1.1. What is persistence?

Almost all applications require persistent data. Persistence is one of the fundamental concepts in application development. If an information system didn't preserve data when it was powered off, the system would be of little practical use. When we talk about persistence in Java, we're normally talking about storing data in a relational database using SQL. We'll start by taking a brief look at the technology and how we use it with Java. Armed with that information, we'll then continue our discussion of persistence and how it's implemented in object-oriented applications.

1.1.1. Relational databases

You, like most other developers, have probably worked with a relational database. Most of us use a relational database every day. Relational technology is a known quantity, and this alone is sufficient reason for many organizations to choose it. But to say only this is to pay less respect than is due. Relational databases are entrenched because they're an incredibly flexible and robust approach to data management. Due to the complete and consistent theoretical foundation of the relational data model, relational databases can effectively guarantee and protect the integrity of the data, among other desirable characteristics. Some people would even say that the last big invention in computing has been the relational concept for data management as first introduced by E.F. Codd (Codd, 1970) more than three decades ago.