Everything You Need to Know Before Starting API Testing

API (Application Programming Interface) is a means of communicating between two separate software systems and enables them to exchange data. These APIs work with the help of functions/sub-routines. Each software system executes these functions to send information across different platforms and perform seamlessly. 

However, since APIs operate in a web environment, releasing them into production without extensive testing can create multiple problems for your web service. Errors during communication can crash the application and ground the service to a halt.

API Testing is critical for ensuring the best performance for software systems and maintaining high-quality software. To help our readers understand how API testing works, we have compiled an API testing tutorial. Read our API testing tutorial to learn the basics of API Testing, its types, and API testing best practices.

Table of Contents

  1. What is API Testing?
  2. Types of API Testing
  3. 5 Benefits of API Testing
  4. Getting Started with API Testing
  5. How to Perform API Testing
  6. Conclusion

What is API Testing?

API testing is a software testing technique for validating APIs before we introduce them to a live environment. We perform API Testing to check the reliability, functionality, performance, and security of these interfaces.

Instead of using standard user inputs from keyboards and seeing their outputs, we test APIs using software calls. After sending the responses, we note their outputs and the system’s response. 

As API tests are mostly based on performance and functionalities, we don’t concentrate on the look and feel of an application as we do in GUI Tests. It mostly concerns the business logic layer in the software architecture.

API testing tutorial

Types of API Testing

API testing typically involves the following practices:

Functional Testing

As the name suggests, functional testing is centered on the functionality of an API. Therefore, during functional testing, testers need to evaluate whether the core functions of an application are performing optimally. 

UI Testing

Many people interexchange UI (User Interface) testing with UX (user experience) testing. However, in terms of API testing, testers perform UI testing to implement end-to-end integration tests and ensure that components of the user interface function as needed.

Compatibility Testing

Compatibility testing is the type of mobile app testing that focuses on the non-functional aspects of the application. As the name suggests, the procedure measures the compatibility of mobile apps against specific software, network, and hardware requirements.

Performance and Load Testing

Performance testing is meant to evaluate how APIs function under a particular workload of users. It helps testers ensure that the application doesn’t malfunction during its operation.

Security Testing

The level of security in APIs is fundamental to its success. Security testing helps developing teams evaluate the authenticity, integrity, and confidentiality of the API.

Unit Testing

Unit testing is the type of API testing where testers evaluate individual functionalities an API and see if these functions or sub-routines are working properly on their own.

Runtime/Error Detection

Monitoring APIs to identify problems such as exceptions and resource leaks.

Interoperability and WS Compliance Testing

Interoperability and WS Compliance Testing is a sub-group of API testing that relates directly to SOAP APIs. Interoperability between SOAP APIs is essential for ensuring conformance to the Web Services Interoperability profiles. Testers evaluate WS-* compliance to verify standards such as WS-Discovery, WS-Federation, WS-Trust, WS-Addressing, WS-Policy, and WS-Security.

Penetration Testing

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

Stress and Fuzz Testing

Stress testing is used to measure an API’s performance against extreme workloads, such as high data processing or traffic. The primary objective of this test is to identify the breaking point of the API. We also input forcibly into the system to create a forced crash.

5 Benefits of API Testing 

The API test codes are used to send different parameter combinations using APIs and are checked using value-based input conditions. It is the only to secure reliable and scalable connections between different software systems. Here are some of the main benefits of API Testing:

Access to Application without User Interface

API testing provides allows testers to test the application without requesting users to interact with a potentially disparate system. This means they can identify issues within the software before sending out components to users. 

Protection from Malicious Code and Breakage

During API tests, testers outline a secure system, conditions, and inputs for all APIs. This safeguards APIs from malicious code and breakage. As a result, API testers can push the application to its limits and address critical vulnerabilities.

Time Efficiency Vs. Functional and Validation Testing

Compared to functional and validation testing, API testing is less time-consuming. 

Cost-Effective / Reduces Testing Cost

The cost of API testing is significantly lower than GUI automated tests because it usually needs less code. As a result, API testing ensures faster test results, optimized test coverage while keeping testing costs minimal.

Furthermore, testing the API level functionality allows testers to evaluate the overall build strength before executing GUI tests. By detecting errors early, API testing can reduce manual testing costs. At the same time, API test automation increases the scope and depth of tests.

