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If you ever read tech magazines or blogs, you’ve probably seen the abbreviation API. It sounds solid, but what does it mean?
Let’s start with a simple example: human communication. We can express our thoughts, needs, and ideas through language, gestures, or facial expressions. Interaction with computers, apps, and websites requires user interface components a screen with a menu and graphical elements, a keyboard, and a mouse.
Software or its elements don’t need a graphical user interface to communicate with each other. Software products exchange data and functionalities via machine-readable interfaces APIs (application programming interfaces).
What is an API?
API is the acronym for Application Programming Interface, which is a software intermediary that allows two applications to talk to each other. Each time you use an app like Facebook, send an instant message or check the weather on your phone, you’re using an API.
(OR)
An API is a set of programming code that enables data transmission between one software product and another. It also contains the terms of this data exchange.
Application programming interfaces consist of two components:
- Technical specification describing the data exchange options between solutions with the specification done in the form of a request for processing and data delivery protocols
- The software interface is written to the specification that represents it
The software that needs to access information or functionality from another software, calls its API while specifying the requirements of how data/functionality must be provided. The other software returns data/functionality requested by the former application.
And the interface by which these two applications communicate is what the API specifies.
The Red Hat specialists note that APIs are sometimes considered contracts, where documentation is an agreement between the parties: “If party first sends a remote request structured a particular way, this is how the second party’s software will respond.” The API documentation is a manual for developers that includes all necessary information on how to work with the API and use the services it provides. We will talk more about the documentation in one of the next sections.
Each API contains and is implemented by function calls language statements that request software to perform particular actions and services. Function calls are phrases composed of verbs and nouns, for example:
- Start or finish a session
- Get amenities for a single room type
- Restore or retrieve objects from a server.
Function calls are described in the API documentation.
APIs serve numerous purposes. Generally, they can simplify and speed up software development. Developers can add functionality from other providers to existing solutions or build new applications using services by third-party providers. In all these cases, specialists don’t have to deal with source code, trying to understand how the other solution works. They simply connect their software to another one. In other words, APIs serve as an abstraction layer between two systems, hiding the complexity and working details of the latter.
Types of APIs
APIs by availability aka release policies
In terms of release policies, APIs can be private, partner, and public.
1. Private APIs: These application software interfaces are designed for improving solutions and services within an organization. In-house developers or contractors may use these APIs to integrate a company’s IT systems or applications, build new systems or customer-facing apps leveraging existing systems. 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 the API usage.
2. Partner APIs: Partner APIs are openly promoted but shared with business partners who have signed an agreement with the publisher. 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. At the same time, it can monitor how the exposed digital assets are used, ensure whether third-party solutions using their APIs provide a decent user experience, and maintain corporate identity in their apps.
3. 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 and commercial ones. The Open API Definition suggests that all features of such an API are public and can be used without restrictive terms and conditions. For instance, it’s possible to build an application that utilizes the API without explicit approval from the API supplier or mandatory licensing fees. The definition also states that the API description and any related documentation must be openly available and that the API can be 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.
APIs by use cases
APIs can be classified according to the systems for which they are designed.
Database APIs: Database APIs enable communication between an application and a database management system. Developers work with databases by writing queries to access data, change tables, etc. The Drupal 7 Database API, for example, allows users to write unified queries for different databases, both proprietary and open-source (Oracle, MongoDB, PostgreSQL, MySQL, CouchDB, and MSSQL).
Another example is ORDS database API, which is embedded into Oracle REST Data Services.
Operating systems APIs: This group of APIs defines how applications use the resources and services of operating systems. Every OS has its set of APIs, for instance, Windows API or Linux API (kernel user-space API and kernel internal API).
Apple provides API references for macOS and iOS in its developer documentation. APIs for building applications for Apple’s macOS desktop operating system is included in the Cocoa set of developer tools. Those building apps for the iOS mobile operating system use Cocoa Touch a modified version of Cocoa.
Remote APIs: Remote APIs define standards of interaction for applications running on different machines. In other words, one software product accesses resources located outside the device that requests them, which explains the name. Since two remotely located applications are connected over a communications network, particularly the internet, most remote APIs are written based on web standards. Java Database Connectivity API and Java Remote Method Invocation API are two examples of remote application programming interfaces.
Web APIs: This API class is the most common. Web APIs provide machine-readable data and functionality transfer between web-based systems which represent client-server architecture. These APIs mainly deliver requests from web applications and responses from servers using Hypertext Transfer Protocol (HTTP).
Developers can use web APIs to extend the functionality of their apps or sites. For instance, the Pinterest API comes with tools for adding users’ Pinterest data like boards or Pins to a website. Google Maps API enables the addition of a map with an organization’s location.
API specifications (protocols)
The goal of API specifications is to standardize data exchange between web services. In this case, standardization means the ability of diverse systems, written in different programming languages and/or running on different OSs, or using different technologies, to seamlessly communicate with each other.
Several API specifications are in place.
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.
RPC is also known as a subroutine or function call. One of two ways to implement a remote procedure call is SOAP.
