API Testing 101: With Types, Benefits, Steps, Tools, and More.

Applications typically rely on using APIs that enable their user-facing side to communicate with backend servers. APIs are the digital workhorses that ferry data back and forth to apps. 88% of all global apps use APIs in some form. Regardless of what architecture or framework a software uses, it’s safe to assume that APIs are lifelines for any software. 

But have you ever wondered what makes these API data exchanges blazing fast, error-free, highly secure, and reliable enough? API testing does all this heavy lifting to ensure that software APIs perform as expected, safeguarding the overall app experience.

API testing is the process of evaluating the correct functioning of software APIs. Using API testing, developers can ensure that the APIs operate as intended, handle various inputs correctly, and software components communicate with each other without issues.

API testing aims to verify that the software APIs work well in terms of functionality, reliability, performance, and security. Moreover, it ensures that the APIs adhere to the original requirements and project specs. You can perform API testing using manual and automation testing methods.

Also Read: Getting Started with Software Testing

How API Testing Fits into SDLC?

In a typical SDLC, API testing is usually conducted towards the end of the development phase — as soon as the testing phase starts. However, with the increased complexity and testing requirements of present-day apps, this approach is slowly getting overshadowed by shift left testing, which focuses on running the QA and testing activities right from the product planning and design phases. This is especially true with projects using the Agile development methodology.

Shift left uses a proactive and iterative testing approach that involves QA teams right from the planning phase in the SDLC aiming to prevent and resolve defects as soon as they get introduced. This pushes API testing lifecycles to the early stages of SDLC.

Also Read: QA Best Practices for Agile Testing 

Here’s a list that shows the most common API testing types:

Unit Testing 

Unit testing verifies the correct functioning of the individual functions or methods in an API to ensure that each unit of code works well. It includes verifying a single endpoint responds appropriately based on the request. Moreover, it confirms the proper responses of parameterized endpoints too. 

API Documentation Testing 

Testing API docs ensures the correctness of the documentation for the API under test. An API doc is the go-to resource for developers, so it must be clear and up-to-date.

Functional Testing 

Functional testing is the process of checking whether or not the API performs its intended functions correctly. It typically includes:

• Contract testing —  verifies the correctness of content and formats API responses defined by the SLAs (service level agreements) between API producers and consumers.
• Component testing  — assesses individual components or modules of a system to ensure they function correctly in isolation.
• Scenario testing — evaluates the system's behavior in specific user scenarios or workflows. It tests the system's functionality from an end-user perspective or based on user journeys.

Also Read: What is an End User and How to Identify the Correct End User Type?

Integration Testing 

Integration testing ensures that different APIs in an app work well together. It also involves checking the correctness of data flow and exchange. 

Also Read: Unit Testing vs. Integration Testing

Regression Testing 

It verifies that recent changes or updates to the API do not break the existing app functionalities. 

Read More: What is Regression Testing? Explained with Steps, Examples, and More.

Security Testing 

Security testing confirms that the APIs have all the safeguards in place and that the API’s authentication and authorization workflows are correct. API penetration testing, rate limit testing, and input validation testing are prime examples of API security testing. 

End-to-end Testing 

End-to-end testing involves validating various application flows that use multiple APIs and endpoints. This typically includes testing specific user journeys in a fully integrated system to find API defects for prompt resolutions.

Read More: What is End-to-End Testing? Explained

Load Testing 

Load testing is the process of evaluating the API performance under high traffic and variable loads. It helps identify performance bottlenecks, error rates, and response times by simulating large volumes of user requests.

APIs are the linking chains that enable an app’s components to communicate and are crucial for ensuring a fully-fledged and top-notch system. Here are some reasons that make API testing worth your time, efforts, and investment:

Ensures a Quality Assured Product 

Well-tested APIs ensure that users have a positive experience with the software. When an API is consistent, high-performing, secure, and reliable, it promises a quality-assured product and happier end users.

Also Read: Quality Assurance vs. Quality Control

Helps Prevent Defects From Early On and Saves Dollars 

API testing with the shift left testing approach, rivets quality into the product at all stages. This helps identify and fix API issues as soon as they are introduced leading to significant cost savings associated with bug fixing and reworks at later stages.

Also Read: What is Bug Lifecycle and How to Run it?

Works Well With the Agile Development Practices 

API testing aligns perfectly with the iterative approach for Agile development methods. It’s even more beneficial when a product follows an "API-first" strategy, which focuses on preventing defects from creeping in by including API lifecycles since the product planning phases itself.

