What do we Persist?

Despite the Oxford English Dictionary's pedantic insistence that persist is an intransitive verb, programmers like to talk about persisting an object. When we persist an object, we store it away so that we can get at it later. You'll find us talking about persisting our money for a rainy day.

Typically, we persist an object or a "web" of connected objects. A web of objects consists of a starting object (called the focus) and all the objects connected to that focus. The web also includes all the objects connected to these other objects, and so on. After a change is made to the state of the object, it may be persisted again.

The operations that need to be available to persist an object or a web of objects are similar to those that are classically used in relational database management systems:

These form the acrostic CRUD: Create, Read, Update, Delete

The way these operations are implemented depends on the type of persistence storage used. A number of different persistence mechanisms will be considered in the following pages, but the essential issues are the same across all techniques.

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