Most applications need to persist some kind of data. In this chapter, we'll be looking at some approaches to data persistence for Node.js applications.
The default choice for persistence for a long time has been the traditional relational database. You may have used RDBMSs (relational database management systems) such as Microsoft SQL Server, Oracle, MySQL or PostgreSQL. These systems are often categorized as SQL databases since they all use SQL as their primary query language.
More recently, there has been a proliferation of so-called NoSQL databases. This umbrella term isn't particularly useful as a category. Some NoSQL databases have no more in common with each other than with traditional relational databases.