A lot of services would be fine with some big traditional databases - StackOverflow is one example that scaled vertically instead of horizontally. But a lot of

A lot of services would be fine with some big traditional databases - StackOverflow is one example that scaled vertically instead of horizontally. But a lot of services would also get by with a distributed DB with weak integrity guarantees. Does it really matter if a few upvotes get lost on a busy post? By the time you notice sporadic integrity issues in your DB, the startup will probably have failed or raised enough money to not care.