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Sameeksha Medewar | 10 Dec, 2022

What is API Testing? Understanding API and How It Works

Are you trying to figure out how to perform API testing? Maybe you want to get started with it, but you’re unsure where to begin? Don't fret. From different types of API testing methods to tools for API testing, you’ll learn all you need to know about API testing in this article.

But before we dive in, first things first:

What is an API?

An API, or Application Programming Interface, is like a middle man between two separate systems. Its main job is to deliver a user response to a system and send the system’s response back to a user. 

For instance, ProcurementExpress (PE) has an API that allows it to connect with Xero seamlessly. API defines data formats for PE's and Xero's users as well. This way, the two platforms can exchange data with each other. How did that happen? API has a set of algorithms that tells PE and Xero how to talk to each other. Pretty cool, right?

In short, API has an interface that allows you to make a specific “request” to process information. It also allows you to use programming languages like JSON.

how an api works

What is API Testing?

Like anything in programming, an API must be tested before users can use it. Hence, we call the process of testing “API testing.” If you are unsure what software testing is, check out our software testing guide.

Developers perform API testing to confirm it ticks all the boxes. They include user-friendliness, reliability, performance, and security. Besides, API testing exposes inconsistencies, bugs, and any other performance-related issues.

Typically, all applications come with three separate layers:

  • User interface layer
  • Application user interface
  • Software architecture layer, which is where you perform API testing

Why is API Testing Crucial?

The rise of cloud applications has increased the use of APIs and API testing. Most applications that businesses and individuals use rely on interconnected APIs. So, API testing is vital for some of the following reasons:

  • API testing enhances the application's performance.
  • API testing lets you exchange data via XML and JSON.
  • You can use API testing to check the application's core functionality, identify minor errors, and check a system's strength.
  • You can use API's features to create automated tests with high test coverage. This includes positive and negative cases as well.
  • API testing saves you time. For instance, you will save up to eight hours just by doing API testing. In contrast, you will spend countless hours if you do UI testing only.
  • A major advantage of performing API tests is that you can catch errors early on. 
  • You don't have to wait for development to build your API tests. Plus, you can develop them to be compatible with your test cases.

How to Approach API Testing

You need to follow specific steps to ace the API testing process. Naturally, it would help to begin the API testing with defined goals. Or at least with a thorough understanding of how APIs work. 

Some questions to consider before doing API testing are:

  • What program will I be following?
  • Which tools will I use for testing?
  • What's the first thing I should do before testing the API?
  • Which error message will appear on every unsuccessful test?
  • Which message will appear on every successful request?

Once you have a handle on the answers to the above questions, you’re ready to use one of the 7 API tests.

Types of API Tests

Validation testing: With this type of testing, you’ll use basic questions. Examples of qualifying questions that you may use include:

  • Did we build the product correctly? 
  • Is the API we are using efficient enough to resolve issues? 
  • Did we experience any significant coding stumbling blocks? If so, what are those stumbling blocks, and how will we remove them? 
  • Do we have any long-winded codes that are time-consuming to fix and would push the API in the wrong direction? 
  • And most importantly, is the API storing the data accurately?
  1. Functional testing: This test will ensure that the API performs what it needs to do. How do you do that? By analyzing specific functions within the codebase. This will ensure that the API manipulates errors accurately.
  2. Load testing: This kind of API test determines how many requests an API can handle. In most cases, you will use load testing after every codebase. Your goal is to confirm if the API will function well under pressure.
  3. Reliability testing: This test helps you develop consistent results across all platforms.
  4. Security testing: Suppose you want to test the security of an API. Where exactly do you begin? You start to check the level of penetration of your API. From there, you confirm authorization checks and user rights management.
  5. Penetration testing: You will follow up the security test with the penetration test. Why? Because you must analyze the attack vector from an external standpoint. An example is when you use penetration testing to target the API in its entirety.
  6. Fuzz testing: You will use this test to test the negative behavior of the system. For example, when you attempt to force a crash or overflow of data.

