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What is Application Programming Interface (API)?

Table Of Contents

  • What is API?

  • API as Abstraction

  • Usage of API’s

  • Operating systems

  • Remote APIs

  • Libraries and frameworks

  • Web APIs

  • API Release policies

  • Examples of API

  • API Design

  • API Testing

  • Types of API Testing

  • Advantages of API Testing

  • Challenges of API Testing

  • Frequently Asked Questions about API

Application Programming Interface (API)

Have you ever wondered how an application on your phone connects to a server? Or wondered what is responsible for this action?

For example, when you use an application on your phone, the application will connect to the Internet and send data to the server. The server then retrieves the data, interprets it, performs the necessary operations, and sends it back to the phone. The application then interprets the data and provides you with the required information in a readable manner. All this is done via an API.

Application Programming Interface (API)

What is an API?

Application Programming Interface (API) is a computing interface that shows the collaboration between different software intermediaries. It defines the sorts of calls or demands that can be made, the conventions to follow, the data formats that ought to be utilized, and how to make these calls.

It can likewise provide an extension system so that clients can expand functions to varying degrees and various ways. APIs can be fully customized for components, or they can be designed according to industry standards to ensure interoperability. Through information hiding, the API supports modular programming, which permits clients to utilize the interface, independent of implementation.

API as Abstraction

In terms of software, APIs are everywhere. APIs go hand-in-hand with abstraction; which is one of the fundamental concepts in computer science. Abstraction is only a way to organize the complexity of the system, so complex actions can be handled simply. Think of this abstraction as Amazon Dash Buttons, which are battery-powered button-type circuit boards that can be used to order staples from Amazon.

You order a Dash button from Amazon and used the app on your phone to connect to your Amazon account, a specific product, and your Wi-Fi network. Then, whenever you need more tissues, just click the button. The Dash button connects to the internet and sends a message to place an order on your account. After a few days, the tissues will arrive at your door.

API as Abtraction

Now imagine that as a customer, you have to coordinate all these things. You will never order a tissue because it is too complicated and time-consuming, and you have better things to do. Fortunately, the entire process is abstracted away from you. A long list of interconnected computer systems and manual processes make these tissues appear at your door, but you only need to think about the buttons.

This is what APIs are like for programmers. They require a lot of complexity and define a set of relatively simple interactions; which you can use to complete all operations instead of doing it yourself. In any software project, you may directly use dozens (if not hundreds) of APIs, and each API depends on other APIs, and so on.

Usage of API’s

1. Operating systems

API can specify the interface between the operating system and the application. For example, POSIX specifies a set of popular APIs designed to enable applications written for POSIX-compatible operating systems to be compiled for other POSIX-compatible operating systems. Examples of operating systems that implement the POSIX API are Berkeley and Linux software distributions.

Microsoft has shown a strong commitment to backward compatibility with APIs, especially in its Windows API (Win32) library, so older applications can run on new versions of Windows using executable program-specific settings called "compatibility mode".

The difference between API and Application Binary Interface (ABI) is that API is based on source code, while ABI is based on binary. For example, POSIX provides API, while Linux Standard Base provides ABI.

2. Remote APIs

The remote API permits developers to control remote resources through a protocol (a specific communication standard) that allows various technologies to cooperate, paying little mind to the language or platform.

For instance, the Java database connection API permits developers to use the same feature set to query different types of databases, while the Java remote method call API uses the Java remote method protocol to allow developers to call functions that can be operated remotely but run locally.

In object-oriented programming, the remote API is valuable in maintaining the object abstraction. Method calls executed locally on the intermediary object use the remote protocol to call the relating method on the remote object, and use the outcome as the return value locally.

Modifications to the proxy object will also result in corresponding modifications to the remote object.

3. Libraries and frameworks

APIs are usually related to software libraries. While the library is the actual implementation of this set of rules, the API describes and prescribes the expected behavior. A single API can take the form of different libraries sharing the same programming interface, with multiple implementations.

To use programs written in one language and libraries written in another language, the API is separated from the implementation. For example, Scala developers can take advantage of any Java API, since Scala and Java can be compiled into compatible bytecodes.

The use of API depends on the type of programming language involved. APIs for procedural languages ​​(such as Lua) may mainly consist of basic routines for executing code, processing data, or handling errors, while APIs for object languages ​​(such as Java) will provide specifications for classes and their methods.

