What is API

What is API
API, an abbreviation of application program interface, is a set of routines,protocols, and tools for building software applications. A good API makes it easier to develop a program by providing all the building blocks. A programmer then puts the blocks together.

Most operating environments, such as MS-Windows, provide an API so that programmers can write applications consistent with the operating environment. Although APIs are designed for programmers, they are ultimately good for users because they guarantee that all programs using a common API will have similar interfaces. This makes it easier for users to learn new programs.

Generally speaking, an application programming interface (API) specifies how some software components should interact with each other.

In practice in most of the cases an API is a library that usually includes specification for routines, data structures, object classes, and variables.

An API specification can take many forms, including an International Standard such as POSIX, vendor documentation such as the Microsoft Windows API, the libraries of a programming language, e.g., Standard Template Library in C++ or Java API.

An API differs from an application binary interface (ABI) in that an API is source code based while an ABI is a binary interface. For instance POSIX is an API, while the Linux Standard Base is an ABI.

API in procedural languages

In most procedural languages an API is a specification that documents a set of functions or routines that accomplish a specific task or allow to interact with a specific software component.

This documentation is presented in a human readable format in paper books, or in electronic formats like ebooks or as man pages.

E.g. the math API on Unix systems is a specification on how to use the mathematical functions included in the math library. Among these functions there is a function, namedsqrt(), that can be used to compute the square root of a given number. The Unix command man 3 sqrt presents the signature of the function sqrt in the form:

SYNOPSIS
            #include <math.h>
            double sqrt(double X);
            float  sqrtf(float X);
DESCRIPTION
       sqrt computes the positive square root of the argument. ...
RETURNS
       On success, the square root is returned. If X is real and positive...

this description means that: the sqrt() function returns the square root of a positive floating point number (single or double precision) as another floating point number.

Hence the API in this case can be interpreted as the collection of the include files used by a program, written in the C language, to reference that library function, and its human readable description provided by the man pages.

Similarly, other languages have procedural libraries, e.g. Perl has a dedicated APIs for the same mathematical task that are presented in digital format, with the tool perldoc:

$ 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.

API in object-oriented languages

In its simplest form an object API is a prescription of how objects work in a given object-oriented language: usually it is expressed as set classes with an associated list of class methods.

For example, in the Java language, if a programmer wants to use the class Scanner (a class that reads input from the user in text-based programs), they would import thejava.util.Scanner library in order to use the Scanner class, and later they can use objects of type Scanner invoking some class methods:

import java.util.Scanner;

import java.util.Scanner;
 
public class Test {
   public static void main(String[] args) {
       System.out.println("Enter your name: ");
       Scanner inputScanner = new Scanner(System.in);
       String name = inputScanner.nextLine();
       System.out.println("Your name is " + name + ".");
       inputScanner.close();
  }
}

where the methods nextLine() and close() are part the API for the Scanner class, and hence are described in the documentation for that API.

More generally, 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. This abstract concept is associated with the real functionality exposed, or made available, by the classes that are implemented in terms of class methods (or more generally by all its public components hence all public methods, but also possibly including any internal entity made public, like fields, constants, nested objects, enums, etc.).

The API in this case can be conceived of 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 Stack can simply expose publicly two methods push() (to add a new item to the stack), and pop() (to extract the last item, ideally placed on top of the stack).

In this case the API can be interpreted as the two methods pop() and push(), or, more generally, as the idea that one can use an item of type Stack that implements the behavior of a stack: a pile exposing its top to add/remove elements. The second interpretation appears more appropriate in the spirit of object orientation.

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 (programming language) API include the interface Serializable, which is a marker interface that requires that each class that implements it should behave in a serializedfashion. 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.

Similarly the behavior of an object in a concurrent (multi-threaded) environment is not necessarily determined by specific methods, belonging to the interface implemented, but still belongs to the API for that Class of objects, and should be described in the documentation.

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 libraries and frameworks

An API is usually related to a software library: the API describes and prescribes the expected behavior 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.

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 hands of the framework via inversion of control or a similar mechanisms.

API and protocols

An API can also be an implementation of a protocol.

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. E.g.: RMI is an API that implements the JRMP protocol or the IIOP as RMI-IIOP.

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 different environments. APIs are usually 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.

To enable the exchange of information among systems that use different technologies, when an API implements a protocol, it can prescribe a language-neutral messagge format: e.g. SOAP uses XML as a general container for the messages to be exchanged.

Object API and protocols

An object API can prescribe a specific object exchange format, an object exchange protocol can define a way to transfer the same kind of information in a message sent to a remote system.

When a message is exchanged via a protocol between two different platforms using objects on both sides, the object in a programming language can be transformed (marshalled and unmarshalled) in an object in a remote and different language: so, e.g., a program written in Java invokes a service via SOAP or IIOP written in C# both programs use APIs for remote invocation (each locally to the machine where they are working) to (remotely) exchange information that they both convert from/to an object in local memory.

Instead when a similar object is exchanged via an API local to a single machine the object is effectively exchanged (or a reference to it) in memory: e.g. via memory allocated by a single process, or among multiple processes using shared memory or other sharing technologies like tuple spaces.

API sharing and reuse via virtual machine

Some languages like those running in a virtual machine (e.g. .NET CLI compliant languages in the Common Language Runtime and JVM compliant languages in the Java Virtual Machine) can share API.

In this case the virtual machine enables the language interoperation thanks to the common denominator of the virtual machine that abstracts from the specific language using an intermediate bytecode and its language binding.

Web APIs

Main article: Web API

When used in the context of web development, an API is typically defined as a 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" historically has been virtually synonymous for 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).Part of this trend is related to theSemantic 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.

Web use 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 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.

If content is a direct representation of the physical world (e.g., temperature at a geospatial location on earth) then an API can be considered an "Environmental Programming Interface" (EPI). EPIs are characterized by their ability to provide a means for universally sequencing events sufficient to utilize real-world data for decision making.

Implementations

The POSIX standard defines an API that allows writing a wide range of common computing functions in a way such that they can operate on many different systems (Mac OS X, and various Berkeley Software Distributions (BSDs) implement this interface). However, using this requires re-compiling for each platform. A compatible API, on the other hand, allows compiled object code to function with no changes to the system that implements 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".

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 has 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, andOpenBSD, 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.

APIs and copyrights

In 2010, Oracle sued Google for having distributed a new implementation of Java embedded in the Android operating system. Google had not acquired any permission to reproduce the Java API, although a similar permission had been given to the OpenJDK project. Judge William Alsup ruled in the Oracle v. Google case that APIs cannot be copyrighted in the U.S.

API examples

See also: Category:Application programming interfaces

ASPI for SCSI device interfacing

Cocoa and Carbon for the Macintosh

DirectX for Microsoft Windows

EHLLAPI

Java APIs

ODBC for Microsoft Windows

OpenAL cross-platform sound API

OpenCL cross-platform API for general-purpose computing for CPUs & GPUs

OpenGL cross-platform graphics API

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)