APIs (Application Programming Interfaces) are the glue that holds applications and services together. With the growth of distributed, multi-tier architectures, APIs are a crucial piece of modern applications, usually encompassing a good chunk of an application's logic and business processes. One of the most popular architectural styles for building APIs is REST (Representational State Transfer), known for its simplicity, scalability, and stateless nature. RESTful APIs facilitate communication with internal applications, support data integration workflows, and play significant roles in app development and data pipelines.
However, with the increased complexity and dependency on REST APIs, ensuring their quality, security, and performance is now critical to the software development lifecycle. This is where REST API testing comes into play. Testing your APIs is not just a best practice; it is essential for delivering reliable, secure, high-performing APIs that meet user and business expectations. Comprehensive testing throughout the entire API lifecycle—from design to deployment—ensures that APIs function as intended as they are designed, built, and deployed.
In this article, we’ll dive into REST API testing from various angles, looking at the fundamentals, best practices, and tools that can be used in your testing efforts. Let’s begin by digging into some high-level topics revolving around API testing.
What is API Testing?
API testing focuses on validating the functionality, reliability, performance, and security of application programming interfaces (APIs). Unlike UI testing, which focuses on the visual aspects of an application and related flows, API testing targets the logic and data layers of an application. It involves sending requests to API endpoints and analyzing the responses to ensure they meet the expected criteria in terms of usability, performance, and reliability.
API testing should be a regular part of building APIs, preferably testing at various stages of API development. API testing is vital for:
Core Functionality: APIs are the building blocks of applications. Thorough testing ensures that each API endpoint does its job in terms of required functionality.
Data Accuracy: APIs read and write data. API testing verifies that the data written to a database or returned from one is accurate, complete, and formatted correctly.
Error Handling: Errors can occur unexpectedly. API testing checks how the API handles errors and how they are presented to users.
Security: APIs can be vulnerable to attacks. Testing helps identify security flaws, such as unauthorized access, data leaks, and injection vulnerabilities.
Performance: APIs must be responsive under load and at scale. Performance testing measures how well the API performs under different levels of traffic.
Benefits of REST API Testing
By testing at various stages, API testing brings many benefits to developers and their organizations. A few of the most prominent benefits include:
Early Bug Detection: Catching and fixing bugs early in development is much cheaper, more efficient, and less risky than finding and fixing them in production. API testing helps catch issues before they make their way onto production servers and impact end users.
Better Software Quality: Thorough testing ensures the overall quality and reliability of your APIs for a better user experience.
Better Security: By finding vulnerabilities during testing, you can take proactive measures to secure your APIs to ensure that critical security issues are remedies before they can be exploited in production environments.
Faster Development: Testing APIs early and often allows for faster development cycles. By automating parts of API testing, certain tests can be run in a matter of seconds or minutes, allowing developers to identify and remedy bugs and gaps in functionality early on in the SDLC.
To fully realize these benefits, different types of API testing need to be performed as APIs are developed and published. Let’s take a look at those particulars in the next section.
Types of API Tests
REST API testing involves various types of tests, each serving different purposes. Each testing type helps ensure the overall quality and functionality of your APIs from different angles. Let’s look into some of the most common and essential types of API testing.
Unit Testing
At the most basic level, unit testing involves testing individual components or operations of an API. It ensures that each isolated part of the API works as expected and validates the logic and expected behavior. Developers are usually familiar with this type of testing and the frameworks that can be used to implement it. These tests are usually written by developers and aid in ensuring that the code itself is adhering to the functional needs of the application.
Integration Testing
As the name suggests, integration testing checks how different API components or modules work together and with other parts of the system. Integration tests help to verify the interactions between various endpoints and check for data flow, error handling, and overall system integration. This testing is still likely performed by developers and might include automated tests and manual test cases to ensure that systems can be integrated as required.
Functional Testing
Functional testing involves verifying that the REST API behaves according to its functional requirements. This involves testing the API with different inputs, ensuring it returns the correct output, handles edge cases, and follows the business logic. This may sometimes overlap with the business acceptance testing phase of a project where the group that the application is being built for does preliminary testing to ensure that it fits their needs.
Load Testing
Load testing tests the REST API under expected and peak traffic. Testing REST API performance is vital to ensure stability and responsiveness under different load scenarios. It helps to determine the API’s capacity, response time, and stability under a high volume of requests. For this type of testing, developers and testers usually use an automated approach to produce the API requests to push load onto the endpoint and the server. Quite a few tools exist, some simple ones and more advanced ones, which can help to make load testing easier.