Language binding is also an API. By assigning features and functions of one language to an interface applied in another language, language binding allows the use of libraries or services written in one language when developing in another language. Tools like SWIG and F2PY (Fortran-to-Python Interface Builder) can simplify the process of creating such interfaces.

APIs can also be related to software frameworks: frameworks can be based on multiple libraries that implement multiple APIs, but unlike the normal use of APIs, access to the built-in behavior of the framework is achieved by using new classes to extend its content plugged into the framework.

Moreover, the overall program flow of control can be controlled by the framework through inversion of control or similar mechanisms, and not controlled by the caller.

4. Web APIs

Web API is a defined interface for interaction between an enterprise and an application that uses its assets. This is also a service level agreement (SLA), which is used to specify function providers and expose service paths or URLs to their API users. The API method is an architectural method that develops around a program interface that provides a set of services for different applications that serve different types of consumers.

An API is defined as a set of specifications when used in the context of web development, such as defining the structure for HTTP response messages and request messages, usually in JavaScript Object Notation Format ( JSON) or Extensible Markup Language (XML). For example, the API of a transportation company can be added to an e-commerce-centric website to facilitate shipping services, and automatically include the current shipping prices, while the site developer does not have to enter the shipping price list in the network database.

Although "Web API" is historically synonymous with Web services, the recent trend has shifted from Web services based on Service Oriented Architecture and Simple Object Access Protocol towards resource-oriented architecture and more direct representational state transfer style web resources.

Part of this trend is related to the development of the Semantic Web to the Resource Description Framework (RDF), a concept that promotes the web-based ontology engineering technique. The web API allows many APIs to be incorporated into new applications called Mashups. In the field of social media, Web API enables Web communities to facilitate the sharing of content and data between communities and applications.

This way, the dynamically generated content can be published in one place and updated to multiple websites. For example, the Twitter REST API allows developers to access core Twitter data, while the Search API provides developers with a way to interact with Twitter Search and trends data.

API Release policies

APIs are one of the most popular ways to integrate tech companies. Those who provide and use APIs are members of a business ecosystem.

The policies for the release of the API are:

  • Private: The API is for internal company use only.

  • Partner: This API can only be used by specific business partners. For example, vehicles from rental companies like Uber and Lyft allow certified third-party developers to request rides directly within their apps. This allows the company to conduct quality control by choosing which applications can access the API and provide it with an additional source of income.

  • Public: The API is for public use. For example, Microsoft has disclosed the Microsoft Windows API, and Apple has released its Carbon API and its Cocoa API so that the program could be written for its platform. Generally, not everyone has access to all public APIs. For instance, RESTful APIs are used by Internet service providers such as Voxility and Cloudflare to allow vendors and customers to access network performance, infrastructure information, dashboard controls, or DDoS statistics. Access to these APIs is granted by either "API tokens" or client status checks.

Examples of API

  • DirectX for Microsoft Windows

  • ODBC for Microsoft Windows

  • ASPI for SCSI device interfacing

  • OpenMP API that supports multi-platform shared memory multiprocessing programming in C, C++, and Fortran on many architectures, including Unix and Microsoft Windows platforms.

  • Java APIs

  • Server Application Programming Interface (SAPI)

  • OpenAL cross-platform sound API


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

  • Cocoa and Carbon for the Macintosh

  • OpenGL cross-platform graphics API

  • Simple DirectMedia Layer (SDL)

Public API’s

There are many public APIs for you to interact with, and many of them come from industry giants. The ability to programmatically access specific platform companies' code through APIs makes them a platform.

Some well-known API examples include:

  • The Google API allows you to deliver your code to all Google services from maps to translation. The API is so important to Google that it has acquired Apigee, the leading platform for managing APIs.

  • The Facebook API gives you programmatic access to Facebook's marketing tools and social graph. Facebook has been restricting the user data that you can access through these APIs due to the impact of the Cambridge Analytica scandal.

To understand how the API works, let's dive into two sections: the Java API that Java developers use to interact with the Java platform, and the Twitter API; a public API used to interact with the social networking sites.

1. Java API

The Java API is a library of software configurations, which anyone who installs the Java Development Kit can use directly. These components can perform common tasks and increase productivity in general because programmers don't have to start from scratch every time.