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. Generally speaking, 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.
Representational State Transfer (REST)
The term REST was introduced by computer scientist Roy Fielding in a dissertation in 2000. 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 considered 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 is represented by a unique URL, and one can request this resource by providing its URL.
Web APIs that comply with REST architectural constraints are called RESTful APIs. These APIs use HTTP requests (AKA methods or verbs) to work with resources: GET, PUT, HEAD, POST, PATCH, CONNECT, TRACE, OPTIONS and DELETE.
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.
GraphQL
The need for faster feature development, more efficient data loading due to increased mobile adoption, and a multitude of clients, made the developers look for other approaches to software architecture. GraphQL, initially created by Facebook in 2012 for internal use, is the new REST with organizations like Shopify, Yelp, GitHub, Coursera, and The New York Times using it to build APIs.
GraphQL is a query language for APIs. It allows the client to detail the exact data it needs and simplifies data aggregation from multiple sources, so the developer can use one API call to request all needed data. Another special feature of GraphQL is that it uses a type system to describe data.
Apps using GraphQL control what data they need to fetch from a server, which allows them to run fast even when the mobile connection is slow. You can see how GraphQL, REST, RPC, and SOAP are compared in the linked article.
API Documentation
No matter how many opportunities for creating or extending software products API gives, it would remain an unusable piece of code if developers didn’t understand how to work with it. Well-written and structured API documentation that explains how to effectively use and integrate an API in an easy-to-comprehend manner will make a developer happy and eager to recommend the API to peers.
The API documentation is a reference manual with all needed information about the API, including functions, classes, return types and arguments.
Numerous content elements make good documentation, such as:
- a quick start guide
- authentication information
- explanations for every API call (request)
- examples of every request and return with a response description, error messages, etc.
- samples of code for popular programmatic languages like Python, Java, JavaScript, or PHP;
- tutorials
- SDK examples (if SDKs are available) illustrating how to access the resource, etc.
Documentation may be static and interactive. The latter allows for trying out APIs and see return results and usually consists of two columns: human and machine. The human column contains API descriptions, and the machine one has a console to make calls and contains info that clients and servers will be interested in when testing the API.
What Is an Example of an API?
When you use an application on your mobile phone, the application connects to the Internet and sends data to a server. The server then retrieves that data, interprets it, performs the necessary actions, and sends it back to your phone. The application then interprets that data and presents you with the information you wanted in a readable way. This is what an API is all of this happens via API.
To explain this better, let us take a familiar example.
Imagine you’re sitting at a table in a restaurant with a menu of choices to order from. The kitchen is the part of the “system” that will prepare your order. What is missing is the critical link to communicate your order to the kitchen and deliver your food back to your table. That’s where the waiter or API comes in. The waiter is the messenger or API that takes your request or order and tells the kitchen the system what to do. Then the waiter delivers the response back to you; in this case, it is the food.
Here is a real-life API example. You may be familiar with the process of searching for flights online. Just like the restaurant, you have a variety of options to choose from, including different cities, departure and return dates, and more. Let us imagine that you’re booking you are flight on an airline website. You choose a departure city and date, a return city and date, cabin class, as well as other variables. In order to book your flight, you interact with the airline’s website to access their database and see if any seats are available on those dates and what the costs might be.
However, what if you are not using the airline’s website a channel that has direct access to the information? What if you are using an online travel service, such as Kayak or Expedia, which aggregates information from a number of airline databases?
The travel service, in this case, interacts with the airline’s API. The API is the interface that, like your helpful waiter, can be asked by that online travel service to get information from the airline’s database to book seats, baggage options, etc. The API then takes the airline’s response to your request and delivers it right back to the online travel service, which then shows you the most updated, relevant information.
What an API Also Provides Is a Layer of Security
Your phone’s data is never fully exposed to the server, and likewise, the server is never fully exposed to your phone. Instead, each communicates with small packets of data, sharing only that which is necessary like ordering takeout. You tell the restaurant what you would like to eat, they tell you what they need in return and then, in the end, you get your meal.
APIs have become so valuable that they comprise a large part of many businesses’ revenues. Major companies like Google, eBay, Salesforce.com, Amazon, and Expedia are just a few of the companies that make money from their APIs. What the “API economy” refers to is this marketplace of APIs.
The Modern API
Over the years, an “API” has often described any sort of generic connectivity interface to an application. More recently, however, the modern API has taken on some characteristics that make them extraordinarily valuable and useful:
- Modern APIs adhere to standards (typically HTTP and REST), that are developer-friendly, easily accessible, and understood broadly
- They are treated more like products than codes. They are designed for consumption for specific audiences (e.g., mobile developers), they are documented, and they are versioned in a way that users can have certain expectations of its maintenance and lifecycle.
- Because they are much more standardized, they have a much stronger discipline for security and governance, as well as monitored and managed for performance and scale
- Like any other piece of productized software, the modern API has its own software development lifecycle (SDLC) of designing, testing, building, managing, and versioning. Also, modern APIs are well documented for consumption and versioning.