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May 25, 2023 | 5 Minute Read

What Is API Testing? A Comprehensive Guide

Table of Contents


APIs (Application Programming Interfaces) connect and transfer data across systems and applications, which is why it’s at the center of software development. Testing APIs can enhance the efficiency of the testing strategy, helping organizations deliver software faster.

According to a report by Data Bridge Market Research, the global API testing market is projected to reach USD 4,733 million by 2030, from USD 1,071 million in 2020. This market growth corresponds to the boost in cloud applications and interconnected platforms that require APIs.

API testing also has many other aspects but generally involves requesting API endpoints and validating the response. This response could include checking for communication, security, performance, status, or functional correctness. API testing emphasizes testing business logic, security, performance bottlenecks, and data responses.

Dimpy Adhikary, a skilled Quality Analyst with 10+ years of experience, also highlighted the importance of the API testing during Axelerant’s QA Q&A show with Shweta Sharma.



Learn more about What is API testing and how it’s done.

What Is API Testing?

API testing is a type of software testing during which APIs are tested to determine if expectations are met for reliability, functionality, performance, and security. API testing is done to reveal bugs, inconsistencies, or deviations from the expected behavior.

There are also three separate layers of applications, including:

  • Presentation layer or user interface
  • Business layer or user interface for business logic processing
  • Database layer for modeling and manipulating data

API testing is done at the business layer, the most critical software architecture layer.

Types Of API Testing

There are different types of API testing, including:

  • Functionality Testing: Checks if the API works as intended
  • Reliability Testing: Checks if API can be consistently connected to and leads to reliable results
  • Load Testing: Ensures the performance of API under both normal and peak conditions
  • UI Testing: Involves testing the user interface for the API and other integral parts
  • Validation Testing: Verifies the aspects of product, behavior, and efficiency of an API
  • Security Testing: Verifies that the API is secure against all possible external threats
  • Fuzz Testing: Tests the API in terms of the limits to prepare for the worse-case scenario
  • Penetration Testing: Ensures that all vulnerabilities of an application are detected
  • Contract Testing: Lightweight form of API testing that verifies the content and format of API requests and responses

Apart from this, all these API tests help testers check for:

  • Missing or duplicate functionality
  • Improper messaging
  • Reliability issues
  • Incompatibility of error handling mechanism
  • Multi-threaded issues
  • Security, performance, and security issues


Why API Testing Is Done

There are several reasons why API testing is important, including:

Prompt Validation

With API testing, after the logic is designed, tests can be built to validate the reliability of responses and data. This means testers don’t have to wait for various teams to finish their work or for the entire application to be developed, as test cases are isolated and ready to build immediately.

Efficient Test Maintenance

UIs are constantly changing and moving based on how it’s accessed. This creates issues as tests are continuously rewritten to keep up with the code production. This is why API changes are more controlled and infrequent to help make refactoring tests more efficient.

Quicker Resolution

When API tests fail, it's easier to figure out where the system has broken and the location of the defect. This helps reduce time assessing bugs between builds, integrations, and different team members. An API test's small, isolated footprint is also ideal for quicker MTTR stats, a valuable KPI for DevOps teams.

Optimal Speed And Coverage

500 UI tests might take 50 hours to run. But 500 API tests can be run in 5 minutes. This means that testers can find more bugs in less time while fixing the errors immediately.

For example, the image below shows a simple and standard functional test at the UI level. It shows a user heading to a website, filling out a form, submitting it, and verifying to move to the next screen.


At the UI level, a simple test can pose a couple of challenges, including issues caused due to the physical limits of the browser and network connection. The test would fail if any elements were changed on the screen.

For example, if the 'Dogs' entry covers the 'Cat' entry, the users won't be able to select it. These challenges pose issues at this level. But the build time would quickly halt when one considers the addition of 10,000 different names and combinations through this form. With API testing, this undesirable scenario would boil down to just one step: conducting an API test.

What Are The Consequences Of Not Testing An API?

According to Dimpy Adhikary,



If API testing is not conducted, it becomes difficult to validate the functionalities of APIs. Skipping API testing can also have severe adverse effects on the security of the overall project, which increases the chances of fraud.

How API Testing Is Done At Axelerant

The quality engineering experts at Axelerant follow the steps given below to perform API testing.

Step 1: Specification Review

The first step is to document the API testing requirements. Testers need to find out the answers to questions like:

  • What is the purpose of the API?
  • What is the workflow of the application?
  • Which integrations are supported by the API?
  • What are the features of the API?

All these requirements should be documented to enable efficient API testing.

Step 2: Determine Appropriate Test Strategy

It is crucial to determine the appropriate test strategy before proceeding further. This step usually involves analyzing the API specifications, requirements, and use cases to identify the required testing techniques, tools, and resources.

Step 3: Set Up The Test Environment

Set up the testing environment with the required parameters around the API. This should involve configuring the database and server for the application requirements.

Step 4: Integrate Application Data

Combine the application data with the API tests to ensure the API functions properly against all possible input configurations.

Step 5: Analyze The Type Of API Test Required

After creating the testing requirements and boundaries, decide what the API should be tested for, as there are different API tests.

Step 6: Test Execution, Reporting, And Management

Create the test cases around the requirements and execute them. It is a good practice to identify common parameters and conditions a developer can use while calling the APIs and testing various scenarios.

Document the test results in a test management tool to ensure easy accessibility. By leveraging a test management tool, you can track the test coverage, execution status, and results. It also makes it easy to manage defects discovered during API testing.

10 Best Practices For API Testing

Apart from the steps mentioned above, there are also a few best practices that testers can follow while performing API testing.

  • Start testing first for the typical or expected results.
  • Add stress to the system through a series of API load tests.
  • Try to test for failure.
  • Group test cases by test categories.
  • Automate testing wherever possible.
  • Perform well-planned call sequencing.
  • Prioritize API function calls.
  • Limit the test variables to keep them isolated.
  • Create test cases for all possible API input combinations.
  • Incorporate mocking, stubbing, and driver components into API testing. This helps create controlled testing environments, isolating dependencies, and stimulating specific behaviors for external dependencies.

Dimpy Adhikary also talked about a few other good API testing practices during Axelerant’s QA Q&A show with Shweta Sharma.



How Axelerant Developed And Tested The Donation Platform For Scouts

The experts at Axelerant followed these best practices and more while developing and testing the donation platform and related APIs to support better funding for Scouts, which is a voluntary non-political education movement. Axelerant’s QA team started by creating a quick mapping sheet to relate APIs with respective functionalities. The key was to first identify test scenarios and test with precision.

The team used Postman, Newman, and GitLab CI to validate and automate API endpoints. Around 300+ test scripts were created for verifying the response data, status codes, and the JSON schema.

The platform was developed, tested, and launched within three weeks, allowing the Scouts team to improve funding for projects and manage user permissions without exposing the system.

Want to learn more about API testing best practices or what mistakes you should avoid while testing APIs?



About the Author
Simran Sethi, Senior Content Marketer
About the Author

Simran Sethi, Senior Content Marketer

Simran is an avid reader. She is an active runner, who also likes to swim, dance, and sing. For her, the meaning of life lies in the little things. Kindness, passion, and compassion are her life’s essential values.

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