Back-end: Know your stuff around distributed systems. They could ask anything about this mega-subject: Resilience, Transparency, Scalability, Performance and so on. They are very open to your answers and know there is not only one answer. If you look at their engineering website, you can also see that they work heavily with Amazon Web Services. Although not entirely necessary, it would be nice to find a way to give AWS-related answers as well when asked about concepts of distributed systems.