Information about Persistence (computer Science)

In computer science, persistence refers to the characteristic of data that outlives the execution of the program that created it. Without this capability, data only exists in memory, and will be lost when the memory loses power, such as on computer shutdown.

Computer programming

In programming, persistence refers specifically to the ability to retain data structures between program executions, such as, for example, an image editing program saving complex selections or a word processor saving undo history.

This is achieved in practice by storing the data in non-volatile storage such as a file system or a relational database or an object database. Design patterns solving this problem are container based persistence, component based persistence and the Data Access Object model. When first introduced, the idea was that persistence should be an intrinsic property of the data, in contrast with the traditional approach where data is read and written to disk using imperative verbs in a programming language. This emphasis has largely disappeared, resulting in the use of persist as a transitive verb: On completion, the program persists the data. Examples of persistence are using Java serialization to store Java objects on disk or using Java EE to store Enterprise Java Beans in a relational database.

Built-in to operating systems and programming languages

Orthogonal persistence refers to inherent support provided by a programming language or operating system of a computer that enables the state of programs written in a persistent programming language, or of the operating system itself, to remain persistent even after a crash or unexpected shutdown. The computer stays in the same state even if you turn it off, behaving much like a TV or similar device.

Operating systems that employ this ability include

• KeyKOS
• EROS, the successor to KeyKOS
• Coyotos and
• CapROS, revisions of EROS
• Multics with its single-level store

Programming languages that implement orthogonal persistence include

• PS-Algol
• Smalltalk and Lisp, using images

User interface

Most software applications include persistence as a minimum required feature in order for the software to be considered complete. In this context, persistence operations are usually separated into well-recognized categories, based on the types of data entries stored by the software. These categories are:

• the ability to add (or create) new entries;
• the ability to view (or retrieve) existing entries;
• the ability to edit (or update) existing entries;
• the ability to delete existing entries.

Moreover, although not necessarily a minimum required standard for software, the following operations are also commonly associated with persistence, whenever a large number of entries must be maintained:

• the ability to search for entries;
• the ability to sort entries;
• the ability to filter entries.

Storage devices

Persistent storage (PSD) is the ability for a device to maintain data even when it is turned off. Devices that provide persistent storage capabilities are called non-volatile memory devices. Examples are a hard drive, floppy disk and CD.

Devices that need power to provide data, as does random access memory (RAM), are called volatile memory devices and do not provide persistent storage.

On many early personal data assistants (PDAs), all data was stored in RAM. When the batteries ran out of power, all data was lost. Most newer PDA designs provide persistent storage to prevent the user from losing all data on the device.

See also

• CRUD
• Hibernate (Java)
• Java Persistence API
• Object Prevalence
• Orthogonality
• Persistent data structure
• Service Data Object