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Why is API Testing indispensable for the success of your product?

Introduction to API Testing

 This article is focused on providing detailed information on API Testing. An Application Programming Interface is a set of programs and instructions to allow interaction between two components of a software application or entirely separate software systems. It consists of a set of routines, protocols, and tools for building the software applications.

For example, you use a mobile application to check the weather, the application connects to the internet and sends requests to a defined server. The server receives the request, interprets it, collects necessary information, and sends it back to your phone. The application grabs the response and presents you the information in readable format. The complete transaction happened via API.

What is API Testing?

API testing is a type of software testing that involves testing APIs directly and as a part of integration testing to check whether the API meets expectations in terms of functionality, reliability, performance, and security of an application. In API Testing our main focus will be on a business logic layer of the software architecture. API testing can be performed on any software system which contains multiple APIs. API testing won’t concentrate on the look and feel of the application. API testing is entirely different from GUI Testing.

Testing APIs directly and as a part of integration testing to check whether the API meets expectations in terms of functionality, reliability, performance, and security of an application.

Different types of APIs

Web APIs

Can be accessed using the HTTP protocol. The API defines endpoints, and valid request and response formats. Web APIs include the APIs used to communicate with the browser. They may be services such as web notifications and web storage. Different web APIs feature varying levels of security and privacy, including open, internal and partner APIs. Multiple web APIs can be combined into a composite API – a collection of data or service APIs.

Open/Public/External APIs

Open APIs, also known as External or Public APIs, are available to developers and other users with minimal restrictions. They may require registration, and use of an API key, or may be completely open. They are intended for external users (developers at other companies, for example) to access data or services.

Internal APIs

In contrast to open APIs, internal APIs are designed to be hidden from external users. They are used within a company to share resources. They allow different teams or sections of the business to consume each other’s tools, data and programs. Using internal APIs has several advantages over conventional integration techniques, including security and access control, an audit trail of system access, and a standard interface for connecting multiple services.

Partner APIs

Partner APIs are exposed by/to the strategic business partners. They are not available publicly and need specific entitlement to access them. A partner API, only available to specifically selected and authorized outside developers or API consumers, is a means to facilitate business-tobusiness activities.

Composite APIs

Composite APIs allow developers to access several endpoints in one call. These could be different endpoints of a single API, or they could be multiple services or data sources. Composite APIs are especially useful in microservice architectures, where a user may need information from several services to perform a single task. Using composite APIs can reduce server load and improve application performance, as one call can return all the data a user needs.

Need of API Testing

More IT companies are inclining towards the concept of microservices. Microservices facilitate different datastores corresponding to each section of the application that requires different commands for operations. Companies prefer to use the concept of microservices because it allows quick deployment which further makes the development process smoother. APIs play an important role here. Each section of the applications gets commands through the API only. Hence, API testing is a must to do because it helps to identify the errors or bugs at the very early stage of development. Also, through API testing, we get to know whether the API is effectively interacting with all the sections of the code or not? Here, testers validate the response of the API.

Types of APIs Testing

Functional Testing

Functional testing is meant for testing the selected functions of the application based upon the codes. The API functions are to be tested with specified parameters to ensure that they function well within the application after it goes to the targeted audience’s hands.

User Interface Testing

User Interface testing is meant to examine the easy accessibility of the application for the users. This test focuses on the interface that holds onto the API. Moreover, this test will give a verdict on the usability, health, accessibility, and efficiency of the application.

Security Testing

Security testing is essential within the API testing practices ensuring that the app is safe from external threats. Some of the aspects that are checked within the security testing services are encryption validation, API design for access control, user rights management, and others.

Load Testing

Load testing is imposed to make sure that the entire codebase has the potential to withstand heavy load. All the theoretical assumptions in terms of the load-bearing capacity of the application are also monitored. Hence, load testing is used to check the performance of the application in both normal as well as peak conditions.

Runtime & Error Detection

Runtime and error detection is to define the actual API running potential. This testing technique is meant for monitoring the app performance, identifying the errors, resource leaks, error detection, and other such aspects. The detected errors will be rectified and fixed to ensure that here will be no runtime breakdown.

Validation Testing

Validation testing is mostly for the final steps of API Testing, which is meant for the development process. This testing process is carried out to verify the product, behavior, efficiency, and other such aspects of the application. Hence, this testing assures that the application is correctly developed.

Fuzz Testing

Fuzz testing determines the security audit by identifying any negative behaviors or forced crash situations. This test is determined to ensure that the API limits are optimal for tackling the worst-case scenarios.

Penetration Testing

Penetration testing is a type of in-depth testing used to find vulnerabilities within an application and save it from potential attackers.

Benefits of API Testing

  1. Access to application without user interface:
    The major core advantage of API testing is that it provides access to application without users actually having to interact with a potentially disparate system. This helps the tester to detect and recognize the errors early, instead of them becoming larger issues during GUI testing.
  2. Protection from malicious code and breakage:
    API test requires extraordinary conditions and inputs, which protects the application from malicious code and breakage. Basically, API tests push software to their connective limits. API testing helps remove vulnerabilities.
  3. Time efficiency vs functional and validation testing:
    API testing is far less time consuming than functional and validation testing. 10,000 automated API tests save 3 hours of time on average vs. functional and validation testing.
  4. Reduces Testing Cost:
    API test automation requires less code than GUI automated tests thus providing faster test results and better test coverage. The end result of faster testing is a reduction in overall testing costs. Testing the API level functionality of the application provides an early evaluation of its overall build strength before running GUI tests. Early detection of errors reduces the manual testing cost. API test automation increases the depth and scope of the tests.
  5. Technologically Independent:
    In an API test, the data is interchanged using XML or JSON and composed of HTTP requests and responses. These all are technology independent and used for development. Thus an API test allows you to select any core language when using automated API testing services for your application.

What exactly needs to be verified in API testing? 

Basically, on API Testing Services, we send a request to the API with the known data and we analyze the response.

  • Data accuracy
  • HTTP status codes
  • Response time
  • Error codes in case API returns any errors
  • Authorization checks
  • Non-functional testing such as performance and security testing

Challenges in API Testing

  • Selecting proper parameters and its combinations.
  • Categorizing the parameters properly.
  • Proper call sequencing is required as this may lead to inadequate coverage in testing.
  • Verifying and validating the output.
  • Due to the absence of GUI, it is quite difficult to provide input values.

Types of bugs while performing API Testing

  • Stress, performance, and security issues
  • Duplicate or missing functionality
  • Reliability issues
  • Improper messaging
  • Incompatible error handling mechanism
  • Multi-threaded issues
  • Improper errors

API Testing best practices

  • Test for the expected results
  • Add stress to the system by sending a series of API load tests
  • Group API test cases by test category
  • Create test cases with all possible inputs’ combinations for complete test coverage
  • Prioritize API function calls to make it easy to test
  • Create tests to handle unforeseen problems
  • Automate API testing wherever it is possible

API Testing Tools

Popular tools which can ease the API testing process are:

  • Postman
  • SoapUI
  • Katalon Studio
  • APIgee
  • Tricentis Tosca
  • JMeter
  • Rest-Assured


API consists of a set of classes/functions/procedures which represent the business logic layer. If the API is not tested properly, it may cause problems not only in the API application but also in the calling application. It is an indispensable test in software engineering.



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