Application Programming Interface or, most commonly said, API is a communication channel between two apps regardless of the programming languages used to create them.

Application Programming Interfaces (APIs) are typically used by online products that provide a complex service, utilizing online resources to provide basic information. The added value comes from how their software uses this information to create a new product that their users will find helpful. Internal APIs developed by the company’s developers are frequently used to create this one-of-a-kind product.

Therefore, API testing is essential to the software development process, enabling us to test the application’s business logic before the UI is complete. In addition, API testing verifies the functionality, reliability, performance, and security of programming interfaces.

Instead of standard user inputs (keyboard) and outputs, API Testing employs software to send calls to the API, receive output, and record the system’s response. API tests are distinct from GUI tests in that they do not focus on the appearance and feel of an application. Instead, it concentrates primarily on the software architecture’s business logic layer.

Failure of any of the APIs involved in creating the final product means that the product will be unavailable or partially unavailable to the company’s user base.

As a result, all APIs that are critical components of a company’s product must be monitored to take suitable actions during an outage.

Understanding the API’s purpose will provide a solid foundation for adequately preparing your test data for input and output. This step also aids in the definition of the verification strategy. For example, you will validate the responses against a database for some APIs, whereas, for others, it is preferable to validate the responses against other APIs.

• API TESTING is software testing that verifies Application Programming Interfaces (APIs).
• It concentrates mainly on the software architecture’s business logic layer.
• The final product will look to the company’s user base to be partially or wholly down if any of the APIs used in its creation fails.
• As a result, it is necessary to monitor all APIs that are essential parts of a company’s product to respond appropriately during an outage.

The API testing flow is straightforward, with three main steps:

• Send the request along with the needed input data.
• Obtain the response containing output data.
• Check that the response was as expected in the requirement.

The most critical aspects of API testing are not sending requests or receiving responses. Instead, they are the management of test data and verification. It is common to find that testing a few initial APIs, such as login, querying some resources, and so on, is quite simple. However, the task of testing APIs becomes increasingly difficult as they progress.

As a result, the API testing task is easily underestimated. You would eventually find yourself deciding on a good test data and verification methods approach.

The returned data in a testing project has similar structures but not the same. Furthermore, deciding which one to use would be difficult if you had to validate the JSON/XML data key by key or use object mapping to leverage the power of the programming language. Therefore, an actual development project is strongly advised when considering API automation testing. It should be extendable, reusable, and easy to maintain.

• Contrast the entire content of the response body with the expected information.

This method is appropriate for a straightforward response with static content. However, dynamic information, such as date and time, increasing ID, and so on, will cause problems with the assertion.

• Compare the response’s attribute values.

It is simple to obtain the value of a given key or attribute for responses in JSON or XML format. As a result, this method helps verify dynamic content or individual values rather than the entire content.

• Compare matching with regular expression.



In addition to verifying individual attribute values, this method is used to verify data responses with a specific pattern to handle complex dynamic data.

API testing best practices include the following:

• When defining test cases, categorize them.
• Include the chosen parameters in the test case.
• To ensure complete test coverage, create test cases for every possible API input combination.
• API load tests should be used to assess the system’s stress.
• Failures in APIs should be tested.
• Tests should be repeated until a failed output is obtained.
• The API should be tested to ensure that it consistently fails to identify problems.
• To monitor the API throughout production, reuse and repeat test cases.
• Use both automated and manual tests to produce more reliable results.
• When testing the API, keep track of what works and doesn’t.
• Call sequencing should be carried out with a rigid plan.

An API is a collection of classes, functions, and procedures representing the business logic layer. If the API is not adequately tested, it may cause issues not only in the API application but also in the calling application. Therefore, it is a must-have test in software engineering.