Technology Independent

API test interchange data using XML or JSON. These tests comprise of HTTP requests and responses, as well. Since both these components are technology-independent, API tests are often technology independent and you can leverage any core language when during automated API testing.

benefits of API testing

Getting Started with API Testing

API testing has 4 key stages.

Mapping the System into Representative Components

Before writing tests, it’s important to have a clear picture of which testing suites your system needs and how will they respond to API testing. This is why you should look at the entire system holistically before you break it into concrete components needed in concrete tests.

Choosing the Test Type and Parameters for Each Component

After we have a clear idea of all critical system components and the integration between them, it’s time to formulate tests that cover all the possible iterations and usages you can expect between those these components. 

Combining the Results from Each Component into One Big Picture

After each testing cycle, you must identify which components are working according to their functionality. You also have to identify components that didn’t work, when those components stopped working, and most importantly, why they didn’t work.

Continuous Testing

Once you put tests and reporting mechanisms in place, it’s time to move on to continuous testing. Being a part of DevOps, continuous testing allows us to pursue testing alongside product development and gives us insight into the product’s condition at a minimal cost.

How to Perform API Testing 

In API Testing, we must make two or more applications interact with each other using an API. Therefore, we must use the testing tool to drive API and write your own code to test the API. Here are 4 steps you should follow when performing API testing.

Set-Up API Test Environment

In API Testing, GUI is not available, so software testers must configure an initial environment for invoking APIs. You must define a required set of parameters, use them to test the application, and finally evaluate the test result.

For beginners, configuring the API testing environment can seem a bit complex, but configuring database and server according to the application requirements can make things easier. After installation, we can check whether the API Function needs to be called back or the API is still working.

Define and Tests Types of Output of an API

The output of any API usually consists of:

  1. Data type 
  2. Status (say Pass or Fail)
  3. Call another API function.

To understand how API applications operate, you must understand how each output functions.

Data Type

Example: There is an API function for multiplying two integers.

Long mul (int a, int b)

We have to submit both numbers as input parameters. The output of both integers will be the product between them. So the output should also be an integer itself and follow the expected outcome. You must call the function in the following manner:

mul (545, 654)

In case, the system faces exceptions, you handle the number that exceeds the integer limit.

Status (say Pass or Fail)

Consider the below API function –

  1. Lock()
  2. Unlock()
  3. Delete()

When either of the function executes successfully the returning output of the function will be True. Similarly, when these functions are stopped because of an error, the output value will be false. To make the test case more accurate, we can call the functions from either script and check those functions for changes later. 

Calling of another API / Event

In some APIs, the output of one API is simply calling other APIs or functions.

For instance, when we use the first API function for deleting a certain record in the table, that function can call the Database refresh function after deleting that record.

To test APIs accurately, it’s important to define what kind of outputs will be dealing with. Based on what you define as expected outputs, you can create exception handling methods for all other outputs.

types of API testing

Creating Test Cases for API Testing

When creating test cases for APIs, it’s important to remember the following things: 

Return Value Based on Input Condition

Return values based on input conditions are comparatively easier to test because we can define inputs more easily and authenticate results.

Does Not Return Anything

You must check the behavior of API on the system when there is no return value.

Triggering Other API/Event/Interrupt

In case an API output triggers an event or interruption, then you should track those events and interruption listeners.

Update Data Structures

If you update the data structure of the API, it will definitely affect the outputs or the system, so it’s important to authenticate before you make these changes.

Modify Certain Resources

In case an API call modifies any resources, you should validate those resources after accessing the respective resources.

Execute the Test Cases

Once you create test cases, you should 

Compare Expected and Actual Results


API testing is essential for creating web and microservices and establishing links between different platforms and systems. Without it, APIs and API testing, you cannot build robust applications with different functionalities while ensuring that these functionalities are easy to test and integrate.

Performance Lab is a testing service dedicated to ensuring the best software quality for our clients. We have served over 500 companies across a wide variety of domains that range from finance and healthcare to retail and technology. 

With years of experience at our disposal, our testers have learned how to ensure seamless communication between different software systems with the help of API testing. Our in-depth knowledge of various testing procedures helps us find hidden issues in different components of the software and deliver the best solution for your business.

To learn more about our company, feel free to visit our website.

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