API Testing: Benefits, Types, Approaches, & More

August 25th, 2022 by inflectra

A crucial part of software development, the quality of your API testing can have a huge impact on the final product, as well as the timeline and budget. APIs help pieces of software communicate and share data, meaning that testing them to ensure quality and reliability is paramount. To understand all of the benefits that come from this testing, we’ll go through some of the most significant factors before detailing the different types of testing that might match your situation. Lastly, we’ll cover how to approach the testing process and the common types of API technologies that you might come across or use.


Benefits of API testing

There is a multitude of advantages that API testing brings to your development process. Let’s discuss some of the individual benefits:

Time effective

API tests require less scripting efforts as compared to GUI tests. GUI test time is spent on loading and rendering web pages and UI elements that might not even be needed in the end, which can slow down the testing process. API testing reduces all these tasks to one and as a result, the testing process is faster with better coverage.

Testing cost reduction

 API testing can provide faster results with early defect diagnosis, which reduces overall testing costs.

Access without UI

API testing allows access to the application without a user interface or users interacting with the system. API tests can run without needing to experience the software application, which gives an early insight into defects and errors. This allows for early issue resolution before the GUI is impacted.

Integratable testing

API testing allows for highly integrable tests. This is especially beneficial if you plan to perform functional GUI tests following your API testing.

Improves testing coverage

Testing the code-level functionality of an application provides an early evaluation of its overall build strength. This exposes small errors before they become larger problems during GUI testing. Core access enables testing in tandem with development, fostering communication and improved collaboration between both teams.


An API test exchanges data using XML or JSON. These transfer modes are completely language-independent, meaning that you can select any core programming language.

What are the types of API Testing?

While there are plenty of reasons for you to invest in API testing, there are many different types that you can implement. From the most basic to much more complex and niche applications, we’ll discuss each of the most common ones you might run into.

1. Functional testing

Functional tests validate the software system against functional specifications and requirements. To test each function of the API, a tester provides appropriate input parameters in XML or JSON payload, then verifies the output against the functional requirements.

2. Security testing 

API security testing reveals any risk, vulnerability, or threat within the API. Security testing identifies all potential loopholes and API weaknesses that can result in a loss of information, revenue, and reputation if misused by an unauthorized user.

Sample process for security testing API endpoints.

3. UI testing 

UI testing is a test of the user interface for the API and other integral parts. UI testing focuses more on the interface which ties into the API rather than the API testing itself. Although UI testing is not a specific test of API in terms of the codebase, it still provides an overview of the health, usability, and efficiency of the frontend and backend.

4. Penetration testing 

Penetration testing emulates an external attacker or malicious insider specifically targeting a custom set of API endpoints and attempting to undermine the security to impact the confidentiality, integrity, or availability of an organization’s resources.

5. Load testing

Load testing emulates a constant steady stream of traffic at both normal and peak conditions. This ensures that the APIs do not contain memory leaks or other similar defects that might cause issues after running for a prolonged period of time.

Sample load testing plan where you have multiple scenarios against different API endpoints.

Load testing often happens after a specific unit, or the whole codebase has been completed. 

6. Runtime/error detection testing

Runtime error detection testing is related to the actual running of the API and the universal results of utilizing the API codebase. This test focuses on monitoring, execution errors, resource leaks, or error detection.

7. Fuzz testing

Fuzz testing or noise testing sets operation parameters to unexpected values to cause unexpected behavior and errors in the API backend. This helps discover bugs and potential security issues.

8. Validation testing

Validation testing helps ensures that the software meets the business requirements and is seen as an assurance of the correct development. Validation testing occurs among the final steps and plays an essential role in the development process. It verifies the aspects of product, behavior, and efficiency.

9. Interoperability and WS compliance testing

 Web Services and Interoperability Compliance testing pertains to SOAP APIs and guarantees that they comply with Web Services Interoperability Profiles. The compliance is checked to see if the predetermined requirements are met.

API Testing Approach

API testing is performed by submitting requests to the software using the application’s programming interface of the application and then checking if it returns the expected data.

Create API testing requirements

To be able to plan API tests, first, you need to determine testing boundaries and requirements. This includes:

  • The purpose of the API 

  • The workflow of the application

  • The functions of the API

  • APIs this API interacts with

  • Aspects of testing

  • The problems that are being tested for

  • Testing priorities

  • The desired output

  • The definition of Pass and Fail

Knowing the purpose of the API will set a firm foundation to prepare your test data for input and output.

Establish the API testing environment 

Setting up an API test environment requires the configuration of the servers, databases, and every resource the API interacts with, depending on the software requirements.

Make a trial API call 

Before diving into thorough testing, make an API call just to check that nothing is broken and the API is operational.

Define the input parameters

Plan all possible input combinations to use them in test cases and authenticate the results to determine whether the API performs as expected.

Create API test cases

Execute test cases and compare actual results with the expected ones. A good practice is to group them by test category.


Examples of API test cases include:

  • Testing value in response to the input condition: define the input and authenticate the output, which can be any type of data or status

  • Checking the behavior of the API in the system when there is no return value in response

  • Tracking events and interruptions triggered by the output

  • Authenticating the effect on the system after data structure updates

  • Validating resources modified by the API call

Considerations for Performing API Tests

The following factors should be considered when performing API testing:

  • Understand API requirements: ensure you know the purpose of the API and the workflow of the application. 

  • Specify the API output status: you need to verify the response status code in API testing. All API response status codes are separated into five classes.

There are five values for the first digit: 

  • 1xx (Informational): The request is received and continues to be processed

  • 2xx (Successful): The request is successfully received, understood, and accepted

  • 3xx (Redirection): Further action needs to be taken to complete the request

  • 4xx (Client Error): The request contains the wrong syntax or cannot be fulfilled

  • 5xx (Server Error): The server fails to fulfill an apparently valid request

The first digit of the code defines the class of the response. Use these codes to determine the output of your API. Determine if the code follows global standard classes or if the code is specified in the requirement.


Analyze the validation response: During the API testing process, a request is raised to the API with the known data. This way, you can analyze the validation response. While testing an API, you should consider: 

  • Accuracy of data

  • Schema validation

  • HTTP status codes

  • Data type, validations, order, and completeness

  • Authorization checks

  • Implementation of response timeout

  • Error codes in case API returns, and

  • Non-functional testing like performance and security testing


Focus on functional APIs: focus on the simple APIs with only one or two inputs. These are necessary access points to further APIs. By focusing on these before moving on to test the others, you ensure that the initial points of access work.

Common Types of API Technologies

Over the years, APIs have evolved from simple code libraries that applications could use to run code on the same computer to remote APIs that can be used to allow code on one computer to call code hosted somewhere else.

Here is a quick list of the more common API technologies that exist in approximate chronological order:

  • TCP/IP Sockets

  • Remote Procedure Call (RPC)

  • Common Object Request Broker Architecture (CORBA)

  • Java Remote Method Invocation (RMI) and Enterprise Java Beans (EJBs)

  • Microsoft Distributed Component Object Model (DCOM) – also known as ActiveX

  • Web Services (SOAP then REST)

Get Started With API Testing Today

If you’re ready to add seamless and comprehensive API testing to your software development pipeline, Rapise provides all this and more. From automated testing and playback to reporting and cross-platform functionality, it empowers your software to be the highest quality possible while making your life as easy as possible. Click here to get started with a free trial today!

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