The efficiency of API can make or break your product. API acts as a bridge between the database and the user interface of your application. Therefore, API testing in product development has rightfully gained importance over the last few years. While the benefits of API testing are many, it is easy to land in pickle over the incorporation and management of API testing in continuous development and agile environments.

Here, let’s better understand what is API testing, its benefits, various API testing tools, and processes in detail. Read on!

What is API Testing?

As the name suggests, API testing is a process that helps in ensuring the quality of the software product. API testing includes various QA actions, including API calls, outputs, and response validation, using multiple parameters. Here, we are more inclined towards finding data accuracy, HTTP status codes, data format, and error codes.


The approach that we utilize to test an API depends on the type of product you have created. Whether you have a web services API, OS API, DB API, or API to access resources remotely, API testing is still necessary.

What are the Different Types of API Testing?

Now that the question ‘what is API testing’ is answered, let’s have a look at the types of API testing. Below, we have discussed 8 API tests that answer common concerns due to which we are using REST API testing in the first place. The concerns are:

  • Understanding the return value of API according to input
  • Verifying whether API is returning the wrong answer or nothing
  • Checking whether API is calling some other API or invoking an event
  • Ascertaining whether the API is connected to the data structures or not

Now, let’s see the types of API testing that you can use.

1. Functionality Testing

Functional testing is a type of API testing, which verifies the operational requirements of the product. These functions use particular scenarios to make sure that the API is working as per the expected parameters. Errors are corrected or managed if the result is not as expected.

For instance, if you are testing an API that is created for ordering music on an online platform, functional testing can check the layered scenarios. It indicates that the user generally searches by track or the artist’s name, and a functional test would use multiple scenarios to understand if the right input is delivered.

2. Reliability Testing

When we connect an API to more than one device, it is necessary to check if it could have any disconnections. Reliability testing allows us to check exactly that. Through this test, you can see if the API will perform without failure in any particular environment and offer consistent results.

3. Load Testing

As the name suggests, load testing checks if the API has the power to handle a certain amount of load. It reviews how a particular API behaves under higher loads than it was meant to tackle.

In this type of test, measure the response times, check severe conditions, analyze throughput, and evaluate other similar factors. The entire goal is to see how the system would react to understand the high load scenario.

4. Creativity Testing

The creativity of any API is its ability to be used in unique ways. That is what creativity testing checks in an API. It evaluates whether we can use a particular API in multiple ways or not. If not, creativity testing reveals that the API is poorly written.

5. Security Testing

Security testing includes the security needs of the API. It contains permissions, authentications, and access controls.

  • The authentication that you need for the API
  • The encryption necessary to keep sensitive data safe
  • The authorization checks and controls in place for resource management

Collectively, these three reveal the security structure of an API.

6. Proficiency Testing

Proficiency testing is related to the performance of the API. It measures the ability it offers to the developers and the degree of the feasibility of conducting API actions.

7. API Documentation Testing

Documentation is often ignored but is crucial for the development team. The API document contains the procedure for using the API. Every complex requirement, technical need, and engineering need is described within the API documentation. Understanding whether this document can guide the user to extract value from the API or not is the actual purpose of API documentation testing.

8. Negative Testing

Negative testing checks if your API is designed to handle unexpected and invalid user responses gracefully. For instance, if the user types a number in a letter field, what would you say to the user, or what message would you display? You can simply show “Invalid response. Please enter a letter.”

Similarly, every wrong user input is covered in this test.

Benefits of Using API Testing in Development

There’s an array of benefits of including API tests to your test-driven development. The entire product development is improved, which helps in delivering high-quality services to the end-user.

There are five significant reasons why the API testing process is crucial for an organization. Read on to understand these benefits.

1. Quality

The rule of thumb is that you should develop API tests during the development lifecycle, and these tests should consecutively check the API. If you fail to achieve that, you would have no option other than developing API tests once you complete the development. The fault behind this is that you are more focused on how the API should perform instead of checking other possible loopholes.

2. Performance

API tests evaluate how the API integrates with other APIs, its functionality, and its performance if you think about it. Things that API testing process helps in:

  • You can verify the code and find if there are any bugs in the code. It saves us from changing a significant part of the code later.
  • You can expose and find issues in the coding, whether through automated or manual tests.
  • API testing understands the core functions of your product and judges if the functionality is according to the user’s expectation.

API testing is an indispensable activity that ensures top performance of your end product. Through all of the above measures, API testing is directly or indirectly improving how your API performs.

