-store contexts specifically. LeetCode system design problems are a starting point but they don't have the MongoDB flavor. Designing Data-Intensive Applications (Kleppmann) chapter on replication is solid background.
Total loop was 5 rounds: phone screen, online assessment, system design, two behavioral rounds. I'll post separately on the OA. Happy to answer questions about the system design round.