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(last modified iunie 22, 2023)

Application Programming Interface (API)

Application Programming Interface (API) reprezintă un set de definiții de sub-programe, protocoale si unelte pentru programarea de aplicații si software. Un API poate fi pentru un sistem web, sistem de operare, sistem de baze de date, hardware sau biblioteci software. De exemplu, când este vorba despre interfața dintre programele de aplicație și sistemul de operare, acesta stabilește în amănunt modul în care programele de aplicație pot accesa (apela) serviciile sistemului de operare sub care rulează.

Sursa: wikipedia


An application programming interface (API) is a way for two or more computer programs to communicate with each other. 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 or use 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 the 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.

There are APIs for programming languagessoftware libraries, computer operating systems, and computer hardware. APIs originated in the 1940s, though the term did not emerge until the 1960s and 1970s. Contemporary usage of the term API often refers to web APIs,[2] which allow communication between computers that are joined by the internet. Recent developments in APIs have led to the rise in popularity of microservices, which are loosely coupled services accessed through public APIs.[3]


In building applications, an API simplifies programming by abstracting the underlying implementation and only exposing objects or actions the developer needs. While a graphical interface for an email client might provide a user with a button that performs all the steps for fetching and highlighting new emails, an API for file input/output might give the developer a function that copies a file from one location to another without requiring that the developer understand the file system operations occurring behind the scenes.[4]

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[5]

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.[6]

1940s and 1950s

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.[6]

Wilkes and Wheeler’s 1951 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.[6]

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.[7]

1960s and 1970s

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.[8][6] 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.[7]

The term was introduced to the field of databases by C. J. Date[9] in a 1974 paper called The Relational and Network Approaches: Comparison of the Application Programming Interface.[10] 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.[5]

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.[11]

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 1980s, 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 CORBACOM, and DCOM competed to become the most common way to expose API services.[12]


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.[13] XML and JSON web APIs saw widespread commercial adoption beginning in 2000 and continuing as of 2022. 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 recasts the API as an open, distributed data interface rather than a software behavior interface.[14] 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.[12] When used in this way, the term API has overlap in meaning with the term communication protocol.


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.[15]

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.[16][17] Hyrum’s law [18] 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.” Meanwhile, several studies show that most applications that use an API tend to use a small part of the API.[19]

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.[20]

Tools such as SWIG and F2PY, a Fortran-to-Python interface generator, facilitate the creation of such interfaces.[21]

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.[22][23]

Operating systems

An API can specify the interface between an application and the operating system.[24] POSIX, for example, provides a set of common API specifications 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.[25]

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”.[26]

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.[27][28]

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.[29][30]

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.[31]

Web APIs

Main article: Web API

Web APIs are a service accessed from client devices (Mobile Phones, Laptop, etc.) to a web server using the Hypertext Transfer Protocol (HTTP). Client devices send a request in the form of an HTTP request, and are met with a response message usually in JavaScript Object Notation (JSON) or Extensible Markup Language (XML) format. Developers typically use Web APIs to query a server for a specific set of data from that server.

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).[32] 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.[33]

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.[34] 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.[35]


The design of an API has a significant impact on its usage.[4] First of all, the design of programming interfaces represents an important part of software architecture, the organization of a complex piece of software.[36] 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.[37] Aside from the previous underlying principle, other metrics for measuring the usability of an API may include properties such as functional efficiency, overall correctness, and learnability for novices.[38] One straightforward and commonly adopted way of designing APIs is to follow Nielsen’s heuristic evaluation guidelines. The Factory method pattern is also typical in designing APIs due to their reusable nature.[39] Thus, the design of an API attempts to provide only the tools a user would expect.[4]

Synchronous versus asynchronous

An application programming interface can be synchronous or asynchronous. A synchronous API call is a design pattern where the call site is blocked while waiting for the called code to finish.[40] With a asynchronous API call, however, the call site is not blocked while waiting for the called code to finish, and instead the calling thread is notified when the reply arrives.


API security is very critical when developing a public facing API. Common threats include SQL injectionDenial-of-service attack (DoS), broken authentication, and exposing sensitive data.[41] Without ensuring proper security practices bad actors can get access to information they should not have or even gain privileges to make changes to your server. Some common security practices include proper connection security using HTTPS, content security to mitigate data injection attacks, and requiring an API key to use your service.[42] Many public facing API services require you to use an assigned API key, and will refuse to serve data without sending the key with your request.[43]

Sursa: wikipedia

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