Interview was mostly about leaning about your thinking and approach towards a problem. How do you come up about handling a problem related to scale, are you able to modularize your components.
There were 3 rounds :
1) Round one was simple Java based question, not much about algorithms, data structures. It was more about coming up with right states as Enums, designing right processing flow, handling edge cases.
2) This was system design round about developing some component which may be used in booking.com. It was more about handling large scales, retries, failures, external system integration etc.
3) Third round was cultural fit round to evaluate how you handle pressure, failure, team etc.