How To Proceed With API Testing? A Detailed Guide

API Testing Guide

API or Application Programming Interface is the foundation of data exchange and communication between two different software as well as between the database layer and UI layer of an application. In the article “What Is API Testing?”, you have already seen how important API testing is and got to know a lot of details about it. Now, such an important form of testing will surely need a properly structured step-by-step process to get successfully executed, and we are going to discuss the same here.

API tests are responsible for testing the performance, functionality, reliability, and security of the programming interfaces. And, the tricky part is that these tests are mostly not performed by providing user inputs through traditional methods. Rather some API calls are made and their responses are checked for correctness. So, let’s dive into the detailed process that can get you the best results of successful API tests.

Required Set-up For API Test Environment

  • API tests are somehow different from other tests because other tests are mostly performed on the components of the GUI, but API tests are performed when GUI is not available. And, the more complex part is that even if the GUI is not available, the API tests need an environment that invokes API with a required set of parameters.
  • You need to configure the database and server as per the application requirements.
  • When you are done with installing the API, its functions should be called to test them.

Types of Outputs of An API

If you are willing to test an API, you must totally know which forms of output an API can produce. So, an API can produce the following 3 types of output.

  1. Any type of data
  2. Status (e.g. Pass or Fail)
  3. Call another API function

Examples of these output types are -

Any Type of Data

Suppose you are adding two integers with the following command -

Long add (int a, int b)

The output of the summation of these two integer numbers is expected to be a number. So, the call can be like this -

Add (2323, 7878)

If the result exceeds the integer limit, you must have properly declared conditions to handle that situation.


API functions like Delete(), Lock(), Unlock(), etc. can return any of the values between True or False as output.

However, a better test case should check for changes either in the database or the GUI after calling the API functions.

Calling of Another API/Event

As a series of events happen in an application to complete a task, an API may need to call another API to perform the next task. For example, after you finish uploading a photo to a virtual album, the next API function should refresh the album to show the newly added photo there. So, the API of the first function should complete its task and call the next API.

Challenges of API Testing

API tests may become challenging in many ways. We have jotted down a few of them below.

  • The main challenges that you will always encounter during Web API testing are parameter combination, parameter selection, and call sequencing.
  • These tests are performed when any GUI is unavailable so, it becomes difficult to put the input values for the tests.
  • The testers find it difficult to validate and verify the output in a different system.
  • The testers should know parameters selection and categorization.
  • You need to test the exception-handling function.
  • The testers need to have significant coding knowledge.

The Methods To Get Rid of Some Common Challenges

Choosing The Right Testing Tool

You already know that automating your tests is one of the most efficient approaches for every form of test, and not just for API tests. For that, you need to use a powerful test automation tool that suits all your testing requirements, but the tricky part is that there are numerous popular tools available out there and you are very likely to get confused about selecting the best one for you.

Now, often people have a misconception that the most expensive one is the best, but that theory is completely wrong. You must select an automated testing tool based on its suitability to the testing needs of your business. You may refer to our article “Top 5 Automated API Testing Tools In 2022 | Latest Picks” for a clearer understanding of the best-suited one for you.

Always Having Clearly Stated Test Specifications

It is almost impossible to conduct complex tests without knowing their proper specifications. For example, a form has an input field for entering every applicant’s date of birth and the process should proceed further only if the entered date of birth states his/her age as 18 or higher on the day of filling the form.

Now, if all the above-mentioned specifications are not available to the testers, how can the tests be properly created to verify the correctness of the functions? Hence, the testers must always know the test specifications in detail to check if the conditions are properly met to make the API calls.

Properly Following The Learning Curve

Effective execution of the API tests and achieving the best results need you to have a lot of knowledge about the product as well as about the testing process. On top of that, you already know that testing APIs is significantly different from testing GUI-based applications.

Now, based on different choices made by you about hiring specialists for API testing or having an existing team member working on the tests with a new tool, the learning curve can be low to medium. Hence, based on the specifications of your product and your testing approach, a suitable learning curve must be followed.

Updating The Existing Skill Set

This point can be considered an extension of the previous learning curve point. The basis of this point is that even if the tester has an existing skill set for a different type of test, he/she has to update the skill set to become capable of performing the API tests. For example, if a tester is coming from the GUI testing team, he/she must adapt the method of testing to the unavailability of GUI. Also, gaining knowledge about certain programming languages and relevant things will always be there.

Types of Bugs API Tests Detect

  • Unused Flags
  • APIs that fail to handle error conditions efficiently.
  • APIs’ reliability issues that cause difficulties in connecting and getting responses from them.
  • Multi-threading issues.
  • Security flaws in APIs.
  • Missing or duplicate functionality.
  • APIs’ improper errors or warnings to a caller.
  • APIs’ response issues that are shown by their high response time.
  • Improperly structured response data in JSON or XML.
  • APIs handling valid argument values incorrectly.