Faster Development Cycles With Continually Improved Product 

Automation tools integrated with CI/CD pipelines enable developers to rapidly release the latest product improvements and features in an iterative manner. API testing can ensure that these changes are implemented correctly, preventing regressions and bugs within the API.\

Also Read: Top 10 Automation Testing Tools: What Works Best For You?

Here’s a step-by-step guide to help you run API testing from scratch:

Step 1. Understanding API Specifications

Before you start, it's best to understand API protocols and specifications, which typically include request/response formats, authentication methods, and error codes. For instance, APIs typically follow specs using the:

• REST (Representational State Transfer) — uses standard specs for handling HTTP methods (GET, POST, PUT, DELETE) and is stateless.
• SOAP (Simple Object Access Protocol) —  uses XML for message formatting and relies on other application layer protocols for message negotiation and transmission.

Step 2. Designing Test Cases

Once you’re familiar with the API specs, you can start designing test cases. For example, you can consider design:

• Positive test cases — to verify the API behaves as expected with valid inputs.
• Negative test cases — to test how the API handles invalid inputs or requests.
• Edge test cases — to assess the API's performance under extreme conditions or unusual inputs.
• Security test cases —  to check for vulnerabilities, such as SQL injection or improper authentication.

Read More: How to Write Test Cases? Step-by-Step Guide with Examples

Step 3. Setting Up Test Environments

Prepare your testing environment to mimic production as closely as possible. This includes configuring the:

• Development environment — is where the API is built and where initial testing activities occur.
• Staging environment —  is a replica of the production environment for more extensive testing.
• Test data — to create or use existing datasets that reflect real-world scenarios.

Step 4. Executing Tests and Collecting Results

Use tools or scripts to execute your test cases. Some popular tools for API testing include Postman, JMeter, REST Assured, and SoapUI (all covered in the next section). Collect results systematically, noting the status of each test case (pass/fail) and any discrepancies.

Also Read: Types of Software Testing Tools: What Works Best For You?

Step 5. Validating Responses and Handling Errors

After executing tests, validate the API responses against the expected results. Some key aspects may include checking the:

• Status codes —  to ensure the correct HTTP status codes are returned (e.g., 200 for success, 404 for not found).
• Response body —  to verify that the data returned matches the expected format and content.
• Error and exception handling —  to check how the API responds to invalid requests and whether error messages are informative and human-readable.

Among many, here are the most prominent API testing tools:

Postman

Postman is a pretty popular tool for API development and testing. It is widely used for both manual and automation API testing activities. Originally developed as an extension for Chrome, it is available as an app for popular operating systems (MacOS, Windows, and Linux). Postman can also be used as an extension for the Microsoft VS Code text editor or directly from the browser.

Top Features

• Easy setup and intuitive user interface
• Supports popular API protocols, such as REST, SOAP, GraphQL, RAML, and WSDL.
• Automation tests can be written in JavaScript, with built-in support for assertions to validate API responses
• Support for CI/CD pipelines and third-party service integrations.
• Auto-generation of Docs capabilities with OpenAI 3.0 definition support.
• Support for creating and running mock servers.
• Can be configurated easily for multiple test environments.
• Includes in-built monitoring and reporting features.
• Collection Runner for executing a series of requests in a specific order.
• Postman visualizer for creating request-response visualizations. 
• Pricing: Free tier available with three-paid tiers (for advanced features)

Apache JMeter

Apache JMeter is an open-sourced Java tool typically used for load and performance testing with solid API testing capabilities.

Top Features

• Available as a Java-based app (jar format).
• Easy to set up with an intuitive user interface. 
• Known for load and performance testing of web apps (both static and dynamic resources).
• Multi-language support: Java, PHP, Node.js, ASP.Net, and others
• Protocol support for HTTP, HTTPS, SOAP, REST, FTP, LDAP, TCP, and others. 
• Multi-threading support.
• Caching and replay features available for test result analysis.
• Scriptable Samplers available for languages like BeanShell and Groovy. 
• Numerous plugins are available to extend JMeter’s testing capabilities.
• Can be integrated with Jenkins, Gradle, and Maven with CI pipelines for running API tests.
• Command-Line Interface (CLI) for integration with CI/CD pipelines and automated testing.
• Huge community support.
• Pricing: Free.

Swagger

Swagger is an open-source tool that offers a suite of tools for API design, documentation, and testing to create and maintain RESTful APIs.