3. Agility

There’s no doubt in the fact that the usage of agile methodology is increasing as we speak. Just conducting GUI tests can’t give us the required results. It also indicates that it is not right to allow the testers to get to work once developers have finished working. That is simply not efficient.

You need to start your API testing early in the development cycle to ensure every endpoint of development is covered and checked. The agile nature of API testing itself offers better management abilities to the team and better performance/quality to the product.

4. Test Coverage

When you need to create a quality product, you need to cover every software failure point. It helps in building customer trust and improving the reliability of the product.

Fortunately, API testing, when conducted during the development phase, can uncover issues in the network, API, services, and the server. These issues may not be as easily solved once the product has moved to deployment.

This is because once your product reaches the production phase, you already have the responsibility of creating new tests for future and evolving use cases. You have tests during development and tests during production to finally offer the product that the user is looking forward to. API testing creates an agile environment to achieve that.

5. Test Reuse

One of the benefits of an API testing tool that you will observe is the ability to run tests in multiple environments. You can duplicate and reuse tests and also share these tests. For instance, your QA team can build API tests for a staging environment, which would be utilized by the DevOps team in the production environment. The reusability of the analysis adds to the efficiency and quality of the API testing process.

Top 10 API Testing Tools

It is necessary to use the right API testing tools for API test automation and efficient outcomes. For this reason, we discuss the ten best API testing tools below.


1.Katalon Studio

Katalon Studio is an automation tool for multiple applications, such as mobile, web, desktop, and API. It supports Rest and SOAP and empowers your team with CI/CD related integrations.


SoapUI is a dedicated and headless tool for functional testing, which empowers its users to test SOAP, Rest, and Web Services APIs without any hassle. It supports data-driven testing along with CI/CD integrations.

Additionally, you can reuse any test code created in SoapUI.

3.Tricentis Tosca

Tricentis Tosca is another amazing tool for DevOps and Agile. It supports various protocols such as TIBCO EMS, IBM MQ, etc. You can reuse the test code for test automation in this API testing tool too.


Apigee is a fantastic API testing tool to test performance and measure the quality of the product. It also supports APIs created with the help of other applications like Apigee, Swagger, and more. Through this tool, you can track the API’s performance with error rates, API traffic, and response times measures.


Postman API testing is a browser plugin, which is now available for both Windows and Mac operating systems. It is excellent for testers and QA teams that are not interested in dealing with integrated development environment coding, the same language that the developers used.

API testing using Postman is easy and hassle-free for the QA team.


JMeter was originally developed for load testing. But, it is often used for functional testing. It supports test result replay and dynamic performance testing of resources. On this tool, you can work with CSV files and find parameters for API tests.


With Rest-Assured, you can test Rest APIs with ease. It has multiple features, and the user doesn’t need to write code from scratch. The user doesn’t have to have expert-level knowledge of HTTP to use this API testing tool.


The Assertible API testing tool is designed for reliability and automation. Through this tool, you can automate your API tests in the CI/CD pipeline. The QA team can also make specific changes to the tests, which would be automatically updated.


Swagger API testing tool is perfect for performance, security, and functional testing. Its platform makes it easier for the QA team to manage, create, and use API tests in the CI/CD pipeline rather quickly.

10.Karate DSL

Karate is a stable and unique tool for API testing. It checks the performance of the product and self-writes step definitions in Karate. As the tool automatically manages that requirement, you can directly start testing your product.

For beginners, Karate is the right choice.

API Testing – Best Practices

Before you start using API testing tools, we have a few suggestions. Below, we have discussed the ten best practices for API testing. These tips would help you put the right foot forward.

  1. Firstly, test the expected outcome or typical outcome.
  2. Use stress by including multiple load tests in the system.
  3. Always test for failure. You need to know the failure points of your API.
  4. Use group tests according to different categories.
  5. Use API function calls to make it simpler for the team to accelerate testing.
  6. Use the maximum number of variables and keep it in maximum isolation.
  7. Check how the API manages unknown loads and problems.
  8. Use pre-planned call sequencing.
  9. Use test cases for every input combination of the API.
  10. Automate API tests through API testing tools wherever it is possible.


API testing tests a very crucial layer – where logic processing decides the transactions and working of the app. Robust API testing in product development ensures quality product, shorter time to market, and flawless performance.

Utilizing API testing early in your product development cycle is key in saving time and resources in the long run, and keeping the stakeholders satisfied.

Originally published here.