Security Testing
Security testing is essential to protecting REST APIs from vulnerabilities and malicious attacks. This involves testing for common security risks, such as injection attacks, cross-site scripting (XSS), authentication and authorization issues, and data exposure. Luckily, OWASP publishes a list of the top 10 API vulnerabilities that many security testing tools use as a guide to help developers automate detecting the most pressing vulnerabilities. Security testing can involve automated testing, such as using a Dynamic Application Security Testing (DAST) tool, or manual penetration testing performed by API security testers.
Each type of API testing is essential in the overall testing strategy for REST APIs. By combining these tests, organizations can be confident in their API’s functionality, performance, reliability, and security. Testing does come with its share of challenges, though. In the next section, we’ll examine some of the difficulties in testing REST APIs and how to overcome them.
Challenges in Testing REST APIs
While REST APIs are at the core of most modern architectures and have many benefits, testing them can be challenging. Let’s look at some of these challenges in more detail below.
API Parameter Combinations
REST APIs can use multiple parameters within an API call, including query parameters, request body fields, and request URIs. Combining these parameters creates numerous possible scenarios that need testing, and testing all combinations efficiently can be challenging. With the advent of AI and testing tools, this can be easier, but it still represents a challenge to test every combination for more complex APIs.
Validating API Parameters
Another challenge is validating the parameters passed to REST APIs. Incorrect data types, invalid values, or unexpected parameter combinations can cause errors or unpredictable behavior. Validating REST API parameters is essential to addressing these issues. However, it is challenging to validate parameters and ensure any incorrect parameters are handled correctly. Testing as many variants as possible to test all logic paths and parameter validation is important to prevent vulnerabilities and make the API robust.
Maintaining the Data Format Schema
REST APIs use data format schemas like JSON or XML, the most popular of which is documenting the APIs through an OpenAPI spec. However, as the API evolves, maintaining the schema’s consistency and accuracy is challenging. When the schema changes, developers and testers may be required to update test cases and validation mechanisms based on the latest spec. This can introduce errors and gaps in testing if not managed carefully.
Testing API Call Sequences
In many cases, REST API calls need to be executed in a specific sequence to achieve the desired outcome. For APIs that require calls to be chained together to test a specific logic flow, testing these call sequences manually can be challenging and error-prone. Testing REST API call sequences is important to detect any errors, logic flaws, or security issues, especially in multithreaded operations. Automating the testing of complex call sequences is important to ensure functionality requirements are met and prevent regressions.
API Testing Setup
Setting up a REST API testing environment can involve many manual steps, especially for larger projects. Configuring test data, managing dependencies, and integrating with other systems can be time-consuming. This includes managing licenses and subscriptions for testing platforms, as well as deploying and configuring these platforms, all of which put extra work on the plates of developers and testers.
Not All Testing Provides Code-Level Insights
Traditional black box testing tools may not provide deep insights into the internal workings of REST APIs, making it difficult to identify the root cause of errors. Coverage-guided testing and white box testing can help overcome this challenge by providing more detailed error reports and better test coverage.
These challenges show the complexity of testing REST APIs. However, by using the right strategies and tools, these challenges can be overcome. Despite the challenges, API testing is a must-have for building robust, secure, and high-performing APIs. In the next section, we will examine some API testing strategies for overcoming these challenges and creating a holistic testing strategy.
API Testing Strategies
When it comes to API testing, certain strategies can work well across the board. Of course, the exact strategies you implement will depend on factors such as API complexity, infrastructure and programming languages you’ve used, and even your team's level of expertise. Let’s take a look at a few strategies you can implement to make your testing comprehensive and efficient.
Fuzz Testing
Fuzz testing, also known as fuzzing, is a technique where you inject invalid, unexpected, or random data into the API request. This helps to find vulnerabilities and unexpected behavior. Feedback-based fuzzing is a more advanced approach in which factors such as code coverage measure testing progress. Feedback-based fuzzing solutions use this info to guide the generation of new test cases based on feedback from the existing test cases. Overall, fuzzing helps to find bugs and security issues caused by unexpected inputs and makes the API more robust and stable.
Automating API Testing
Although necessary for certain types of testing, manual testing of APIs is time-consuming and error-prone. This is especially true for large and complex APIs. Implementing test automation as often as possible is the key to a scalable and consistent testing strategy. Automating API testing allows faster execution of test cases, more test coverage, and early detection of regressions. Automation tools can simulate scenarios, validate responses, and generate detailed reports to inform developers and stakeholders about the current state of the application being tested. Adding automated methods helps make the API testing process more efficient and faster.