Top Features

• Widely used for API documentation capabilities.
• Interactive user interface available (using Swagger UI) for visualizing and interacting with API endpoints directly.
• Automation testing can be done with the Ready API.
• Swagger Editor available for designing and writing API with OpenAI specs.
• Uses OpenAPI Specification (OAS) to help define APIs in a standardized format.
• Swagger Codegen for generating server stubs and client SDKs with OpenAPI Spec definitions.
• Includes features for API governance and API monitoring.
• Supports API versioning.
• Offers role-based access controls and collaboration features with SwaggerHub.
• Supports mock server creation capabilities for running simulated tests.
• Huge user base and strong community support available.
• Strong documentation and excellent resources are available for using Swagger.
• Pricing: Free to use. Paid tools are available for pro-features and enterprise users.

REST Assured  

REST Assured is a widely recognized open-source tool designed for testing RESTful APIs using Java for API testing with an intuitive interface. With its focus on REST APIs, it is particularly valuable for teams using Behavior-Driven Development (BDD) methodology. 

Top Features

• Offers easy-to-read Gherkin syntax that simplifies the writing of tests.
• Supports all standard HTTP methods, such as GET, POST, PUT, DELETE, PATCH, OPTIONS, and HEAD requests.
• Enables the configuration of common request settings for increased reusability across tests.
• Allows validation of various response elements, including status codes, headers, and payloads.
• Built-in support for parsing and validating JSON and XML responses.
• Works well with popular Java frameworks like JUnit and TestNG.
• Open-source and backed by a huge community support.
• Pricing: Free.

Karate Labs  

Karate Labs is yet another powerful open-source tool that combines API testing and automation with a focus on BDD.

Top Features

• Easy to learn and get started with.
• Supports popular API formats, such as JSON, XML, YML, SOAP, and REST.
• Uses Gherkin syntax for writing tests, making it accessible for non-technical team members.
• Enables testing of both APIs and web apps within the same framework.
• Includes features for API testing, API performance testing, and API mocking (open-sourced)
• Integrations available for popular tools, such as GraphQL, Playwright, Selenium WebDriver, Gradle, and Jenkins.
• Includes features for Async API testing, support for Swagger, and OpenAI specs (closed-sourced)
• Supports parallel test execution.
• Support for API mocking and UI testing.
• Support for DevOps tools integration for API testing.
• Logging and reporting features available.
• Pricing: Open source version available for free. Three paid tiers for advanced features.

Apigee API Management 

Owned by Google, Apigee is a comprehensive API management platform that offers tools for API design, security, analytics, and monitoring. It is particularly suited for companies looking to manage and optimize their API development and testing ecosystems.

Top Features

• Supported architectures: REST, SOAP, gRPC, and GraphQL.
• Provides detailed analytics and reporting on API usage and performance.
• Offers several security features for SSL encryption, OAuth 2.0, API key validation, and others.
• Includes features for AI/ML.
• Features for PCI, SOC2, and HIPAA compliance.
• Includes a customizable developer portal for API documentation and developer engagement.
• Features tools for traffic management, including caching and rate limiting.
• Easily integrates with various cloud services.
• Scalability features are available with multi-cloud deployment support. 
• Pricing: Paid plans with a 60-day free trial available.

SoapUI  

SoapUI is a widely used open-source tool for testing SOAP and REST APIs known for its features that support functional testing, performance testing, and security testing of APIs.

Top Features

• Supports both SOAP and REST API testing with extensive capabilities.
• Graphical user interface with drag-and-drop features.
• Allows creation for multiple test environments.
• Allows for advanced scripting and debugging.
• Includes built-in security testing capabilities to identify vulnerabilities in APIs.
• Supports load and performance testing.
• Integrates with CI/CD pipelines.
• Pricing: Free version available. Paid plans are also available (with Ready API).

Playwright  

Playwright is an open-source automation library developed by Microsoft for testing web apps across different browsers. 

Top Features

• Supports testing across multiple browsers including Chrome, Firefox, and Safari.
• Provides a rich API for interacting with web elements and controlling browser behavior.
• Multi-language support: JavaScript, Java, Python, and others.
• API mocking feature is available.
• Can be integrated with CI/CD workflows for continuous testing.
• Clear documentation is available.
• Pricing: Free to use.

Cypress  

Cypress is a modern JavaScript testing framework primarily used for end-to-end testing of web apps that also boasts features for running API testing and integration testing.

