Difference between revisions of "Application programming interface"

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An '''application programming interface''' ('''API''') is a particular set of rules and specifications that software programs can follow to communicate with each other.  It serves as an interface between different software programs and facilitates their interaction, similar to the way the user interface facilitates interaction between humans and computers.
{{wikipedia::API}}
 
An API can be created for applications, libraries, operating systems, etc., as a way of  defining their "vocabularies" and resources request conventions (e.g. function-calling conventions). It may include specifications for routines, data structures, object classes, and protocols used to communicate between the consumer program and the implementer program of the API.<ref>{{cite web|
title=Definition of: API|
url=http://www.pcmag.com/encyclopedia/term/37856/api|
publisher=PC Magazine|
accessdate=28 June 2009}}</ref><ref>{{cite web|
first=David|
last=Orenstein|
url=http://www.computerworld.com/action/article.do?command=viewArticleBasic&articleId=43487|
title=QuickStudy: Application Programming Interface (API)|
publisher=Computerworld|
date=10 January 2000|
accessdate=04 June 2009}}</ref>
 
==Concept==
An API can be:
* general, the full set of an API that is bundled in the libraries of a programming language, e.g. Standard Template Library in C++ or Java API.
 
* specific, meant to address a specific problem, e.g. Google Maps API or Java API for XML Web Services.
 
* language-dependent, meaning it is only available by using the syntax and elements of a particular language, which makes the API more convenient to use.
 
* language-independent, written so that it can be called from several programming languages. This is a desirable feature for a service-oriented API that is not bound to a specific 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 facilitates this functionality. Google Maps' API controls what information a third-party site can use and how they can use it.
 
API may be used to refer to a complete interface, a single function, or even a set of APIs provided by an organization. Thus, the scope of meaning is usually determined by the context of usage.
 
==Advanced explanation==
An API may describe the ways in which a particular task is performed.
In procedural languages like C language the ''action'' is usually ''mediated'' by a function call.
 
For instance: the <code>math.h</code> include file for the C language contains the definition of the function prototypes of the mathematical functions available in the C language library for mathematical processing (usually called <code>libm<code>).
This file describes how to ''use'' the functions included in the given library: the function prototype is a signature that describes the number and types of the parameters to be passed to the functions and the type of the return value. 
 
The ''behavior'' of the functions is usually described in more details in a human readable format in printed books or in electronic formats like the man pages: e.g. on [[Unix]] systems the command
 
<code>man 3 sqrt</code>
 
will present the signature of the function <code>sqrt</code> in the form:
<source lang="C">
SYNOPSIS
            #include <math.h>
            double sqrt(double X);
            float  sqrtf(float X);
DESCRIPTION
      DESCRIPTION
      sqrt computes the positive square root of the argument. ...
RETURNS
      On success, the square root is returned. If X is real and positive...
</source>
 
That means that the function returns the square root of a positive floating point number (<code>single</code> or <code>double</code> precision) as another floating point number.  Hence the API in this case can be interpreted as the collection of the included files used by the C language and its human readable description provided by the man pages.
===API in modern languages===
Most of the modern programming languages provide the documentation associated with an API in some digital format that makes it easy to consult on a computer. E.g. perl comes with the tool perldoc:
<source lang="perl">
$ perldoc -f sqrt
      sqrt EXPR
      sqrt    #Return the square root of EXPR.  If EXPR is omitted, returns
              #square root of $_.  Only works on non-negative operands, unless
              #you've loaded the standard Math::Complex module.
</source>
 
python comes with the tool pydoc:
 
<source lang="python">
$ pydoc math.sqrt
Help on built-in function sqrt in math:
math.sqrt = sqrt(...)
    sqrt(x)
    Return the square root of x.
</source>
 
ruby comes with the tool <code>ri</code>:
<source lang="ruby">
$ ri Math::sqrt
------------------------------------------------------------- Math::sqrt
    Math.sqrt(numeric)    => float
------------------------------------------------------------------------
    Returns the non-negative square root of _numeric_.
</source>
 
Java comes with the documentation organized in html pages (JavaDoc format), while Microsoft distributes the API documentation for its languages (Visual C++, C#, Visual Basic, F#, etc...) embedded in [[Visual Studio]]'s help system.
 