The Benefits of API Testing

There are numerous benefits of using API testing — let’s look at some of them:

  • API testing guarantees users three key things: safety, reliability, and scalability.

  • API automation saves you a lot of time and money. As a developer, you won’t waste time figuring out how to test an application.

  • One of the significant benefits of API testing is that you can access the app without a User Interface (UI). What’s more, you'll pinpoint errors in the early stages and avoid wasting time and money.

API and Web Service Introduction

How to Perform API Testing

Follow the below steps to perform API testing: 

  • Document all the API testing requirements. This will help you to plan your API tests. Ask yourself: what will we use the API for? Does our application have a workflow? If not, why? Which applications can integrate with our system? 
  • Set up a testing environment. Once you have documented your testing requirements, take a look at your testing environment. Are all your databases and server applications up to scratch? See if there is any configuration that you still need to perform, and do it pronto! 
  • Integrate application data. Next up, put together your application data with API tests to ensure everything looks good. Remember, your API features should perform well against any configuration.
  • Decide on the type of API test. Once you have integrated your application data, you need to decide the kind of API test you would use. We have highlighted some of the different types of API tests above.
  • Consider text execution and reporting. If you want to execute your API testing right, you can't ignore text and reports. The easiest way to go about doing this is by creating test cases. Think about scenarios that the ordinary user may face when requesting the API. Test those scenarios and come up with a report. 

API Testing tools

Consider using one of the following API testing tools to perform your API testing like a pro. 

1. Postman

postman api

When Postman was first introduced to the market, it was a Google Chrome plugin. Today, you can use Postman to test any API. Interestingly, Mac and Windows users can use it too. You can use Postman to automate tests, perform debugging, and run requests. 


  • Postman interface allows you to extract web API data.
  • You can use it to write Boolean tests.
  • It comes with built-in tools, collections, and workspaces.
  • Postman supports various formats, including RAML and Swagger.


While you can use it for free, additional features cost $12.

2. SoapUI

SoapUI’s API

You can use this nifty tool to run web services and APIs tests. What's good about it is that it provides a free package and a fixed package. Developers can use its free package to grant users access to the full source code. 


  • It allows you to reuse security scans and load tests for functional test cases.
  • It has point-and-click and drag-and-drop functionality for simple and quick test creation.
  • It supports asynchronous testing.
  • It has data-driven testing functionality.
  • It allows you to synthesize data generation.


You can use it for free. However, if you wish to enjoy other benefits, the cost starts at €640 a year.

3. Apigee


Apigee is another API testing tool that you should consider, offered by Google. It’s a cross-cloud API testing tool that boasts a myriad of features. You use it to measure and test API performance.


  • Apigee has a functionality that can track error rates, API traffic, and response times.
  • You can use it to create proxies with the help of "OpenAPI Specification" and their deployment in the cloud.
  • It is compatible with APIs containing huge data.


You can use Apigee for free.

4. Karate DSL

karate api

Are you familiar with Karate DSL? If you aren’t, you should give it a try. There is a lot you can do with this cool API testing tool, from creating scenarios for API-based BDD tests to creating definitions for scenarios. 


  •  Karate DSL has sync capability-based support for WebSocket.
  •  It allows writing tests, even for technophobes.
  •  It supports multithreaded parallel execution and configuration staging or switching. 


Karate DSL has open-source pricing, which means it is available to any developer. 

Last Thoughts 

Every business will need integration at some point in time. And integrations can't be seamless without API testing. Most companies today use API integration platforms like Zapier, which allow you to integrate with other cutting-edge technologies. 

For any developer who wants to perform API testing right, it’s important to nail the basics. So, invest time in familiarizing yourself with all the tricks and tools of performing API testing. Also consider taking a course to better yourself with indepth API knowledge


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By Sameeksha Medewar

Sameeksha is a freelance content writer for more than half and a year. She has a hunger to explore and learn new things. She possesses a bachelor's degree in Computer Science.

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Disclosure: is supported by its audience. When you purchase through links on our site, we may earn an affiliate commission.

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