What is API Testing? A Simple Guide on How to Perform API Testing

The concept of API’s pre-dates even the advent of personal computing and Web, it’s been around since the 1960s. Without APIs, the digital experiences that we experience every day wouldn’t be possible. API’s are responsible for everything from driving information-rich marketing campaigns to checking the weather on a mobile app. These API’s are tested before they are put to use and we call the process ‘API Testing’. In this article, we will explore ‘What is API Testing?’ in detail. If you are new to software testing, be sure to also read the Beginners’ Guide for Software Testing.

Let’s take a look at topics covered in this article:

• What is an API?
• What is API Testing?
• Why should we do API Testing?
• How to Perform API Testing?
• Types of API Testing
• List of Popular API Testing Tools
• Challenges of API Testing

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.

Suppose you are searching for a hotel room through an online travel booking site. Using the site’s online form, you will fill the necessary information like the city you want to stay in, check-in and check-out dates, number of guests, and number of rooms. Then you click “search.”. But what’s going on between entering your information to receiving your hotel choices? APIs, that’s what! The site aggregates information from many different hotels. When you click “search,” the site then interacts with each hotel’s API, which delivers results for available rooms that meet your criteria. And all this happens in seconds because of an API, which acts like a messenger that runs back and forth between applications, databases, and devices.

But, if an API doesn’t work efficiently and effectively, it will never be adopted, regardless if it is free or not. To avoid that, API’s are tested before they are put to use.

What is API Testing?

API testing is a type of software testing where application programming interfaces (APIs) are tested to determine if they meet expectations for functionality, reliability, performance, and security.

In simple terms, API testing is intended to reveal bugs, inconsistencies or deviations from the expected behavior of an API. Commonly, applications have three separate layers:

• Presentation Layer or user interface
• Business Layer or application user interface for business logic processing
• Database Layer for modeling and manipulating data

API testing is performed at the most critical layer of software architecture, the Business Layer. It is in the business layer, business logic processing is carried out, and all transactions between User Interface(UI) and database happen. So, making sure that, API offers complete intended functionality allows for easy future expansion of the software product. More reasons as to why API testing is important are discussed next in this ‘What is API Testing?’ article.

Why should we do API Testing?

API testing is an important activity that testing teams should focus on. It offers a number of advantages over other kinds of testing

Language Independent: Data is exchanged via XML and JSON, so any language can be used for automation, independent from the languages used to develop the application.

GUI Independent: API testing can be done to check the core functionality of the application to expose small errors and to evaluate a build’s strength. Also, we can test the APIs without a user interface.

Improved Test Coverage:  Most API’s have features, that allow a tester to create automated tests with high test coverage including positive and negative cases. We can rarely find test cases that can’t be automated.

Reduce Testing Cost: With API testing we can find minor bugs before the GUI testing. Usually, these minor bugs will become bigger during GUI testing. So finding those bugs while performing API testing will be cost effective.

Enables Faster Releases: The test cases that take about 8-10 hrs when executing UI regression testing take only about 1-2 hrs with API testing. This allows companies to release builds faster with API testing.

So, API testing is a really important phase of the testing process. But how is API testing carried out? Is there a specific procedure?

How to Perform API Testing?

Listed below are the steps that you can follow to perform API testing:

API Specification Review

The first step is documenting the API testing requirements. What is the purpose of the API? What is the workflow of the application? Which integrations are supported by the API? What are the features of the API? Documenting all these API testing requirements is the first thing you need to do. This will help you in planning API tests throughout the testing process.

Setting Up Test Environment

Next step is setting up a testing environment, with the required set of parameters around the API. This involves configuring the database and server for the application requirements.

Integrating Application Data

In this step, you need to combine your application data with the API tests to ensure that the API functions as expected against all possible input configurations.