===API in object-oriented languages===
In [[object oriented]] languages, an API usually includes a description of a set of class definitions, with a set of behaviors associated with those classes.  A ''behavior'' is the set of rules for how an object, derived from that class, will act in a given circumstance. This abstract concept is associated with the real functionalities exposed, or made available, by the classes that are implemented in terms of class methods.
 
The API in this case can be conceived as the totality of all the methods publicly exposed by the classes (usually called the class ''interface''). This means that the API prescribes the methods by which one interacts with/handles the objects derived from the class definitions.
 
More generally, one can see the API as the collection of all the ''kinds'' of objects one can derive from the class definitions, and their associated possible behaviors. Again: the use is mediated by the public methods, but in this interpretation, the methods are seen as a ''technical detail'' of how the behavior is implemented.
 
For instance: a class representing a <code>Stack</code> can simply expose publicly two methods <code>push()</code> (to add a new item to the stack), and <code>pop()</code> (to extract the last item, ideally placed on top of the stack).
 
In this case the API can be interpreted as the two methods <code>pop()</code> and <code>push()</code>, or, more generally, as the ''idea'' that one can use an item of type <code>Stack</code> that implements the behavior of a stack: a pile ''exposing'' its top to add/remove elements.
 
This concept can be carried to the point where a class interface in an API has no methods at all, but only behaviors associated with it. For instance, the Java language and Lisp API include the interface <code>Serializable</code>, which requires that each class that implements it should behave in a serialized fashion. This does not require to have any public method, but rather requires that any class that implements it to have a representation that can be ''saved'' (serialized) at any time (this is typically true for any class containing simple data and no link to external resources, like an open connection to a file, a remote system, or an external device).
 
In this sense, in object oriented languages, the API defines a set of object behaviors, possibly mediated by a set of class methods.
 
In such languages, the API is still distributed as a library. For example, the Java language libraries include a set of APIs that are provided in the form of the JDK used by the developers to build new Java programs. The JDK includes the documentation of the API in JavaDoc notation.
 
The quality of the documentation associated with an API is often a factor determining its success in terms of ease of use.
 
===API and protocols===
An API can also be an implementation of a protocol.
 
In general the difference between an API and a protocol is that the protocol defines a standard way to exchange requests and responses based on a common transport, while an API provides a library to be used directly: hence there can be no ''transport'' (no information physically transferred from some remote machine), but rather only simple information exchange via ''function calls'' (local to the machine where the elaboration takes place).
 
When an API implements a protocol it can be based on proxy methods for remote invocations that underneath rely on the communication protocol.
The role of the API can be exactly to hide the detail of the transport protocol.
 
Protocols are usually shared between different technologies (system based on given computer programming languages in a given operating system) and usually allow the different technologies to exchange information, acting as an abstraction/mediation level between the two ''worlds''. While APIs are specific to a given technology: hence the APIs of a given language cannot be used in other languages, unless the function calls are wrapped with specific adaptation libraries.
 
==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, which is usually 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.<ref>
{{cite web
|first      = Djamal
|last        = Benslimane
|coauthors  = Schahram Dustdar, and Amit Sheth
|title      = Services Mashups: The New Generation of Web Applications
|url        = http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4620089&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D4620089
|work        = IEEE Internet Computing, vol. 12, no. 5
|publisher  = Institute of Electrical and Electronics Engineers
|pages      = 13–15
|date        = September 2008
}}
</ref> Web APIs allow the combination of multiple services into new applications known as  Mashup_(web_application_hybrid)|mashups.<ref>
{{citation
|first      = James
|last        = Niccolai
|title      = So What Is an Enterprise Mashup, Anyway?
|url        = http://www.pcworld.com/businesscenter/article/145039/so_what_is_an_enterprise_mashup_anyway.html
|work        = [[PC World (magazine)|PC World]]
|date        = 23 April 2008
}}</ref>
 
