When I started to develop Web applications in the mid- 1990s, I naïvely thought that if I needed to persist data across sessions, I could and should use a file. It quickly was explained to me that using the filesystem for such purposes was not a good idea, and that instead I should be using my server’s relational database. Using a database made it possible for me to access the same data from a number of different servers. Moreover, using a relational database meant that I could store my data in an appropriate format, using a data type (for example, a number or a text string) that would express my intent more clearly.