Top Features

• Easy setup with a user-friendly interface.
• Uses Node.js
• Provides real-browser environments for testing apps.
• API testing support is available.
• Can be used for component testing (functional testing) and end-to-end-testing 
• Time travel feature that allows users to see snapshots of the application at each step of the test execution.
• Built-in Waits feature that allows for automatic waits for elements to appear, reducing flakiness in tests.
• Network traffic control capabilities to stub and control network requests for testing.
• Can be integrated with CI/CD pipelines for automated testing.
• Mocking feature available
• Good community support and widely adopted by the JavaScript community.
• Extensive and easy-to-use docs.
• Pricing: Free tier available with paid plans for advanced features.

Katalon Studio  

Katalon Studio is an automation testing solution that supports API, web, mobile, and desktop app testing. It’s a feature-rich tool and a good choice for both beginners and experienced testers.

Top Features

• Provides an intuitive and modern interface.
• Supports REST, SOAP, GraphQL.
• Includes features to import tests from WSDL, Postman, and Swagger.
• Dedicated feature for schema validation and parameterization
• Support for data injection from CSV, databases, and Excel files.
• Built-in keywords for common testing actions, reducing the need for coding.
• Support for Behavior Driven Development (BDD).
• CI/CD and DevOps pipeline integrations are available.
• Provides detailed reporting and analytics features for test results.
• Provides hybrid testing strategy for end-to-end and API testing.
• Pricing: Free version available. Paid plans (for advanced features).

Also Read: Top 24 Chrome Extensions for Software Testers

Here are some common API testing questions that you may come across during interviews: 

1 - What is an API?

An API, or Application Programming Interface allows different software components to talk to each other. APIs define a set of rules and protocols for how these applications can request and exchange information. When you use a web service or a mobile app, you are often interacting with an API without even realizing it. Using APIs, developers can access specific features or data from servers without needing to understand how everything works behind the scenes.

2 - What are the common types of API protocols?

Common API protocols include:

• REST — stands for Representational State Transfer, is widely known for its simplicity, and uses standard HTTP methods like GET, POST, PUT, and DELETE.
• SOAP — or Simple Object Access Protocol, is a bit more complex and relies on XML for messaging, often used in enterprise settings.
• GraphQL — API architecture allows you to fetch data from different resources using a single query and present data in a graph format. 
• gRPC — is a modern API framework that uses Protocol Buffers for data serialization, making it ideal for high-performance apps, especially for ones using the microservice architecture.

3 - What is the difference between REST and SOAP APIs?

4 - What are the common status codes you encounter in API testing?

Common status codes you would typically encounter while running API tests include:

• Status Code 200 —  Successful request.
• Status Code 201 —  New resource created. 
• Status Code 400 — Bad request from the client.
• Status Code 401 — Authentication is required. 
• Status Code 404 — Resource not found.
• Status Code 403 — Resource access is forbidden.
• Status Code 500 — Points to a server error.

5 - Why is documentation important while writing APIs?

Documentation serves as an important resource for people using the API. Good documentation explains how to access the API, what endpoints are available, and the data formats needed for requests and responses. Moreover, it helps understand the API functions and limitations, which reduces the chances of errors or misuse.

6 - What is API versioning?

API versioning is the practice of managing changes to an API over time while ensuring that existing front-end clients that rely on the older version can still function properly. You can implement versioning in various ways, such as through the URL (like /v1/resource) or by using request headers. This way, clients using the previous API version can continue using it without breaking the app until it gets replaced by the new API version.

7 - What are the most common bugs you would find while API testing?

While working with the APIs, here are the bugs you may typically come across:

• Incorrect status codes
• Unclear error messages while handling errors
• Data validation issues where the API accepts invalid input or fails to handle edge cases properly 
• Authentication bugs if the API does not enforce security measures correctly 
• Performance issues like slow response times or request timeouts
• Incorrect schema

8 - What are test environments in API testing? 

Test environments in API testing are setups used to evaluate how well an API functions before it goes live. These environments mimic production settings but are isolated to prevent any impact on live systems. Typically, you use different environments for development, testing, and staging, ensuring that changes are validated at each stage.

9 - What are the most common HTTP requests in REST APIs?

10 - What is rate limiting?

Rate limiting is a technique used to control how many requests a client can make to an API within a specific time frame. When a client exceeds the allowed number of requests, the API responds with an error code, like 429 Too Many Requests. This helps prevent abuse, ensures fair usage, and protects the server from being overwhelmed by too many requests. 

Here’s a list of best practices to ensure effective API testing:

Run Tests At All Stages of the API Lifecycle

Run API tests throughout the API lifecycle to ensure a high-quality end product. API lifecycle starts from the API planning and design phase and goes all the way to deploying it to products for end users to consume. 