==Use of APIs to share content==
The practice of publishing APIs has allowed web communities to create an open architecture for sharing content and data between communities and applications. In this way, content that is created in one place can be dynamically posted and updated in multiple locations on the web.
#Photos can be shared from sites like Flickr and Photobucket to social network sites like Facebook and MySpace.
#Content can be embedded, e.g. embedding a presentation from SlideShare on a LinkedIn profile.
#Content can be dynamically posted. Sharing live comments made on Twitter with a Facebook account, for example, is enabled by their APIs.
#Video content can be embedded on sites which are served by another host.
#User information can be shared from web communities to outside applications, delivering new functionality to the web community that shares 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.<ref>
{{cite web
|title      = Dynamic Community content via APIs
|date        = 26 October 2009
}}
yes
</ref>
 
==Implementations==
The [[POSIX]] standard defines an API that allows a wide range of common computing functions to be written in a way 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 to the system implementing that API.  This is beneficial to both software providers (where they may distribute existing software on new systems without producing and distributing upgrades) and users (where they may install older software on their new systems without purchasing upgrades), although this generally requires that various software libraries implement the necessary APIs as well.
 
Microsoft has shown a strong 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".<ref>
{{cite web
|author      = Microsoft
|title      = Run Older Programs On Windows XP
|url        = http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/appcompat.mspx
|archiveurl  = https://web.archive.org/web/20110320094425/http://www.microsoft.com/windowsxp/using/helpandsupport/learnmore/appcompat.mspx
|publisher  = Microsoft
|pages      = 4
|language    = EN
|date        = 15 October 2001
|archivedate = 20 March 2011
|accessdate  = 26 October 2013
}}</ref>
 
Apple Inc. has shown less 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. {{Citation needed|date=January 2010}}
 
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 standardize 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 Unixes, such as FreeBSD, NetBSD, and 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==
The two options for releasing API are:
 
#Protecting information on 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. This gives companies quality control privileges and can provide them with potential licensing revenue streams.
#Making 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 platforms.
 
A mix of the two behaviors can be used as well.
 
==ABIs==
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.<ref>{{cite web|
first=Nick|
last=Stoughton|
url=https://db.usenix.org/publications/login/2005-04/openpdfs/standards2004.pdf|
title=Update on Standards|
publisher=USENIX|
format=PDF|
date=April 2005
accessdate=04 June 2009}}</ref>
 
==API examples==
* [[Advanced SCSI Programming Interface|ASPI]] for [[SCSI]] device interfacing
* DirectX for Microsoft Windows
* [[EHLLAPI]]
* Java APIs
* [[OpenGL]] cross-platform graphics API
* [[OpenAL]] cross-platform sound API
* [[OpenCL]] cross-platform API for general-purpose computing for CPUs & GPUs
* [[OpenMP]] API that supports multi-platform shared memory multiprocessing programming in C, C++ and Fortran on many architectures, including Unix and Microsoft Windows platforms.
* Simple DirectMedia Layer (SDL)
* Talend integrates its [[data management]] with Business process management (BPM) from Bonita Open Solution
* Windows API
 
==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]] opensource interfaces bindings generator from many languages to many languages (Typically Compiled->Scripted)
* F2PY:<ref>[http://www.f2py.org/ F2PY.org]</ref> Fortran to Python interface generator.
 
==External links==
* [http://lcsd05.cs.tamu.edu/slides/keynote.pdf How to design a good API and why it matters-PDF]
* [http://www.lior.ca/publications/api_design.pdf How to Write an API]. Practical example with detailed code and motivation.
* [http://java.sun.com/developer/technicalArticles/WebServices/soa/ Service-Oriented Architecture (SOA): The Road to Enterprise Application Integration (EAI)]
* [http://isotc.iso.org/livelink/livelink.exe/fetch/2000/2489/186491/186605/Jtc1_Directives.pdf?nodeid=3959538&vernum=0 ISO/IEC JTC 1 Directives, 5th Edition Version 3.0, Annex J: Guidelines for API standardization]
 
==Notes==
==Notes==
 
This article is a direct transclusion of [https://en.wikipedia.org/wiki/API the Wikipedia article] and therefore may not meet the same editing standards as LIMSwiki.
The bulk of this article is reused from [http://en.wikipedia.org/wiki/Application_programming_interface the Wikipedia article].
 
