It's just a spectrum. At one end, where all we do is write, we use log files. At the other end, where all we do is read, we cache static content at network hu

It's just a spectrum. At one end, where all we do is write, we use log files. At the other end, where all we do is read, we cache static content at network hubs. There's a lot in between. An RDBMS is one tradeoff, 'NoSQL' represents another. They simply imply different types of read/write tradoffs.