Went through the Booking.com senior engineer system design interview earlier this year. Posting because the prep advice I found online was generic and not super useful for their specific flavor.
What they asked: design a hotel search and availability system. Not surprising given the domain. But the interesting thing is they pushed it toward real distributed systems problems pretty quickly: how do you handle inventory consistency when two users are booking the same room at the same time, how do you make search fast over millions of properties with dynamic filters, what does your caching strategy look like.
Format: one hour, one interviewer plus a shadow. The interviewer asked me to share my screen and use a whiteboard tool (they suggested Miro but were fine with whatever). First 5-10 minutes was scoping and requirements, then I drove.
What they care about at senior level:
They specifically want to see you reason about scale. Booking.com handles enormous traffic and their SWE system design bar reflects that. I got asked follow-ups about how my approach would hold up at 10x traffic, what would break first, where the bottlenecks were. They also asked about operability: how would you monitor this, how would you know it's broken before users do.
The behavioral component was woven in: I got asked at least twice about a real system I'd built and what I'd do differently. So they're not just testing abstract design, they want to see if you've shipped things.
What tripped me up: I underinvested in the data model early on. Spent a lot of time on the service architecture and they had to redirect me back to the schema. I'd say spend the first 15 minutes on data model and API design, not just boxes-and-arrows architecture.
Comp context: the role was senior SWE, Amsterdam base. I'll post comp separately but it's competitive by European standards, less so if you're comparing to US TC.
Feel free to ask specifics. I took notes during debrief and remember most of it.