Plan Your Tests Well 

Define clear objectives and strategies for testing your API to ensure test comprehensive coverage. This includes identifying which endpoints to test, determining expected outcomes, and establishing performance metrics to measure API’s effectiveness in a production setup.

Keep Your API Tests Organized

Maintain separate test suites for different API tests to streamline your testing efforts. It allows you to avoid running obsolete tests or those already executed. Additionally, it makes it easy to identify which tests correspond to specific APIs, aiding newcomers to the team or those transitioning from different projects.

Create And Maintain Dedicated Test Environments 

Run tests in dedicated test environments that closely mirror the production settings before making the changes live. This helps avoid system downtimes, contain issues and fix them rapidly, and ensure consistent API behavior.

Ensure Maximum Test Coverage  

Testing all endpoints (with unit testing), key user journeys (with end-to-end testing), and other prominent API testing methods ensures maximum test coverage. Include tests for edge cases and negative scenarios to validate for a wide range of inputs.

Use Automation Tools Wherever Possible

API automation tools allow you to run repetitive and long-running tests that require little to no human supervision. This leads to faster and more efficient testing cycles and helps cover scenarios best suited for the human mind and expertise. Automation tools can easily be integrated with CI/CD pipelines which makes them even more desirable to push frequent API updates and new version releases.

Use Mock Servers 

With mock servers, you can simulate various API responses, including error scenarios, without relying on external systems or services. This approach is particularly useful while APIs are still in development, allowing teams to continue the API testing process. Additionally, using mock servers makes it easier to obtain controlled outputs and test APIs under extreme conditions without the risk of breaking the production API.  

Maintain Clear Documentation 

Proper documentation of test cases, results, and procedures to ensure that team members and stakeholders easily understand testing efforts. Moreover, it simplifies the process of onboarding new members and enables faster KT sessions.

Write Tests for Errors And Exceptions 

Writing and running error handling tests enables the APIs to handle adverse or unexpected situations, such as invalid inputs, network failures, or server errors, without crashing or returning misleading responses.

Create Reusable Subtests 

Modular subtests can easily be tweaked for common scenarios. Rather than spending your valuable resources on writing repetitive test logic, you can create subtests that can be reused across diverse test suites for running API tests.

Incomplete Documentation

Incomplete or poorly written API docs can be a roadblock while using the API docs for running tests. It’s best to invest in thorough documentation practices. Use tools that can auto-generate docs from code and maintain up-to-date changelogs to reflect any updates or changes in the API.

Handling Diverse API Protocols 

APIs can use diverse protocols like REST, SOAP, and GraphQL, each with unique requirements and behaviors. To address this challenge, standardize testing practices across different protocols. Utilize tools that support multiple API types to ensure consistency in testing methodologies.

Changing Schemas and API Versioning

APIs often undergo numerous revisions. This can lead to schemas and API version changes that can break existing implementations. Use version control and backward compatibility strategies to handle such scenarios.

Also Read: GitHub vs. GitLab: What Works Best For You? 

Testing Complex Parameters

APIs can use diverse (and complex) parameters to control usage and behavior. This can pose a challenge while running tests. Write test cases that handle various parameter combinations. Use automated testing tools to streamline the process and ensure thorough coverage.

Managing Data Consistency

Consistent and correct data flow across different environments and test cases is crucial for reliable API testing. You can achieve this by mapping data flows, using realistic test data, and implementing automated checks to ensure data integrity throughout the testing process.

Choosing The Right Tools

With a wide variety of options, choosing the right tools for API testing can be daunting at times. Before fixating on a specific tool, assess your needs, such as protocol support and integration capabilities. Consider using solutions that align with your needs and provide comprehensive testing features to simplify the process.

Negative Test Scenarios and Edge Cases

Testing negative scenarios and edge cases can be a pain to simulate but are equally important. Design test cases that cover unexpected inputs and error conditions to ensure they aren’t overlooked while considering positive test cases.

Gaps in Technical Knowledge

Gaps in technical knowledge and expertise can also hinder effective API testing. Invest in training and development programs to enhance skills and knowledge among team members.

Also Read: Professional Courses and Certifications for QA Testers & Developers

Multiple Dependencies

APIs often rely on other services and components with complex dependencies. Use mocking and virtualization techniques to simulate dependent services. This allows for isolated testing of the API without interference from external systems.

Underdeveloped APIs

Testing underdeveloped APIs is another common hurdle. It's best to adopt an iterative testing approach that focuses on testing available endpoints while providing feedback to developers to ensure that the API development progresses in sync with the QA team members in the loop.