One of the primary components used in the program in the list, and as you might expect, it can track the list of items. The Java API defines the operations you can perform on the list: adding items, sorting the list, determining if an item is in the list, and so on. It also specifies how to perform these operations. To sort the list, you need to select the list sorting method: alphabetical order, number descending order, lightest to the darkest color, etc.

2. The Twitter API

Twitter API is a JSON web API that allows developers to interact with Twitter data programmatically. Unlike the Java API included in the Java Development Kit, Twitter API is a web API. You must submit a request for a service hosted by Twitter to access it online.

With a web-based API (like Twitter), your app can send HTTP requests, just like a web browser. However, for people to understand, the response is not sent in the form of a web page, but rather returned in a format that can easily be parsed by the app. For this, there are various formats, and Twitter uses a popular and easy-to-use format called JSON.

Tweets are one of the key elements of Twitter. The Twitter API tells you how to use Tweets and perform these operations; create Tweets, search for Tweets, and bookmark Tweets. To search for Tweets, you need to define your search criteria: the hashtags or words you want to search for, language, and geographic location.

API Design

In API design, taking into account the context usually means that you follow accepted best practices and draw inspiration from other APIs that users may be familiar with. Suppose the library you are building provides a new kind of list for Java applications, maybe the library is dedicated to handling very large lists. The API of the List may include an add method that behaves the same as the Java List add method. This way, users can easily adapt to your library because they already know how to use it.

In API design, it is important to understand users and keep their needs in mind. If you understand the pain points of users and help them avoid this pain, your API will satisfy them. You may choose to break other well-designed API rules for the same reason. Using JSON as the exchange format is the de facto standard when writing a Web API. Although, JSON is too verbose and cumbersome if the target users for your API are scientific users; who will be retrieving large amounts of data. Therefore, you can choose to use a binary format such as GRIB; even though this is an extremely uncommon choice in the general sense.

APIs are a significant part of software design, and they are present at every level of the software stack. They provide a way to manage and define abstractions by telling us how we can deal with software components and how to implement them. Well-designed APIs support efficient, smooth, and easy adoption and use, while poorly designed APIs cause headaches every time they are used.

API Testing

API testing is a type of software testing that involves directly testing application programming interfaces (APIs) and as part of integration testing to determine whether they meet expectations for functionality, reliability, performance, and security. Because the API lacks a GUI, API testing is performed at the message layer. API testing is now considered to be the key to automated testing because API has now become the main interface of application logic, and because GUI testing is difficult to maintain, the release cycle is a short and agile software development and DevOps are often used and frequently changed.

API Testing

Softwares used in API testing are; API Fortress, SOAtest, SoapUI, and Swagger.

Types of API Testing

  1. Unit testing – This is used in single operations to test their functionalities.

  2. Functional testing - Testing functions of a wider range of scenarios, usually using unit testing as the basis for end-to-end testing. Including test case regression, verification, execution, and definition testing.

  3. Load testing - Often reuse functional test cases to verify functionality and performance under load.

  4. Runtime error detection - monitor the execution of automatic or manual tests of the application to expose issues such as resource leaks, race exceptions, and conditions.

  5. Security testing - Includes penetration testing and fuzz testing as well as identity verification, control access, and encryption.

  6. Web UI testing - performed as part of the end-to-end integration test, the test also covers the API, allowing the team to verify the GUI project in a larger transaction

  7. Penetration testing - Testing web applications, networks, or computer systems to find vulnerabilities that can be exploited by attackers.

Advantages of API Testing

API testing does not require a user interface to access the application. This way, you can detect small problems before they turn into big problems during GUI testing.

API testing is usually less time-consuming than GUI testing because it uses less code. As a result, it provides a more efficient and effective test range.

Another advantage is that you can use XML or JSON to transfer data. These exchange modes have nothing to do with language, allowing users to choose any coding language when choosing automated testing services. Also, API testing is easily integrated with GUI testing.

Challenges of API Testing

Organizations will face many challenges to successfully adopt an efficient API testing process, some of them are:

1. Tracking API Inventory: Many APIs involved in an application run independently of each other. When performing API testing, testers have to keep up with rapid updates and how these updates affect the entire application. Maintaining an API inventory is an important activity – without it, the test will fail or fail to evaluate the latest changes to the API and application.

2. Understanding of business application logic: API usually has many rules and guidelines to regulate its usage, such as display policy, storage policy, copyright policy, and rate limit. Based on the overall business logic, a set of business rules is defined for the development, use, and integration of APIs. API testers lack knowledge and understanding of this business logic and rules, leading to ambiguity about the test objectives.

