Automated API Testing: Complete Guide 2023

September 27, 2023

Automated API testing ensures your software's reliability faster and more efficiently.

Automated API testing ensures your software's reliability faster and more efficiently.

Application Programming Interfaces, aka APIs, are the "glue" that allows different software components, such as operating systems and applications or web apps and external services on the Internet, to communicate and function seamlessly. In today's hyperconnected, tech-hungry world, technology keeps changing and we depend more and more on systems connected via APIs. Automated API Testing is a key part of the software development process - one that ensures APIs' reliability, functionality, and performance. 

What is Automated API Testing? 

Automated API testing is a type of software testing that uses special software tools to send calls to APIs, receive responses, and then evaluate the response against pre-defined criteria. The purpose is to ensure that the APIs meet their requirements and function as expected. Automated API tests can be run more frequently, allowing for faster development cycles and greater accuracy than manual testing. This approach ensures that APIs, which enable communication between different software components, function correctly, reliably, and efficiently. 

Automated API tests also help to reduce the time it takes to test APIs by eliminating the need for manual steps such as setting up the environment or writing code. This makes them ideal for Agile development where quick feedback is important.

Additionally, automated API tests provide a way to track and monitor changes to APIs over time. It is important to have accurate records of how an API has changed since its last release in order to ensure that any new features or bug fixes are working properly. Automated tests can detect errors quickly, making them a valuable tool for identifying bugs before they reach production. Development teams can test early and often, and reduce the risk of introducing bugs into their applications.

How Does API Test Automation Help QA Teams?

API test automation offers numerous benefits to Quality Assurance (QA) teams that contribute to more effective and efficient software testing and development processes.

Let’s look at a quick overview of how API test automation helps QA teams: 

  • Enhances efficiency by accelerating testing processes 
  • Ensures repeatability in test execution 
  • Provides comprehensive test coverage 
  • Facilitates regression testing 
  • Detects issues early in development 
  • Offers rapid feedback to developers 
  • Integrates seamlessly with CI/CD pipelines
  • Optimizes resource allocation 
  • Aids in documentation of expected API behavior 
  • Streamlines test data management 

Overall, automated API tests can save time and money while ensuring that APIs are reliable and secure. They can be used to quickly detect any changes or errors in an API, allowing development teams to quickly address any issues before they have a negative impact on the application. Automated testing also provides a way to track changes over time, providing valuable insights into how an application is performing. 

Automated tests can be used to verify the performance and scalability of an API, ensuring that it will meet the needs of its users. Automated testing offers a way to measure the effectiveness of changes made to an API, allowing development teams to ensure that they are consistently delivering on their promise of quality and reliability.

Key Levers of Automated API Testing 

Although automated API testing offers many advantages, there are two that play a particularly crucial role in keeping software development processes robust and efficient.

Immediate issue detection 

Automated API tests can detect any issues with an application's functionality as soon as they occur. This helps developers identify and address problems quickly, preventing them from becoming more widespread and costly to fix. This approach ensures that potential problems are addressed at the earliest stages of development, contributing to enhanced efficiency and reliability.

For example, consider an e-commerce platform that relies heavily on API interactions for order processing. Through automated API testing, developers can simulate various scenarios, such as high traffic volumes or complex order combinations, to detect any inconsistencies in the API responses. This enables them to promptly identify and rectify any issues, ensuring a seamless shopping experience for customers.

Saving time and resources

Automated API testing streamlines QA efforts by automating repetitive tasks, enabling faster test execution, and freeing up QA teams for strategic testing while reducing retesting needs. With automated API testing, QA teams can quickly and accurately identify any bugs that might be present in the system while ensuring a high level of quality. 

Automation also eliminates the need to manually set up and configure test environments, making it possible for developers to focus on more important tasks rather than manual testing.

Best Practices for API Automated Testing 

Let’s look at some best practices so that you and your teams can make the most of your automated API testing programs:

Don't automate complex tests

It is essential not to automate complex tests as they can lead to overly intricate test scripts that are difficult to maintain. What’s more - they may not provide precise, actionable results, which is simply counterproductive. 

Focus on automating simpler, high-priority tests, allowing for more efficient execution and easier troubleshooting.This is especially important when considering regression tests - ensuring that the most critical functionality of your API is covered and quickly executable in an automated fashion should be a top priority.  

Keep test scripts independent and maintainable

When writing automated API tests, it is important to keep the scripts independent from each other. This means that each script should be able to run on its own without relying on the success of any other test case. 

