Application programming interface

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An application programming interface (API) is an interface that a software program implements in order to allow other software to interact with it; much in the same way that software might implement a user interface in order to allow humans to interact with it.

APIs are implemented by applications, libraries and operating systems to define how other software can make calls to or request services from them.^[1][2][3] An API determines the vocabulary and calling conventions the programmer should employ to use the services. It may include specifications for routines, data structures, object classes, and protocols used to communicate between the consumer and implementer of the API.

Features

An API, like all interfaces, is abstract. The software that provides the functionality described by an API is said to be an implementation of the API.

An API may be:

• Language-dependent; that is, available only in a given programming language, using the syntax and elements of that language to make the API convenient to use in this context.

• Language-independent; that is, written in a way that means it can be called from several programming languages. This is a desired feature for a service-style API that is not bound to a given process or system and may be provided as remote procedure calls or web services.

For example, a website that allows users to review local restaurants is able to layer their reviews over maps taken from Google Maps, because Google Maps has an API that allows it. Google Maps' API controls what information a third-party site can grab, and what can be done with it.

"API" may be used to refer to a complete interface, a single function, or even a set of multiple APIs provided by an organization. Thus, the scope of meaning is usually determined by the person or document that communicates the information.

Web APIs

When used in the context of web development, an API is typically a defined set of Hypertext Transfer Protocol (HTTP) request messages along with a definition of the structure of response messages, usually expressed in an Extensible Markup Language (XML) or JavaScript Object Notation (JSON) format. While "Web API" is virtually a synonym for web service, the recent trend (so-called Web 2.0) has been moving away from Simple Object Access Protocol (SOAP) based services towards more direct Representational State Transfer (REST) style communications^[4]. Web API's allow the combination of multiple services into new applications known as mashups ^[5].

Web Communities use APIs to share content in an Open Architecture

The practice of publishing APIs has allowed web communities to create an open architecture for sharing of content and data between communities and applications. In this way, content that is created in one place can be dynamically posted and/or updated in multiple locations on the web.
1. Photos can be shared from sites like Flickr and Photobucket to Social Network sites like Facebook and MySpace.
2. Content can be embedded, Here is an example of a presentation embedded by SlideShare on Linked-in:Embedded Slide Show in a Profile
3. Content can be dynamically posted. Sharing comments made on Twitter to a Facebook account for example is enabled by Twitter and Facebook's APIs.
4. Video content can be embedded on sites, in this example of an embedded YouTube Video, where the content is served by YouTube on a third party site Embedded Video Content Example
5. User information can be shared from web communities to outside applications, delivering new functionality to the web community that shared its user data via an open API. One of the best examples of this is the Facebook Application platform. Another is the Open Social platform.

^[6].

Implementations

The POSIX standard defines an API that allows a wide range of common computing functions to be written such that they may operate on many different systems (Mac OS X and various Berkeley Software Distributions (BSDs) implement this interface); however, making use of this requires re-compiling for each platform. A compatible API, on the other hand, allows compiled object code to function without any changes, on any system implementing that API. This is beneficial to both software providers (where they may distribute existing software on new systems without producing/distributing upgrades) and users (where they may install older software on their new systems without purchasing upgrades), although this generally requires various software libraries implementing the necessary APIs too.

Microsoft has shown commitment to a backward compatible API, particularly within their Windows API (Win32) library, such that older applications may run on newer versions of Windows using an executable-specific setting called "Compatibility Mode"^[7]. Apple Inc. has shown less propensity to this concern, breaking compatibility or implementing an API in a slower "emulation mode"; this allows greater freedom in development, at the cost of making older software obsolete.

Among Unix-like operating systems, there are many related but incompatible operating systems running on a common hardware platform (particularly Intel 80386-compatible systems). There have been several attempts to standardise the API such that software vendors may distribute one binary application for all these systems; however to date, none of these have met with much success. The Linux Standard Base is attempting to do this for the Linux platform, while many of the BSD Unices (FreeBSD, NetBSD, OpenBSD) implement various levels of API compatibility for both backward compatibility (allowing programs written for older versions to run on newer distributions of the system) and cross-platform compatibility (allowing execution of foreign code without recompiling).

