Comment by georgewfraser

It is amazing how many large-scale applications run on a single or a few large RDBMS. It seems like a bad idea at first: surely a single point of failure must be bad for availability and scalability? But it turns out you can achieve excellent availability using simple replication and failover, and you can get huge database instances from the cloud providers. You can basically serve the entire world with a single supercomputer running Postgres and a small army of stateless app servers talking to it.