==References==
{{reflist|colwidth=50em}}


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[[Category:Software terms]]
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Latest revision as of 21:15, 17 September 2022

For other uses, see API (disambiguation).
"Api.php" redirects here. For the Wikipedia API, see Special:ApiHelp.

An application programming interface (API) is a connection between computers or between computer programs. It is a type of software interface, offering a service to other pieces of software.[1] A document or standard that describes how to build such a connection or interface is called an API specification. A computer system that meets this standard is said to implement or expose an API. The term API may refer either to the specification or to the implementation.

In contrast to a user interface, which connects a computer to a person, an application programming interface connects computers or pieces of software to each other. It is not intended to be used directly by a person (the end user) other than a computer programmer who is incorporating it into software. An API is often made up of different parts which act as tools or services that are available to the programmer. A program or a programmer that uses one of these parts is said to call that portion of the API. The calls that make up the API are also known as subroutines, methods, requests, or endpoints. An API specification defines these calls, meaning that it explains how to use or implement them.

One purpose of APIs is to hide the internal details of how a system works, exposing only those parts a programmer will find useful and keeping them consistent even if the internal details later change. An API may be custom-built for a particular pair of systems, or it may be a shared standard allowing interoperability among many systems.

The term API is often used to refer to web APIs,[2] which allow communication between computers that are joined by the internet. There are also APIs for programming languages, software libraries, computer operating systems, and computer hardware. APIs originated in the 1940s, though the term did not emerge until the 1960s and 70s.

Purpose

An API opens a software system to interactions from the outside. It allows two software systems to communicate across a boundary — an interface — using mutually agreed-upon signals.[3] In other words, an API connects software entities together. Unlike a user interface, an API is typically not visible to users. It is an "under the hood" portion of a software system, used for machine-to-machine communication.[4]

A well-designed API exposes only objects or actions needed by software or software developers. It hides details that have no use. This abstraction simplifies programming.[5]

Metaphorically, APIs connect software like interlocking blocks.

Building software using APIs has been compared to using building-block toys, such as Lego bricks. Software services or software libraries are analogous to the bricks; they may be joined together via their APIs, composing a new software product.[6] The process of joining is called integration.[3]

As an example, consider a weather sensor that offers an API. When a certain message is transmitted to the sensor, it will detect the current weather conditions and reply with a weather report. The message that activates the sensor is an API call, and the weather report is an API response.[7] A weather forecasting app might integrate with a number of weather sensor APIs, gathering weather data from throughout a geographical area.

An API is often compared to a contract. It represents an agreement between parties: a service provider who offers the API and the software developers who rely upon it. If the API remains stable, or if it changes only in predictable ways, developers' confidence in the API will increase. This may increase their use of the API.[8]

History of the term

A diagram from 1978 proposing the expansion of the idea of the API to become a general programming interface, beyond application programs alone[9]

The term API initially described an interface only for end-user-facing programs, known as application programs. This origin is still reflected in the name "application programming interface." Today, the term is broader, including also utility software and even hardware interfaces.[10]

The idea of the API is much older than the term itself. British computer scientists Maurice Wilkes and David Wheeler worked on a modular software library in the 1940s for EDSAC, an early computer. The subroutines in this library were stored on punched paper tape organized in a filing cabinet. This cabinet also contained what Wilkes and Wheeler called a "library catalog" of notes about each subroutine and how to incorporate it into a program. Today, such a catalog would be called an API (or an API specification or API documentation) because it instructs a programmer on how to use (or "call") each subroutine that the programmer needs.[10]

Wilkes and Wheeler's book The Preparation of Programs for an Electronic Digital Computer contains the first published API specification. Joshua Bloch considers that Wilkes and Wheeler "latently invented" the API, because it is more of a concept that is discovered than invented.[10]

Although the people who coined the term API were implementing software on a Univac 1108, the goal of their API was to make hardware independent programs possible.[11]