Deciding Type of API Test

After you’ve created the testing boundaries and requirements, you need to decide what you want to test your API for. There are different types of API tests like functionality testing, validation testing, load testing, security testing, end-to-end testing, fuzz testing and many more. More about API testing types in the next session of this article.

Text Execution & Reporting

Once you have decided on what to test the API for, the next step obviously is to create test cases around that requirements and execute them. A basic guideline is to identify the most common parameters and conditions that an end developer will use when calling the APIs and test these scenarios extensively. And then document the test results for further use.

Well, you can follow the above steps to successfully test APIs. One of the important steps while performing API tests is to decide what kind of test you want to perform. For that, you should be aware of the types of API testing. Let’s check them out next in this ‘What is API Testing’ article.

Types of API Testing

In API testing, the following types of testing can be conducted:

• Functionality Testing – To check if the API works and does exactly what it’s supposed to do
• Reliability Testing – To check if API can be consistently connected to and lead to consistent results
• Validation Testing – Helps verify the aspects of product, behavior, and efficiency of an API
• Load Testing – Is performed to ensure the performance of API under both normal and at peak conditions
• UI Testing – It involves testing the user interface for the API and other integral parts
• Security Testing – To test that the API is secure against all possible external threats
• Penetration Testing – To detect vulnerabilities of an application from an attackers perspective
• Fuzz Testing – To test the API in terms of the limits to prepare for the “worst case scenarios”

Well, these are the most frequently performed API test types. By performing these tests what are you trying to test for? What will you find?

You are checking for:

• Duplicate or missing functionality
• Improper messaging
• Error handling mechanism is incompatible
• Multi-threaded issues
• Security, performance & security issues
• Reliability issues

Since API Testing is gaining popularity, we have many tools available for the same. This ‘What is API Testing?’ article lists some popular API testing tools for your reference.

API Testing Tools

For successfully performing API testing, you will need a tool to structure and manage your test cases. Here are some of the top API testing tools that can be used for Rest API and Soap API:

SoapUI: It is the most widely popular open-source tool for API testing. With SopaUI you can perform functional testing, performance testing, security testing, and data-driven testing. It will provide you with the reports for testing and will allow you to export the data.

Postman: With more than 4 million users worldwide it is one of the most popular API testing tools. It runs on Mac, Windows, Linux & Chrome Apps. Allows you to set up all the headers and cookies your API expects, and then check the response. 

Katalon Studio: It is a robust and comprehensive automation tool for API, Web, and mobile testing. Provides easy deployment by including all frameworks, ALM integrations, and plugins in one package. Also, supports both SOAP and REST requests,also supports various types of commands and parameterization functionalities.

Tricentis Tosca: It is a model based test API automation testing tool from Tricentis but also supports API testing. It supports a wide array of protocols including HTTP(s) JMS, SOAP, REST, IBM MQ, NET TCP, etc.

REST-assured: It is an open-source Java Domain-specific language (DSL) tool that makes testing REST service simple. Supports XML and JSON requests. This tool simplifies things by eliminating the need to use boiler-plate code to test and validate complex responses.

Apart from these popular tools, there are also tools like Apache JMeter, HttpMaster, Parasoft, HP QTP, Karate DSL, and many others. Though these tools make API testing easy, you might still come across some challenges while performing API testing.

Challenges of API Testing

• There is no GUI, which will make it difficult for testers to give input values
• Test case management is difficult since a tester might have to deal with millions of test cases
• The tester must possess expertise in the programming language(s) that are targeted by the API
• Time-consuming, requires a lot of time and resources to develop tools and design tests
• Improper documentation will make it difficult for a test designer to understand the purpose of API calls
• Proper call sequencing is required as this may lead to inadequate coverage in testing
• Exception handling functions need to be tested thoroughly

Regardless of the challenges that we might come across API testing is quite crucial and highly required. If an API breaks because errors weren’t detected, there is the threat of not only breaking a single application but an entire chain of business processes hinged to it. With this, we have reached the end of this ‘What is API Testing?’ article. 

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