Were I asked this, my general approach would be: (1) Given the available building blocks (disks, ram, cpus, etc) what strategies are workable? An example of a

Were I asked this, my general approach would be: (1) Given the available building blocks (disks, ram, cpus, etc) what strategies are workable? An example of a strategy would be "store it in a btree and keep all but the last two levels in ram; therefore we'll need enough ram for the top of the tree and 2x seeks for each lookup." (2) Find existing software that implements a workable strategy if at all possible, or at least the complicated components of it and code the rest yourself. You can think of two extreme answers to this as "buy oracle" and "buy some transistors". Those are both bad answers. The first is a bad answer because, unless the candidate can answer the follow-up question "how does oracle work?" it probably just amounts to wishful thinking and they almost certainly won't be able to say when it will fail. The second is a bad answer because you can't do that in two weeks by yourself. Take for example, Foursquare's recent outage. From what I read online, it seems like they went with "use MongoDB" as a strategy without actually understanding what Mongo was doing under the hood. Presumably they didn't figure out how it would fail (the author's follow-up question, natch) and thus they discovered that the hard way. So they chose bad answer one. On the other hand, had they decided to implement their own database, they'd know all about its characteristics, but they wouldn't have launched yet.