Moreover, maintaining test scripts is key to keeping them functional in the long term. It's recommended to avoid hardcoding values and use variables instead, as well as regularly reviewing and updating test scripts to reflect any changes made to the API. This helps your automated tests remain reliable and accurate over time.

Run parallel tests

Running tests in parallel is a recommended practice that speeds up test execution by distributing the workload across multiple test environments or devices. This not only saves time but also helps identify concurrency-related issues in APIs. Parallel testing is especially useful for large test suites or when aiming for faster feedback in a continuous integration environment.

Use an API testing tool that is compatible with your workflow

Choose an API testing tool that seamlessly integrates with your development workflow and technology stack. Compatibility leads to a smoother adoption of automation. Alignment with your team's existing practices supports efficient collaboration. It also reduces the learning curve and improves the tool's effectiveness in meeting your testing needs.

What are the challenges associated with Automated API Testing? 

While the benefits are clear, it's important to also anticipate the challenges that can occur and consider mitigating measures. In this section, we take a look at some of the main challenges you might expect from automated API testing: 

Speed constraints

Speed limitations in automated API testing can arise due to responsiveness constraints in the API or limitations in the testing tool. This can result in slower test execution and delays in development, which may be especially problematic in fast-paced agile environments where quick feedback and iteration are crucial.

To mitigate these constraints, it's important to perform load testing and identify any bottlenecks or performance issues early on. Utilizing tools specifically designed for API testing can also help improve speed and efficiency by automating repetitive tasks and providing comprehensive reporting.

Difficult maintenance 

Maintaining automated API tests can become challenging as the API and application evolve. Keeping test scripts up-to-date with changing requirements and ensuring their reliability over time requires ongoing effort and careful documentation. Failure to do so can result in outdated and unreliable tests, leading to false results and impacting the overall quality of the API.

To address this challenge, it's essential to establish a proper version control system for test scripts and regularly review and refactor them as needed. This could also involve engaging developers in the testing process to ensure that changes are reflected accurately in the tests.

Complex scalability 

Scaling automated API tests to accommodate a growing number of APIs, endpoints, or test cases can be complex, often requiring additional resources and infrastructure to maintain efficient testing processes as the software ecosystem expands. Not addressing scalability can lead to bottlenecks and delays in test execution, impacting the development timeline.

One approach to addressing this issue is through parallel testing, where multiple tests are run simultaneously on different API endpoints or versions. This helps distribute the workload and reduces overall execution time. Additionally, regularly reviewing and optimizing test cases can also help improve scalability by eliminating redundant or unnecessary tests.

How to Automate API Testing

Now that we’ve explored what the key benefits, challenges and best practices are, let’s go through the steps you need to follow to automate API testing.

Determine which API tests to automate

Identify the specific API test cases that are suitable for automation based on factors like frequency of execution, criticality, and repeatability. These tests should also be stable, meaning that they consistently produce accurate results. Prioritizing the right tests for automation will help you maximize your resources and reduce execution time.

Set up an API test automation environment

Create a dedicated testing environment with the necessary infrastructure, test data, and configurations to support automated API testing. This should include a version control system to manage your test scripts and track changes, as well as testing tools that can integrate with your APIs.

Select a tool for automated API testing 

Choose an appropriate testing tool or framework that aligns with your project's requirements and integrates well with your technology stack. Popular choices include Postman, SoapUI, Karate DSL and many others. Consider factors such as integration capabilities, support for different protocols, ease of use, and reporting capabilities when selecting a tool. Evaluate each tool based on your project requirements, budget and team expertise.

Define your API testing approach 

Define a clear approach for your API testing, including the types of tests to conduct (e.g., functional, integration, performance) and the testing methodologies to follow (e.g., black-box, white-box, or gray-box testing). This will help guide your team in creating effective and efficient test cases and ensure consistency across all tests.

Types of Output from an API

APIs generate diverse types of output depending on their functionalities. It is essential to verify the correct output format for your specific API and handle it appropriately in your tests.

Standard output formats include: 

  • JSON (JavaScript Object Notation), favored for its human-readability and machine parsing capabilities 
  • XML (Extensible Markup Language), known for its hierarchical structure 
  • HTML (Hypertext Markup Language), used for web content delivery 
  • Plain text for more straightforward data 
  • Binary data streams for files 
  • CSV (Comma-Separated Values) for structured data 
  • Custom formats for specialized use cases 
  • Status codes like HTTP status codes for indicating request outcomes 

APIs can also provide media files such as images, audio, or video as part of their output.

