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

Contents 1 Overview 2 Persistence topics 2.1 Built-in to operating systems and programming languages 2.2 User interface 2.3 Storage devices 2.4 Incremental persistence 2.5 Persistence Services 3 See also 4 References

[edit] Overview

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.

[edit] Persistence topics

[edit] Built-in to operating systems and programming languages

Orthogonal persistence refers to either

• 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. Programming languages that implement orthogonal persistence include
  • PS-Algol
  • Smalltalk and Lisp, using images

• operating systems that 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
  • CapROS, revisions of EROS
  • Coyotos, successor to EROS
  • EROS, the successor to KeyKOS
  • KeyKOS
  • Multics with its single-level store
  • Phantom

[edit] 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.

[edit] 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, flash memory 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.

[edit] Incremental persistence

Incremental persistence is simply writing each change to an in-memory-datastructure immediately to disk or other non volatile storage device. When used with object-oriented languages, this is known as object prevalence.

[edit] Persistence Services

Persistence services is when the workflow runtime engine semantics dictate that persistence should occur, the workflow runtime engine will call methods supplied by a persistence service to save state information about the workflow instance. Likewise when workflow runtime engine needs to restore a previously persisted workflow instance, it will call methods supplied by the persistence service to load this state information.

[edit] See also

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

[edit] References

This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources (ideally, using inline citations). Unsourced material may be challenged and removed. (October 2008)