Release policies

Common API publishing policies include:

• Companies do not release API information to anybody outside of the company.
• Companies protect information on their APIs from the general public. For example, Sony used to make its official PlayStation 2 API available only to licensed PlayStation developers. This enabled Sony to control who wrote PlayStation 2 games. Such control can have quality control benefits and potential license revenue.
• Companies make their APIs freely available. For example, Microsoft makes the Microsoft Windows API public and Apple releases its APIs Carbon and Cocoa so that software can be written for their platform.

ABI's

The related term application binary interface (ABI) is a lower level definition concerning details at the assembly language level. For example, the Linux Standard Base is an ABI, while POSIX is an API.^[8]

API examples

Language bindings and interface generators

APIs that are intended to be used by more than one high-level programming language often provide, or are augmented with, facilities to automatically map the API to features (syntactic or semantic) that are more natural in those languages. This is known as language binding, and is itself an API. The aim is to encapsulate most of the required functionality of the API, leaving a "thin" layer appropriate to each language.

Below are listed some interface generator tools which bind languages to APIs at compile time.

• SWIG generates interfaces from many languages for many languages (Typically Compiled->Scripted)
• F2PY: Fortran to Python interface generator.
• XPCOM (Cross Platform Component Object Model) is a cross platform component model from Mozilla.

See also

• API Writer
• Application Binary Interface (ABI)
• Document Object Model (DOM)
• Double-chance function
• List of 3D graphics APIs
• Mashup (web application hybrid)
• Open Service Interface Definitions (OSID)
• Platform-enabled website
• Plugin
• Software Development Kit (SDK)
• Web service

References

1. ^ "Application Program Interface". Free On-line Dictionary of Computing. 1995-02-15. http://foldoc.org/Application+Program+Interface. Retrieved 2009-06-28. 
2. ^ "Definition of: API". PC Magazine. 1996. http://www.pcmag.com/encyclopedia_term/0,2542,t=application+programming+interface&i=37856,00.asp. Retrieved 2009-06-28. 
3. ^ Orenstein, David (2000-01-10). "QuickStudy: Application Programming Interface (API)". Computerworld. http://www.computerworld.com/action/article.do?command=viewArticleBasic&articleId=43487. Retrieved 2009-06-04. 
4. ^ Benslimane, Djamal; Schahram Dustdar, and Amit Sheth (2008). "Services Mashups: The New Generation of Web Applications" (HTML). IEEE Internet Computing, vol. 12, no. 5. Institute of Electrical and Electronics Engineers. pp. 13-15. http://dsonline.computer.org/portal/site/dsonline/menuitem.9ed3d9924aeb0dcd82ccc6716bbe36ec/index.jsp?&pName=dso_level1&path=dsonline/2008/09&file=w5gei.xml&xsl=article.xsl. 
5. ^ "Mashup Dashboard". ProgrammableWeb.com. 2009. http://www.programmableweb.com/mashups. 
6. ^ David Shantz (Oct 26 2009). "Dynamic Community content via APIs" (html). magnity.com. http://www.magnity.com/?id=how. 
7. ^ Microsoft (October 2001). "Run Older Programs On Windows XP" (in EN) (HTML). Microsoft. pp. 4. http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/appcompat.mspx. 
8. ^ Stoughton, Nick (April 2005). "Update on Standards" (PDF). USENIX. https://db.usenix.org/publications/login/2005-04/openpdfs/standards2004.pdf. Retrieved 2009-06-04. 

External links

• Practical API Design book and collaboration site
• How to design a good API and why it matters-PDF
• Service-Oriented Architecture (SOA): The Road to Enterprise Application Integration (EAI)
• ISO/IEC JTC 1 Directives, 5th Edition Version 3.0, Annex J: Guidelines for API standardization
• API functions tutorial for game making