API Testing Tutorial for Beginners

What is an API?

An API (Application Programming Interface) is a computer interface that allows two different software systems to communicate and share data. A software system that performs an API has numerous functions/subroutines that may be performed by another software system. Between two software systems, an API describes the types of requests that can be made, how they may be made, the data formats that can be utilized, and so on.

Shift Left Testing

Shift Left Testing is one of the most common forms of testing that is now being asked in API Testing Interviews. Almost every project that follows the Agile Methodology includes this form of testing.

Prior to the introduction of Shift-Left Testing, software testing occurred only after the development was completed and the code was provided to the testers. This strategy resulted in a last-minute scramble to make the deadline, as well as a significant reduction in product quality.

Besides that, the work required (when errors were discovered at the last stage before production) was enormous, since developers had to repeat the design and coding phases.

What does API Testing Entail?

API Testing is a sort of software test that verifies Application Programming Interfaces (APIs). The goal of API testing is to ensure that programming interfaces are functional, reliable, fast, and secure. Instead of utilizing typical human inputs (keyboard) and outputs, API Testing uses software to make API calls, get output, and record the system's response. API tests are distinct from GUI testing in that they do not focus on the appearance and feel of an application. It focuses mostly on the software architecture's business logic layer.

API automation is a term that refers to the Testing that necessitates the use of an API-enabled application. To test an API, you'll need to do a few things.

  • To drive the API, use the Testing Tool.

  • To test the API, write your own code.

You will learn more about- in this API testing lesson.

  • API Test Environment Setup

  • The Different Types of API Output

  • API Testing Test Cases are a set of scenarios that are used to test APIs.

  • API Testing Methodology

  • There is a distinction between API testing and unit testing.

  • How to Run API Tests

  • API Testing Best Practices

  • Types of Bugs Detected through API Testing

  • How to Automate API Testing

  • API Testing Challenges

API Test Environment Setup

  • API Testing differs from other methods of software testing in that there is no GUI, but you must build up an initial environment that executes API with a specified set of parameters and then checks the test result.

  • As a result, setting up a testing environment for API automation testing seems to be a little difficult.

  • The database and server should be set up to meet the application's needs.

  • After the installation is complete, the API Function should be used to verify that the API is operational.

Different Types of API Output

An API output might be −

  • Any kind of information

  • current situation (say Pass or Fail)

  • Another API function is called.

In this API testing lesson, we'll look at an example of each of the aforementioned Types.

Any Kind of Information

For instance, there is an API method that adds two integer integers.

Long add(int a, int b)

The figures must be entered as input parameters. The final result should be the sum of two integer values. This output must be compared to a predicted result.

It is necessary to make calls, such as

add (1234, 5656)

If the number exceeds the integer limit, an exception must be handled.

Current situation (say Pass or Fail)

Take a look at the API method below −

  • Lock()

  • Unlock()

  • Delete()

They may return any result as an output, such as True (in the event of success) or False (in the case of failure). A more precise Test Case would be to run functions in any of the scripts and then check for changes in the database or the Application GUI.

Another API / Event is being called.

In this scenario, one of the API functions is called, which then calls another function.

For instance, the first API method may be used to delete a specific record from a table, and this function then calls another function to REFRESH the database.

API Testing Test Cases

API testing test cases are based on

  • Return value depending on input condition: it's simple to test since the input can be described and the outcomes can be verified.

  • Does not provide any information: When there is no return value, the system's API behavior must be examined.

  • Activate another API/event/interrupt: If an API's output causes an event or interrupt, the events and interrupt listeners should be monitored.

  • Update data structure: Updating data structure will have an impact or consequence on the system, which should be verified.

  • Modify specific resources: If an API request alters specific resources, it should be checked by accessing those resources

Methodology for API Testing

The API Testing Approach is a predetermined approach or technique that the QA team will use to conduct API testing after the build is complete. The source code is not included in this testing. The API testing method aids in a better understanding of the features, testing approaches, input parameters, and test case execution.

The following points will assist the user in implementing an API Testing strategy −

  • Understanding the API program's functionality and precisely defining the program's scope

  • Write test cases for the API using testing approaches such as equivalence classes, boundary value analysis, and error guessing.

  • The API's input parameters must be carefully considered and described.

  • Carry out the test scenarios and compare the predicted and actual outcomes.

API Testing Vs. Unit Testing

The following table highlights the major differences between API Testing and Unit Testing −

Unit TestingAPI Testing
It is carried out by developers.It is carried out by testers.
Different functions are put to the test.The functionality is tested from beginning to finish.
The source code may be accessed by a developer.The source code is not accessible to testers.
UI testing is also a part of the process.Only API functions are put to the test.
Only the most fundamental functions are put to the test.All functional difficulties are thoroughly examined.
The scope is limited.a broader breadth
Usually performed before check-in.After the build is finished, it is run.

How to Run API Tests?

Apart from the standard SDLC process, API automation testing should include at least the following testing approaches.

  • Discovery Testing − The test group should manually perform the set of calls defined in the API, such as confirming that a given resource accessible by the API may be listed, created, and destroyed as needed.

  • Usability Testing − Usability testing ensures that the API is both functional and user-friendly. And how effectively does the API interface with other platforms?

  • Security Testing − Testing for security involves determining what form of authentication is necessary and if sensitive data is encrypted through HTTP or both.

  • Automated Testing − API testing should result in the establishment of a collection of scripts or a tool that can be used to run the API on a regular basis.

  • Documentation − The test team must ensure that the documentation is complete and sufficient for interacting with the API. The final output should include documentation.

API Testing Best Practices

  • API test cases should be organized into categories.

  • The declarations of the APIs being called should be included at the top of each test.

  • The choice of parameters should be stated clearly in the test case.

  • Prioritize API function calls so that testers may easily test them.

  • Each test case should be as self-contained as feasible and free of dependencies.

  • In your development, avoid "test chaining."

  • When using one-time call functions such as Delete, CloseWindow, and so on, more caution is required.

  • Call sequencing should be carried out and meticulously planned.

  • Create API test cases for all conceivable input combinations of the API to guarantee thorough test coverage.

Types of Bugs Detected through API Testing

  • Fails to gracefully handle error circumstances

  • Flags that have never been used

  • Functionality that is either missing or duplicated

  • Issues with Reliability Connecting to API and receiving a response is difficult.

  • Security Concerns

  • Problems with multithreading

  • Problems with performance. The API response time is really long.

  • Errors/warnings to a caller that isn’t correct

  • Handling valid argument values incorrectly

  • The data in the response isn't organized properly (JSON or XML)

How to Automate API Testing

The lessons that follow give step-by-step guidance to automating API testing.

  • REST Assured API Testing

  • How to use Postman to test an API

  • How can I use UFT to test an API?

API Testing Challenges

API testing has a number of drawbacks, including −

  • Parameter Combination, Parameter Selection, and Call Sequencing are the three main issues in Web API testing.

  • There is no graphical user interface (GUI) to test the program, making it impossible to provide input data.

  • It's a bit tough for testers to validate and verify the results on a separate system.

  • The testers must be aware of parameter selection and classification.

  • It is necessary to test the exception handling method.

  • For testers, coding expertise is required.


The business logic layer is represented by a collection of classes, functions, and methods in the API. If an API is not well tested, it may create issues in both the API application and the caller application. In software engineering, it's a must-do test.