3. Complex agreements: APIs interact with each other through a set of defined rules called agreements or contracts. These protocols are often complex and may hinder the proper integration and testing of communication between components.

4. Impact of change: Whenever there is a new version of the API, it may cause the entire application to go haywire. Because there are multiple related components, implementing changes is usually risky and unpredictable in terms of its impact.

5. Test data management: Many APIs with various parameters require a lot of data to effectively test them. For API testers, maintaining such a large amount of data and ensuring that the data is reusable is a huge challenge. The limited access to source platforms and the diversity of APIs further make it challenging to test data management.

When organizations seek to incorporate API testing into their test automation, they encounter these challenges as they try to build sustainable and maintainable workflows.

Frequently Asked Questions about API

Frequently Asked Questions about API

1. What are some styles for creating a Web API?

Common Web API architecture style is XML / JSON as format language, stateless connection, base URI as the service address, and HTTP for client-server communication.

2. What is the procedure for performing API testing?

When you do an API test, you will first select the group you want to add the API state that you want to test, and then choose the test development mode. Then, you can create test cases for the required API methods, configure the control parameters and test conditions of the application as well as the validation method. Then you can do an API test. After the test is completed, you will check the test report, filter, and sort all API test cases.

3. What is the difference between API and Web services?

Web Services should be connected across the web and have three modes of communication: XML-RPC, REST, and SOAP. They always need a network to operate. However, APIs have multiple methods of communication. A network is not necessary for its operation, and it is not necessary to be connected to it via the web.

4. What is SOAP?

SOAP (Simple Object Access Protocol) is a messaging protocol based on XML. It facilitates the exchange of information between computers. You can use Simple Object Access Protocol API to delete, update, create or find records In more than 20 instances of different calls, the Simple Object Access Protocol API can be used to manage passwords and do searches by modifying the protocol to any language that supports Web services.

5. What is REST API?

Rest (Representational State Transfer) is a set of functions that help developers to implement requests and receive responses. The interaction is carried out through the HTTP protocol. REST is stateless, so the server does not contain status or session data. With the REST API application effectively, you can restart the server in between two calls, you can restart the server between calls. Web services usually use the POST method to perform operations. However, REST uses GET to access the resources.

6. What is the difference between SOAP and REST?

SOAP only supports XML format, while REST supports many different data formats. SOAP cannot support caching, but REST can. SOAP is not as fast as REST and is similar to a desktop application that is tightly connected to the server. REST acts like a browser and uses standard methods. The application must fit into it. Finally, SOAP runs on HTTP but encapsulates messages, while REST uses HTTP headers to store meta-information.

7. What factors help inform your decision on which style of Web services—SOAP or REST—to use?

REST is usually preferred because of its performance, simplicity, scalability, and support for multiple data formats. However, when the service requires a higher level of security and reliability, SOAP is a viable option.

8. What kind of testing environment is needed for API?

Setting up an API test environment can be difficult because the database and server must be configured without using the GUI.

9. Explain API documentation.

Good API documentation is essential to this process. It provides a quick reference when working within the program. It provides planning, delivery layout, content, and detailed description of each function in the system.

Future of APIs Beyond 2020

From connecting the user to the interface of the system, API has come a long way. Now, API is providing support for the interface between subsystems and the interface between systems.

How far can we go?

We can imagine the future of APIs in two ways.

Future APIs will promote smarter forms of communication. Considering the fan-out pattern of communication in many distributed applications, this trend is obvious. Therefore, you can imagine that event-driven smart APIs spreading information across multiple systems.

Another area of impact on APIs is the emergence of the Internet in the future. Through various research projects to improve the Internet, API will produce many synergies. One such initiative is the concept of named data networks (NDN). NDN proposes a different semantics for the Internet data packet format, that is, using unique names instead of IP addresses to identify endpoints and applications. Although NDN and similar ideas are still in their infancy, they hope to fundamentally change the addressability of the Internet and make it more user-friendly based on names. APIs with name-based URLs and resource formats may become high-level abstractions of these underlying network semantics.

In this way, we can foresee the API trends that will develop with the future of the Internet. API will unify various technologies and systems, to realize a truly distributed system communication. They will form the basis of the future Internet communication structure.

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