API | Definition, working, types, protocols, and examples

What is an API


An API is a set of definitions and protocols for building and integrating application software. API stands for an application programming interface.

An application programming interface, or API, enables companies to open up their applications’ data and functionality to third-party developers, business partners, and within their internal departments. This allows services and products to communicate with each other and hold each other’s data and functionality through a documented interface. Developers don’t necessarily know how an API is working; they simply use the interface to communicate with other products and services. This can simplify app development, saving time and money. When you’re designing new tools and products—or managing existing ones—APIs give you flexibility; simplify design, administration, and use; and provide opportunities for innovation.

Because APIs simplify how developers integrate new application components into existing architecture, they help business and IT teams collaborate. Business needs change quickly in response to shifting digital markets, where new competitors can change a whole industry with a new app. In order to stay competitive, it’s important to support the rapid development and deployment of innovative services.

Why do we need APIs

Whether you’re managing existing tools or designing new ones, you can use an application programming interface to simplify the process. Some of the main benefits of APIs include the following:

  • Improved collaboration: The average enterprise uses almost 1,200 cloud applications, many of which are disconnected. APIs enable integration so that these platforms and apps can seamlessly communicate with one another. Through this integration, companies can automate workflows and improve workplace collaboration. Without APIs, many enterprises would lack connectivity and would suffer from informational silos that compromise productivity and performance.
  • Easier innovation: APIs offer flexibility, allowing companies to make connections with new business partners, offer new services to their existing market, and access new markets that can generate massive returns and drive digital transformation. For example, the company Stripe began as an API with just seven lines of code.
  • Data monetization: Many companies choose to offer APIs for free, at least initially, so that they can build an audience of developers around their brand and forge relationships with potential business partners. However, if the API grants access to valuable digital assets, you can monetize it by selling access (this is referred to as the API economy).
  • Added security: As noted above, APIs create an added layer of protection between your data and a server. Developers can further strengthen API security by using tokens, signatures, and Transport Layer Security (TLS) encryption; by implementing API gateways to manage and authenticate traffic; and by practicing effective API management.

How an API works

  1. A client application initiates an API call to retrieve information—also known as a request. This request is processed from an application to the web server via the API’s Uniform Resource Identifier (URI). And includes a request verb, headers, and sometimes, a request body.
  2. After receiving a valid request, the API makes a call to the external program or web server.
  3. The server sends a response to the API with the requested information.
  4. The API transfers the data to the initial requesting application.

While the data transfer will differ depending on the web service being used, this process of requests and responses all happens through an API. Whereas a user interface is designed for use by humans, APIs are designed for use by a computer or application.

Types of API

Private APIs

These application software interfaces are designed for improving solutions and services within an organization. In-house developers may use these APIs to integrate a company’s IT systems or applications, build new systems, or customer-facing apps. Even if apps are publicly available, the interface itself remains available only for those working directly with the API publisher. The private strategy allows a company to fully control API usage.

Partner APIs

Partner APIs are the ones that share with business partners. The common use case for partner APIs is software integration between two parties. A company that grants partners with access to data or capability benefits from extra revenue streams. It can ensure whether third-party solutions using their APIs provide a decent user experience, and maintain corporate identity in their apps.

Public APIs

Also known as developer-facing or external, these APIs are available for any third-party developers. A public API program allows for increasing brand awareness and receiving an additional source of income when properly executed.

There are two types of public APIs – open (free of charge) and commercial ones. All features of these APIs are public and we can use them without restrictive terms and conditions. It’s possible to build an application that utilizes the API without approval from the API supplier or mandatory licensing fees. API description and any related documentation must be openly available. And the API can freely used to create and test applications. Commercial API users pay subscription fees or use APIs on a pay-as-you-go basis. A popular approach among publishers is to offer free trials, so users can evaluate APIs before purchasing subscriptions.