Testing Before Deploying vs. After Deploying
Testing APIs at various stages of development and deployment is also crucial to detecting issues. Since different environments can affect the results of tests, such as performance and security, testing can be done before and after deploying RESTful APIs to various environments, including production. Testing before deploying (pre-production testing) includes unit testing, integration testing, and other types of testing to ensure the API works as expected in a controlled environment. Testing after deploying (post-production testing) is testing the API in the real world, under real conditions. At this point, testing the API for performance under load and finding issues that may occur in production can be performed. Security testing is one type that should be performed pre and post-deployment to production to ensure that vulnerability detection is in force through all environments. Doing pre and post-production testing gives you a full spectrum of API quality and reliability throughout its life cycle.
Having the right tools to implement these testing strategies is important. Luckily, many tools are available to cover all aspects of API testing, from unit testing to security testing. Let’s take a look at some of the top tools next.
Top API Testing Tools
In this blog, we’ve talked about how vital API testing is and the benefits of using automated tools to implement it. In this section, we will look at some of the top tools for testing various aspects of APIs. Let’s take a look at some of the top tools that developers and testers should add to their testing stack.
StackHawk
StackHawk is a Dynamic Application Security Testing (DAST) tool that finds vulnerabilities in running applications, including APIs. It supports testing APIs by crawling API routes or using OpenAPI specifications. It also has built-in API discovery tools to help you find and test all your endpoints. StackHawk can be added directly into CI/CD flows to test REST APIs as developers commit their code, revealing potential security vulnerabilities in real time.
Postman
Most developers are familiar with Postman and its role in API development and testing. Postman is a versatile and widely used API testing tool that simplifies the entire API development life cycle. It allows you to send requests, test and debug responses, automate tests, document your APIs, and even monitor their performance. Its user-friendly interface and wide range of features make it a favorite among developers and testers.
SoapUI
A mainstay for developers for many years, SoapUI is an open-source API testing tool known for its extensive capabilities in testing SOAP (simple object access protocol), REST, and web services. It has a rich set of features for functional testing, security testing, load testing, and test automation. SoapUI is flexible and extensible and can be used for various API testing scenarios.
Apigee
Apigee, now part of Google Cloud, is an API management platform that can also help test APIs at various stages of development. With Apigee, you can design, secure, publish, analyze, and monitor your APIs. It supports multiple API protocols, including REST, SOAP, and GraphQL, and is a full-stack solution for managing and testing your API ecosystem.
Integrate.io
Integrate.io is a data integration platform with a RESTful API connector that simplifies integrating REST APIs into your data pipelines. Its drag-and-drop interface and prebuilt connectors make it easy to connect, transform, and load data from various sources, including REST APIs. As a platform with API management capabilities, integrate.io can help with various facets of API testing.
Swagger UI
Swagger UI is part of the Swagger (now OpenAPI) ecosystem and is an interactive API documentation tool. It also allows API users to visualize and interact with API resources without writing any logic by allowing them to test APIs with a click of a button using sample REST API request examples. In addition to the obvious benefit of interactive API documentation, Swagger UI is useful during development and testing to understand the API structure and functionality.
REST-assured
For those creating APIs in Java, REST-assured is a Java library for testing RESTful APIs. Its domain-specific language (DSL) simplifies creating test cases and validating API responses. REST-assured’s syntax and assertions are intuitive and popular among Java developers. The tool can invoke RESTful APIs and validate the requests and responses based on requirements.
With an abundance of tools available, how do you determine what tool is best for your specific use case? In the next section, we will discuss the factors to consider when evaluating API testing tools and choosing the right one for your needs.
API Testing Tools Evaluation
When choosing which tools are best for your API testing stack, there are quite a few aspects to consider. Let’s take a look at some key considerations, features, and benefits that you should consider when evaluating the tools you are evaluating.
Considerations
Functionality: Does the tool support the types of API tests you need it for(e.g. functional, performance, security)? Does it have features like test automation, data-driven testing, or integration with your CI/CD pipeline?
Ease of Use: Is the tool’s interface user-friendly? Can your team learn and adopt it without extensive training?
Scalability: Can the tool handle the volume and complexity of your APIs? Will it scale as your API portfolio grows?
Security: Does the tool have built-in security features to protect your APIs and data during testing?
Cost: Is the tool’s pricing model within your budget? Consider both upfront costs and ongoing costs like licensing or subscriptions.