Test Cases For API Testing

You have already seen the formats of APIs’ inputs and outputs. Now, those formats are just not for enhancing your knowledge base. They are the foundation for the types of test cases API testing has. Let’s check out the test cases below.

Return Value Based On Input Condition: These test cases are comparatively easier for analyzing. They have their input conditions defined and based on that, their outputs can be theoretically predicted so that it becomes easier for you to check the authenticity of the actual outputs.

Do Not Return Anything: These test cases do not return any value. You have to check the behavior of the API on the system.

Trigger Some Other API/Event/Interrupt: The outputs of some APIs trigger another API or some event or interrupt. In those cases, you have to check those events and interrupt listeners.

Update Data Structure: Updating the data structure will have some effect on the system, and you must have dedicated test cases to check their authenticity.

Modify Certain Resources: Some API calls may modify some resources and that must be validated by specific test cases that access respective resources.

How To Test APIs?

Now, as you have got enough idea about the challenges and respective test cases in API testing, it’s time for you to know how to actually test the APIs. Hence, let’s get started with that.

Discovery Testing

This is the method where the test group should manually execute the set of calls documented in the API. Here it is verified that a specific resource exposed by the API can be listed, created, and deleted as appropriate.

Usability Testing

This type of test ensures that the APIs are functional and user-friendly or not. These tests also check if the APIs integrate well with other platforms.

Security Testing

As the name suggests, this type of API test checks the security aspects of the APIs. They test the type of authentication required and whether sensitive data is encrypted over HTTP or both.

Automated Testing

This is undoubtedly the most efficient form of testing for every type of test (e.g. API testing, UI testing, etc.). This method involves the creation of test scripts or a tool to execute the API functions as planned and test their correctness.


This is one of the most crucial steps for the success of most types of tests. The testing team must have proper documentation that is required to effectively communicate with the APIs and perform the API calls. Also, the tests and their results must be properly documented for in-depth analysis.

API Testing Approach

From all our articles, you may have already understood how important it is to plan the tests before proceeding with developing a tech product. API testing approach is another name that stands by that practice.

It is a testing approach that is predefined from the point of getting started with the build. However, this approach does not involve the source code, and the QA team takes effective steps to clearly understand the testing techniques, APIs’ functionalities, and input parameters to efficiently proceed with executing the test cases.

The API testing approach is dependent on the following points -

  • Clearly understanding the functionality of the API program and properly defining its scope.
  • Applying efficient testing techniques such as boundary value analysis, equivalence classes, error guessing, and writing test cases for the APIs.
  • Properly planning and defining the input parameters for the APIs.
  • Executing the test cases according to test plans.
  • Comparing the expected and actual results.

Differences Between API Testing and Unit Testing

API Testing

Unit Testing

Mostly performed by testers.

Mostly performed by developers.

Does not allow testers to access the source code.

Developers are allowed to access the source code.

End-to-end functionalities are tested.

Every small functionality that consists of a small software module or unit, is tested.

The tests check all functional issues.

The tests check only basic functionalities.

The test cases are concerned with API functions only.

The test cases also involve UI tests.

These tests are usually run after the build is completed.

These tests are usually run before check-in.

These tests have a quite broad scope.

These tests have a limited scope.

API Testing Best Practices

  • The test cases should be grouped based on the test category.
  • Parameter selection should be properly mentioned in the test case.
  • On top of each test, the declarations of the APIs being called should be included.
  • Remember to keep each test case as self-contained and independent from dependencies as possible.
  • API function calls should be prioritized to make it easier for the testers to run the test cases.
  • Remember to take special care while handling one-time call functions like – Delete, CloseWindow, etc.
  • “Test chaining” should be avoided in your development.
  • Call sequencing should be well-planned and carefully performed.
  • For achieving complete test coverage, consider creating API test cases for all possible input combinations of the API.


APIs are those strong bridges that connect the two different layers of your application and establish the communication between two different software to execute a task. From our previous article on “What Is API Testing?” and this guide on how to proceed with it, you may have already acquired all the necessary knowledge to get yourself going with this crucial form of testing.

However, API testing is a huge topic and there will always be new aspects coming up to perform it more efficiently. And, to help you become an expert in it, we will soon be coming up with an article on some advanced API testing tips.

In connection with this, we must again mention our most effective suggestion which is automating all your tests with an efficient test automation tool like Preflight. This highly useful tool exempts you from all your worries about testing and lets every member of your product team create, run, and manage effective test cases. So, if you are already enthusiastic about such an amazing testing experience, we welcome you to try it for free.

For more information about our products, we are all ears to hear from you and encourage you to follow our website. Also, we are always publishing informative technical blogs to help you keep abreast of the latest tech updates.