The term "application program interface" (without an -ing suffix) is first recorded in a paper called Data structures and techniques for remote computer graphics presented at an AFIPS conference in 1968.[12][10] The authors of this paper use the term to describe the interaction of an application—a graphics program in this case—with the rest of the computer system. A consistent application interface (consisting of Fortran subroutine calls) was intended to free the programmer from dealing with idiosyncrasies of the graphics display device, and to provide hardware independence if the computer or the display were replaced.[11]

The term was introduced to the field of databases by C. J. Date[13] in a 1974 paper called The Relational and Network Approaches: Comparison of the Application Programming Interface.[14] An API became a part of the ANSI/SPARC framework for database management systems. This framework treated the application programming interface separately from other interfaces, such as the query interface. Database professionals in the 1970s observed these different interfaces could be combined; a sufficiently rich application interface could support the other interfaces as well.[9]

This observation led to APIs that supported all types of programming, not just application programming. By 1990, the API was defined simply as "a set of services available to a programmer for performing certain tasks" by technologist Carl Malamud.[15]

Screenshot of Web API documentation written by NASA

The idea of the API was expanded again with the dawn of remote procedure calls and web APIs. As computer networks became common in the 1970s and 80s, programmers wanted to call libraries located not only on their local computers, but on computers located elsewhere. These remote procedure calls were well supported by the Java language in particular. In the 1990s, with the spread of the internet, standards like CORBA, COM, and DCOM competed to become the most common way to expose API services.[16]

Roy Fielding's dissertation Architectural Styles and the Design of Network-based Software Architectures at UC Irvine in 2000 outlined Representational state transfer (REST) and described the idea of a "network-based Application Programming Interface" that Fielding contrasted with traditional "library-based" APIs.[17] XML and JSON web APIs saw widespread commercial adoption beginning in 2000 and continuing as of 2021. The web API is now the most common meaning of the term API.[2]

The Semantic Web proposed by Tim Berners-Lee in 2001 included "semantic APIs" that recast the API as an open, distributed data interface rather than a software behavior interface.[18] Proprietary interfaces and agents became more widespread than open ones, but the idea of the API as a data interface took hold. Because web APIs are widely used to exchange data of all kinds online, API has become a broad term describing much of the communication on the internet.[16] When used in this way, the term API has overlap in meaning with the term communication protocol.

Types

Libraries and frameworks

The interface to a software library is one type of API. The API describes and prescribes the "expected behavior" (a specification) while the library is an "actual implementation" of this set of rules.

A single API can have multiple implementations (or none, being abstract) in the form of different libraries that share the same programming interface.

The separation of the API from its implementation can allow programs written in one language to use a library written in another. For example, because Scala and Java compile to compatible bytecode, Scala developers can take advantage of any Java API.[19]

API use can vary depending on the type of programming language involved.

An API for a procedural language such as Lua could consist primarily of basic routines to execute code, manipulate data or handle errors while an API for an object-oriented language, such as Java, would provide a specification of classes and its class methods.[20][21] Hyrum's law states that "With a sufficient number of users of an API, it does not matter what you promise in the contract: all observable behaviors of your system will be depended on by somebody."[22] Meanwhile, several studies show that most applications that use an API tend to use a small part of the API.[23]

Language bindings are also APIs. By mapping the features and capabilities of one language to an interface implemented in another language, a language binding allows a library or service written in one language to be used when developing in another language.[24] Tools such as SWIG and F2PY, a Fortran-to-Python interface generator, facilitate the creation of such interfaces.[25]

An API can also be related to a software framework: a framework can be based on several libraries implementing several APIs, but unlike the normal use of an API, the access to the behavior built into the framework is mediated by extending its content with new classes plugged into the framework itself.

Moreover, the overall program flow of control can be out of the control of the caller and in the framework's hands by inversion of control or a similar mechanism.[26][27]

Operating systems

An API can specify the interface between an application and the operating system.[28] POSIX, for example, specifies a set of common APIs that aim to enable an application written for a POSIX conformant operating system to be compiled for another POSIX conformant operating system.