Compatibility: Is the tool compatible with your existing technology stack, programming languages, and development environment?
Tool Features and Benefits
Besides the above, here are some additional features and benefits that you should look for in any tools that you adopt.
Automated Testing: Support for automating test cases saves time, improves efficiency, and allows continuous testing.
API Design and Documentation: Some tools have features for designing, documenting, and generating API specifications to promote consistency and collaboration.
Security Testing: Look for tools to help you find security vulnerabilities and risks in your APIs.
Reporting and Analytics: Robust reporting and analytics give you valuable insights into test results, allowing you to track progress, identify bottlenecks, and make data-driven decisions.
Collaboration: If you have a team of testers and developers, consider tools that support collaboration, version control, and knowledge sharing.
By evaluating these factors and considering your organization and project needs, you can choose the right API testing tools to support your testing goals and deliver high-quality, reliable, and secure REST APIs.
API Testing Best Practices
When it comes to testing best practices, each type of testing will come with its own guidelines. However, two best practices apply to all kinds of API testing: creating good test cases and automating testing through CI/CD. Let’s explore these in more detail.
Creating Effective Test Cases
Effective RESTful API testing hinges on well-crafted test cases. To achieve comprehensive coverage, a REST API test should consider both expected (positive) and unexpected (negative) behaviors, including edge cases and boundary conditions. Stress-test the API's resilience using data-driven testing with diverse input sets. Prioritize tests that target critical functionalities, potential security vulnerabilities, and performance bottlenecks. Lastly, maintain thorough documentation of test case purpose, steps, expected results, and dependencies for seamless collaboration and maintenance.
Integrating API Testing into CI/CD Pipelines
Integrating API testing into your CI/CD pipeline is critical for continuous quality assurance. Automate your tests to save time, ensure consistency, and enable frequent execution. This practice provides immediate feedback on code changes, allowing developers to quickly identify and resolve issues before making their way onto production servers. By establishing a fast feedback loop through automated testing in your CI/CD pipeline, you can proactively maintain your APIs' quality, reliability, and security.
By following these two best practices across all types of testing, you can build a solid API testing framework for your REST APIs. It’s also essential to research best practices for each specific type of API testing you implement.
Using StackHawk For REST API Testing
With security being a top priority for REST APIs, StackHawk is an excellent choice for testing APIs for vulnerabilities in the OWASP API Top 10 and beyond. Its automated testing, CI/CD integration, and developer-friendly approach align it well with many of the considerations and best practices to look for in an API testing tool. With its modern approach to DAST, it’s an invaluable tool for identifying and mitigating security risks that can impact APIs. Here are a few highlights:
Automated Security Testing Through CI/CD
StackHawk automates testing your APIs for a wide range of vulnerabilities, including those outlined in the OWASP API Top 10. This saves you time and ensures consistent security checks throughout your development cycle. By integrating with your CI/CD pipeline, StackHawk allows you to catch security issues early in the development process on every pull request. This shift-left approach empowers developers to identify and fix vulnerabilities before they reach production, reducing the risk and cost of remediation.
Comprehensive API-Specific Vulnerability Testing
Unlike other DAST solutions that struggle to thoroughly test APIs, StackHawk’s API testing capabilities identify issues like injection attacks, broken authentication, sensitive data exposure, etc. With API discovery capabilities built directly into the platform, ensure even undocumented and shadow APIs are covered. Once a test run is completed, stackHawk provides detailed vulnerability and test reports with actionable insights to help developers prioritize and fix vulnerabilities.
Customizable
StackHawk allows you to tailor its scans to your specific needs. You can configure it to focus on specific vulnerabilities, exclude certain paths, and even create custom test scripts to address unique security concerns. For paths requiring various user authentication and authorization settings, you can use StackHawk’s authenticated scanning capabilities to ensure all API routes and logic paths are included by test coverage.
Want to experience the benefits of StackHawk’s API testing capabilities for yourself? Sign up for a free trial today.
Conclusion
API testing is necessary to ensure your APIs are functional, reliable, secure, and performant. In this post, we looked at the importance of API testing, the types of tests, the challenges, and the best practices for delivering high-quality APIs. Choosing the right tools and approach is the key to implementing a comprehensive API testing strategy. You can fully capitalize on the benefits by implementing various types of API testing early, often, and in an automated manner. It's never too early in the SDLC to start testing your RESTful APIs.
As part of a holistic testing strategy, try StackHawk to experience how modern DAST can augment API security testing. Sign up today for a free trial and begin testing your APIs for the OWASP API Top 10 within minutes.