API specifications/protocols

Remote Procedure Call (RPC)

Web APIs may adhere to resource exchange principles based on a Remote Procedure Call. This protocol specifies the interaction between client-server-based applications. One program (client) requests data or functionality from another program (server), located in another computer on a network, and the server sends the required response.

Service Object Access Protocol (SOAP)

SOAP is a lightweight protocol for exchanging structured information in a decentralized, distributed environment, according to the definition by Microsoft that developed it. This specification contains the syntax rules for request and response messages sent by web applications. APIs that comply with the principles of SOAP enables XML messaging between systems through HTTP or Simple Mail Transfer Protocol (SMTP) for transferring mail.

Extensible markup language (XML) is a simple and flexible text format used for data storage and exchange over the internet. XML defines a set of rules for encoding documents in a format that both humans and machines can read. XML text documents contain self-descriptive tags of data objects, which makes them easily readable.

SOAP is mostly used with web-based software to maintain high security of transmitted data. SOAP APIs are preferred among providers of payment gateways, identity management, and financial and telecommunication services. PayPal public API is one of the commonly known SOAP APIs. It’s also frequently used for legacy system support.

Representational State Transfer (REST)

Unlike SOAP, which is a protocol, REST is a software architectural style with six constraints for building applications that work over HTTP, often web services. The World Wide Web is the most common realization and application of this architecture style.

REST is consider a simpler alternative to SOAP, which many developers find difficult to use. Because it requires writing a lot of code to complete every task and following the XML structure for every message sent. REST follows another logic since it makes data available as resources. Each resource represents by a unique URL, and one can request this resource by providing its URL.

RESTful systems support messaging in different formats, such as plain text, HTML, YAML, XML, and JSON, while SOAP only allows XML. The ability to support multiple formats for storing and exchanging data is one of the reasons REST is a prevailing choice for building public APIs these days.

Social media giants and travel companies provide external APIs to improve their brand visibility even more. Twitter has numerous RESTful APIs; Expedia has both SOAP and RESTful APIs for its partners.


gRPC is an open-source universal API framework that is also classified under RPC. Unlike SOAP, gRPC is much newer, released publicly in 2015 by Google. With gRPC, the client application can directly call methods from a server application located on a different computer as if it was a local object. This makes it easier to create distributed services and applications.

Like SOAP and REST, the transport layer for gRPC is HTTP. However, similar to RCP, gRPC allows developers to define any kind of function calls, rather than selecting from predefined options such as PUT and GET in the case of REST. By default, gRPC uses protocol buffers instead of JSON or XML as the Interface Definition Language (IDL) for serializing structured data. Here, the developer needs to first define the structure of the data they want to serialize. Once the data structures have been specified, they use the protocol buffer compiler to generate the data access classes in the programming language of your choice. Then the data is compress and serialize in binary format at runtime.

Some common API examples

Here are some popular examples of application programming interfaces you may encounter:

Universal logins

A popular API example is a function that enables people to log in to websites by using their Facebook, Twitter, or Google profile login details. This feature allows any website to use an API from one of the more popular services to quickly authenticate users.

Third-party payment processing

For example, the “Pay with PayPal” function you see on eCommerce websites works through an API. This allows people to pay for products online without exposing any sensitive data or granting access to unauthorized individuals.

Travel booking comparisons

Travel booking sites aggregate thousands of flights, showcasing the cheapest options for every date and destination. This service is only possible through APIs that provide application users with access to the latest information about availability from hotels and airlines.

Google Maps

One of the most common examples of a good API is the Google Maps service. The app utilizes other APIs and features to provide users with directions or points of interest. Through geolocation and multiple data layers, you can communicate with the Maps API when plotting travel routes or tracking items on the move, such as a delivery vehicle.


Each Tweet contains descriptive core attributes, including an author, a unique ID, a message, a timestamp when it was posted, and geolocation metadata. Twitter makes public Tweets and replies available to developers and allows developers to post Tweets via the company’s API.

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