Linux and Berkeley Software Distribution are examples of operating systems that implement the POSIX APIs.[29]

Microsoft has shown a strong commitment to a backward-compatible API, particularly within its Windows API (Win32) library, so older applications may run on newer versions of Windows using an executable-specific setting called "Compatibility Mode".[30]

An API differs from an application binary interface (ABI) in that an API is source code based while an ABI is binary based. For instance, POSIX provides APIs while the Linux Standard Base provides an ABI.[31][32]

Remote APIs

Remote APIs allow developers to manipulate remote resources through protocols, specific standards for communication that allow different technologies to work together, regardless of language or platform. For example, the Java Database Connectivity API allows developers to query many different types of databases with the same set of functions, while the Java remote method invocation API uses the Java Remote Method Protocol to allow invocation of functions that operate remotely, but appear local to the developer.[33][34]

Therefore, remote APIs are useful in maintaining the object abstraction in object-oriented programming; a method call, executed locally on a proxy object, invokes the corresponding method on the remote object, using the remoting protocol, and acquires the result to be used locally as a return value.

A modification of the proxy object will also result in a corresponding modification of the remote object.[35]

Web APIs

Main article: Web API

Web APIs are the defined interfaces through which interactions happen between an enterprise and applications that use its assets, which also is a Service Level Agreement (SLA) to specify the functional provider and expose the service path or URL for its API users. An API approach is an architectural approach that revolves around providing a program interface to a set of services to different applications serving different types of consumers.[36]

When used in the context of web development, an API is typically defined as a set of specifications, such as Hypertext Transfer Protocol (HTTP) request messages, along with a definition of the structure of response messages, usually in an Extensible Markup Language (XML) or JavaScript Object Notation (JSON) format. An example might be a shipping company API that can be added to an eCommerce-focused website to facilitate ordering shipping services and automatically include current shipping rates, without the site developer having to enter the shipper's rate table into a web database. While "web API" historically has been virtually synonymous with web service, the recent trend (so-called Web 2.0) has been moving away from Simple Object Access Protocol (SOAP) based web services and service-oriented architecture (SOA) towards more direct representational state transfer (REST) style web resources and resource-oriented architecture (ROA).[37] Part of this trend is related to the Semantic Web movement toward Resource Description Framework (RDF), a concept to promote web-based ontology engineering technologies. Web APIs allow the combination of multiple APIs into new applications known as mashups.[38]

In the social media space, web APIs have allowed web communities to facilitate sharing content and data between communities and applications. In this way, content that is created in one place dynamically can be posted and updated to multiple locations on the web.[39] For example, Twitter's REST API allows developers to access core Twitter data and the Search API provides methods for developers to interact with Twitter Search and trends data.[40]

Design

The design of an API has significant impact on its usage.[5] The principle of information hiding describes the role of programming interfaces as enabling modular programming by hiding the implementation details of the modules so that users of modules need not understand the complexities inside the modules.[41] Thus, the design of an API attempts to provide only the tools a user would expect.[5] The design of programming interfaces represents an important part of software architecture, the organization of a complex piece of software.[42]

Release policies

APIs are one of the more common ways technology companies integrate. Those that provide and use APIs are considered as being members of a business ecosystem.[43]

The main policies for releasing an API are:[44]

  • Private: The API is for internal company use only.
  • Partner: Only specific business partners can use the API. For example, vehicle for hire companies such as Uber and Lyft allow approved third-party developers to directly order rides from within their apps. This allows the companies to exercise quality control by curating which apps have access to the API, and provides them with an additional revenue stream.[45]
  • Public: The API is available for use by the public. For example, Microsoft makes the Windows API public, and Apple releases its API Cocoa, so that software can be written for their platforms. Not all public APIs are generally accessible by everybody. For example, Internet service providers like Cloudflare or Voxility, use RESTful APIs to allow customers and resellers access to their infrastructure information, DDoS stats, network performance or dashboard controls.[46] Access to such APIs is granted either by “API tokens”, or customer status validations.[47]

Public API implications

An important factor when an API becomes public is its "interface stability". Changes to the API—for example adding new parameters to a function call—could break compatibility with the clients that depend on that API.[48]

