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Why API Testing is important?

From social messaging, online shopping to business applications, APIs are used everywhere. After all, Application Programming Interface (API) is the versatile and powerful mediator to connect diverse as well as heterogenous applications in order to interoperate with each other.

What is API Testing?

API testing is the process of sending requests to API and monitoring the responses to ensure that the API performs as expected on multiple factors such as performance, security and reliability along with its functionality. This testing process starts at the early stage of the development life cycle and continues until the production release.

Why API Testing is important?

With the abundant number of applications being churned out every single day, it is crucial to make sure that the APIs on which they are rely on, are providing 100% accurate data at all times. The only way to ensure this is through API testing.

Let’s have a look on some major reasons which makes API Testing so indispensable:

· Ensure the correct implementation of business logic and rules at the API level.

· By enabling developers and testers to detect errors at early stage of Software Development Life Cycle, API testing confirms that the data sharing functions and application endpoints work as expected.

· Ensure the data passed through the APIs is inline with compliance and security. This is done by providing a specific set of parameters to mimic the phishing attacks and check how APIs responds to such attacks.

· Ensure easy expansion of application by checking the dependency on APIs in terms of buildup of the application.

· Ensure the API can handle desired load. This is achieved by progressively increasing the user requests to test how API handles that load and the failure rate is measured for each load.

· Ensure frictionless API performance across all the platforms including desktop, mobile and web. For example, if there is a big sale on footwear website due to which high volume of API calls are expected. The API is tested to handle the traffic regardless from whichever mobile or web platforms API calls are coming.

How to test the APIs?

· Understand the requirements by reviewing API specification.

· Determine the API testing requirements, testing priorities and create a test plan.

· Define the scenarios, the input parameters and positive as well as negative tests.

· Set up the environment according to the testing requirements such as configuring the database and the system.

· Select an API testing tool as per the requirements to structure and manage the test cases.

What are the types of API Testing?

· Unit Testing: These tests are written close to the code and run with each build of the application. All the unit tests should pass while running the application automatically.

· Integration Testing: Ensures all the APIs and the other modules connected to each other are able to communicate properly by logically sanitizing the traffic. Also, if any expansion occurs in future, it should be seamless.

· Performance Testing: Ensures the APIs are consistently working during traffic spikes and multiple heavy processes running simultaneously.

· Load Testing: Determines how API handles large volume of requests over a short period of time and there are no memory leaks or similar issues.

· Security Testing: Based on the risk analysis, this testing is done to determine how APIs react with Cyber attacks and similar threats by emulating these threats. According to the outputs of such emulation tests, the security measures such as encryption methodologies, access control and authentication are enforced and updated.

· Validation Testing: This testing occurs at the later stage of the development to ensure the API is efficient and meets the business requirements.

Testcases for API Testing

· Validity of response headers

· Verification of negative test cases response

· Verification of response HTTP status code

· Verification of the handling of APIs error codes

· Verification of data parameters and APIs

· Validation of response payload

· Validation of keys with the minimum and maximum range of APIs

· Verification of response data

· Data integrity test cases

· End to End CRUD flows

· Test cases to conduct schema validation of JSON, XML

What are the API testing tools?

These are the common API tools for REST and SOAP APIs:

· REST-Assured: An Open-source Java Domain-specific language (DSL) tool which supports XML and JSON requests to test REST API.

· Postman: An API development environment which can run on Windows, Linux, Mac and Chrome. We can set up all the headers and cookies expected for the API in Postman.

· SOAPUI: An Open-source tool on which various types of API testing can be performed such as functional testing , performance testing ,security testing as well as data driven testing.

· Katalon: A comprehensive API, Web, Desktop testing as well as Mobile testing tool which can be used on Windows, Mac and Linux platform. We can use this tool to automate the API testing.

· ReadyAPI : It is the platform for the functional, security, and load testing of RESTful, SOAP, GraphQL and other web services. It is considered best for functional testing and load testing of APIs and web services.

What are some best practices for API testing?

Here are a few best practices for API testing to get more testing in less time, save your money and ultimately give good quality product:

· Document Everything: One of the important factors for the success of the tests is how much closely they are related business requirements. As API testing does not have any GUI reference, it is essential to refer the document which specifies all fields and its parameters. It is also important to create an exhaustive report for the failed test cases.

· Test cases creation: Create API test cases for maximum possible input combinations of the API. Every test case should be independent and self-contained. Group the API Test cases by test category. Every test should contain API declarations on the top. The parameter selection for each test should be mentioned in every test.

· API Function calls: API function calls should be prioritized. The sequencing of API calls should be well planned. One-time call functions such as Close Window, Delete and other similar ones should be handled with extra care.


API Testing is an important part of Testing Life cycle which ensures that the quality of the overall integrated system. As we use numerous services everyday which are relied on many interconnected APIs. API testing is crucial for the overall success of API and the product as whole.

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