Test Cases for API Testing

Let’s get you set up with a few sample test cases for automated API testing. These test cases cover scenarios commonly encountered in API testing: verifying user authentication and validating the accuracy of data retrieval from an API. 

Depending on your specific API and application requirements, you can create additional test cases to cover various functional, edge, and error cases.

Test Case 1: Verify User Authentication API

Objective: To ensure the user authentication API functions correctly by allowing valid users to log in and rejecting invalid login attempts.

Test Scenario: Verify Successful User Login

  • Preconditions: A valid user account exists with known credentials (username and password).
  • Steps:
  • 1.1 Send a POST request to the user authentication API with the valid username and password.
  • 1.2 Receive the API response.
  • Expected Result: The API response should contain a status code 200 (OK) and a JSON or XML response indicating successful login, along with an authentication token.

Test Scenario: Verify Unauthorized User Login Attempt

  • Preconditions: A user account exists with invalid credentials (e.g., incorrect password).
  • Steps:
  • 1.1 Send a POST request to the user authentication API with the invalid username and password.
  • 1.2 Receive the API response.
  • Expected Result: The API response should contain a status code 401 (Unauthorized) and a message indicating that the login attempt is unauthorized, without providing access to the system.

Test Case 2: Validate Product Information API

Objective: To verify that the product information API provides accurate and complete details of a product when queried.

Test Scenario: Retrieve Product Information

  • Preconditions: The product database contains a known product with specific details (e.g., name, price, description).
  • Steps:
  • 1.1 Send a GET request to the product information API for the known product.
  • 1.2 Receive the API response.
  • Expected Result: The API response should contain a status code 200 (OK) and a JSON or XML response with the product's correct information, including name, price, and description.

Test Scenario: Product Not Found

  • Preconditions: No product with the specified identifier exists in the database.
  • Steps:
  • 1.1 Send a GET request to the product information API with an identifier for a non-existent product.
  • 1.2 Receive the API response.
  • Expected Result: The API response should contain a status code 404 (Not Found) and a message indicating that the requested product does not exist in the database.

FAQ about Automated API Testing 

What is API test automation?

API test automation is the practice of using specialized tools and scripts to automatically test APIs. It allows for rapid and consistent verification of API behavior, to automatically test the functionality, reliability, and performance of APIs.

How does automated API testing help businesses?

Automated API testing helps businesses by improving software quality, lowering testing costs, speeding up development cycles, and driving customer satisfaction with reliable, high-performing software products. Automated API testing can also uncover security vulnerabilities in APIs that may go undetected otherwise. 

Ultimately, automated API testing helps businesses create better software products faster and with fewer risks.

What’s the difference between API testing and Unit testing? 

API Testing focuses on testing the interfaces between different software components or services, ensuring they communicate and function correctly. It verifies if the APIs work as intended, i.e. whether they are accurately handling requests and returning responses.

On the other hand, Unit Testing is a type of testing at the code level, where QAs test individual components or functions of a software application in isolation. It checks the correctness of specific code units, often without considering their interaction with external components. 

While API Testing tests the interactions between components, Unit Testing verifies the internal logic of individual units. Both approaches are essential to guarantee quality software products and should be combined for an effective QA process. 

What types of bugs does API testing detect?

API testing can detect a wide range of bugs, including problems with authentication, incorrect responses or data formats, and incorrect parameters. It can also be used to verify the performance of the API in terms of response time and data throughput. Furthermore, if there are any security vulnerabilities associated with the API requests, they will be revealed during testing. 

Here are a few notable examples of bugs that can be detected using API testing:

  • Fails to handle error conditions gracefully
  • Unused flags
  • Missing or duplicate functionality
  • Reliability issues (difficulty in connecting and getting a response from API)
  • Security issues
  • Multi-threading issues
  • Performance issues (e.g. API response time is very high)
  • Improper errors/warnings to a caller
  • Incorrect handling of valid argument values
  • Response data is not structured correctly (JSON or XML)

About the author

author photo: Tamas Cser

Tamas Cser


Tamas Cser is the founder, CTO, and Chief Evangelist at Functionize, the leading provider of AI-powered test automation. With over 15 years in the software industry, he launched Functionize after experiencing the painstaking bottlenecks with software testing at his previous consulting company. Tamas is a former child violin prodigy turned AI-powered software testing guru. He grew up under a communist regime in Hungary, and after studying the violin at the University for Music and Performing Arts in Vienna, toured the world playing violin. He was bitten by the tech bug and decided to shift his talents to coding, eventually starting a consulting company before Functionize. Tamas and his family live in the San Francisco Bay Area.

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