When parts of a publicly presented API are subject to change and thus not stable, such parts of a particular API should be documented explicitly as "unstable". For example, in the Google Guava library, the parts that are considered unstable, and that might change soon, are marked with the Java annotation @Beta.[49]

A public API can sometimes declare parts of itself as deprecated or rescinded. This usually means that part of the API should be considered a candidate for being removed, or modified in a backward incompatible way. Therefore, these changes allow developers to transition away from parts of the API that will be removed or not supported in the future.[50]

Client code may contain innovative or opportunistic usages that were not intended by the API designers. In other words, for a library with a significant user base, when an element becomes part of the public API, it may be used in diverse ways.[51]

On February 19, 2020, Akamai published their annual “State of the Internet” report, showcasing the growing trend of cybercriminals targeting public API platforms at financial services worldwide. From December 2017 through November 2019, Akamai witnessed 85.42 billion credential violation attacks. About 20%, or 16.55 billion, were against hostnames defined as API endpoints. Of these, 473.5 million have targeted financial services sector organizations.[52]

Documentation

API documentation describes what services an API offers and how to use those services, aiming to cover everything a client would need to know for practical purposes.

Documentation is crucial for the development and maintenance of applications using the API.[53]

API documentation is traditionally found in documentation files but can also be found in social media such as blogs, forums, and Q&A websites.[54]

Traditional documentation files are often presented via a documentation system, such as Javadoc or Pydoc, that has a consistent appearance and structure.

However, the types of content included in the documentation differs from API to API.[55]

In the interest of clarity, API documentation may include a description of classes and methods in the API as well as "typical usage scenarios, code snippets, design rationales, performance discussions, and contracts", but implementation details of the API services themselves are usually omitted. It can take a number of forms, including instructional documents, tutorials, and reference works. It'll also include a variety of information types, including guides and functionalities.

Restrictions and limitations on how the API can be used are also covered by the documentation. For instance, documentation for an API function could note that its parameters cannot be null, that the function itself is not thread safe.[56] Because API documentation tends to be comprehensive, it is a challenge for writers to keep the documentation updated and for users to read it carefully, potentially yielding bugs.[48]

API documentation can be enriched with metadata information like Java annotations. This metadata can be used by the compiler, tools, and by the run-time environment to implement custom behaviors or custom handling.[57]

It is possible to generate API documentation in a data-driven manner. By observing many programs that use a given API, it is possible to infer the typical usages, as well the required contracts and directives.[58] Then, templates can be used to generate natural language from the mined data.

Main article: Google LLC v. Oracle America, Inc.

In 2010, Oracle Corporation sued Google for having distributed a new implementation of Java embedded in the Android operating system.[59] Google had not acquired any permission to reproduce the Java API, although permission had been given to the similar OpenJDK project. Judge William Alsup ruled in the Oracle v. Google case that APIs cannot be copyrighted in the U.S. and that a victory for Oracle would have widely expanded copyright protection to a "functional set of symbols" and allowed the copyrighting of simple software commands:

To accept Oracle's claim would be to allow anyone to copyright one version of code to carry out a system of commands and thereby bar all others from writing its different versions to carry out all or part of the same commands.[60][61]

Alsup's ruling was overturned in 2014 on appeal to the Court of Appeals for the Federal Circuit, though the question of whether such use of APIs constitutes fair use was left unresolved.[62][63]

In 2016, following a two-week trial, a jury determined that Google's reimplementation of the Java API constituted fair use, but Oracle vowed to appeal the decision.[64] Oracle won on its appeal, with the Court of Appeals for the Federal Circuit ruling that Google's use of the APIs did not qualify for fair use.[65] In 2019, Google appealed to the Supreme Court of the United States over both the copyrightability and fair use rulings, and the Supreme Court granted review.[66] Due to the COVID-19 pandemic, the oral hearings in the case were delayed until October 2020.[67]

The case was decided by the Supreme Court in Google's favor.[68]

Examples

Main category: Application programming interfaces

See also

References

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Further reading

Notes

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