Even though I followed pretty much the format from the "System Design Interview" book (by Alex Hu): I asked about the predicted scale and their answer it was that "it should scale hugely" (even in the problem they asked, they were talking about "galactical scale") Initially, I presented a high level system with just few components: LB, service layer, database layer. I proposed after that to dive into different components, and taking into consideration the scale it needed to be use multiple strategies for failures, caching, distributed database, etc. In the end, the feedback was that the system was too complex. Also, I've got the question "in how much time would you build a such system?" I would not expected a simple system to scale "galactically".