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What is API testing? Benefits, Types, and Best Practices

Javeria Rahim

Associate Manager SEO

June 20th, 2024

What is API testing?

API testing is the process of verifying that APIs are working and performing as expected. Developers and QA engineers test APIs by sending requests to various API endpoints and validating the responses against expected outcomes. The goal is to determine if the APIs meet the set standards for functionality, performance, scalability, and security. This process includes testing the “happy path” scenarios and negative cases to ensure proper security checks are in place. You can automate these tests by using an API testing tool.

It’s essential to test the API as a whole by simulating expected user journeys and server loads to ensure it can handle real-world usage. For instance, for a typical user journey where a user logs in, searches for a product, and adds it to a cart, the ideal API test would be to mimic these steps by sending requests that replicate those actions. Since real-world applications often experience fluctuations in user traffic, the test should also simulate these peaks and server loads to help identify potential bottlenecks or slowdowns.

A typical API testing strategy specifies the number and combination of endpoints, the expected results, the ideal response time, and the criteria for an unsuccessful response. Ideally, there should be several test cases to analyze APIs against all possible scenarios. In recent years, there’s been a shift toward running these tests earlier in the API lifecycle to identify and resolve issues before they turn into bigger problems.

Why is API testing important?

APIs allow systems and applications to communicate with each other and provide access to their core functionality to end users. Many businesses rely on APIs to generate revenue and create a seamless customer experience. However, there are times when APIs don’t function as intended, disrupting the business. This failure could happen for many reasons, such as a server being unresponsive, invalid inputs, insufficient permissions, and more.

Therefore, API testing is a non-negotiable stage in the API management process and it’s imperative to ensure an API’s quality and reliability. It helps identify bugs and inconsistencies before making APIs available to the end users. According to the 2022 Global Industry Analysts Inc. Report, the global API testing market will grow to 1.8 billion USD by 2026. The figures are hardly surprising, given the increasing importance of APIs in modern software development.

Here are some benefits of API testing:

  • Improved reliability: Reliable APIs, through functional testing, help reduce downtime, minimize the risk of errors, and improves their overall quality, resulting in a better user experience.
  • Enhanced security: Penetration testing helps establish security by restricting unauthorized access, enabling better protection of a business’s data assets. It can help protect your business and users from cyber threats and data breaches.
  • Increased efficiency: Performance testing improves the efficiency of APIs and helps determine if they can handle high loads and demanding conditions. It ensures that your APIs don’t break down frequently.

How is API testing different from API monitoring?

Together, API testing and API monitoring help ensure that APIs continue to deliver reliable performance. While they both complement each other, they serve different purposes. Developers run API tests to identify and fix issues before deploying their APIs to production. In contrast, API monitoring continues after deployment and focuses on observing the behavior of APIs in a production environment to ensure they are working as expected over time.

What are the different types of API testing?

There are several different methods to test APIs, each focusing on different aspects of an API’s functionality. Let’s check out the different types:


Types of API testing


Functional testing

Functional testing confirms that an API does what it is supposed to do by verifying that the endpoints correctly process requests and return appropriate responses. It includes testing the input and output of an API, as well as checking its behavior under different conditions. It also includes checking parameter validations for the correctness of data types and values, categorizing valid or invalid requests, and checking that appropriate error messages are sent.

There are different categories of functional API testing, including:

  1. Unit testing: It involves testing one component of an API, such as a specific method or function.
  2. Integration testing: It checks how an API works with other components or systems.
  3. System testing: It tests the entire API to ensure it is working as intended.
  4. Error handling testing: It ensures that an API is capable of handling all the errors. This includes testing for scenarios such as missing or invalid input values and unexpected conditions.

Performance testing

As the name suggests, performance testing evaluates the performance of an API on several factors, such as: response time, capacity, and scalability. It is an imperative step to ensure that your API can handle high loads under demanding conditions.

Performance testing can include:

  1. Load testing: Tests an API’s performance under expected loads, such as the number of concurrent users or requests.
  2. Stress testing: Tests an API’s performance by gradually increasing the load to identify the maximum capacity of the API.
  3. Endurance testing: Tracks an API’s performance over a long period of time, such as several hours or days, to ensure it can handle the expected workload over an extended period.
  4. Spike testing: Ensures API’s ability to handle sudden spikes in workload.
  5. Response time testing: Measures an API’s response time to requests and verifies it meets the expected performance targets.

Security testing

API security testing helps verify that your API can protect sensitive data and systems from unauthorized access. API security tests include evaluating API’s authentication and authorization mechanisms and testing for vulnerabilities.

API testing for security can also include:

  1. Authenticity testing: Tests the API’s authentication mechanisms to ensure they are secure and prevent unauthorized access. For example, protection from hackers who are trying to impersonate another user to gain access to their sensitive data.
  2. Authorization testing: Tests the API’s authorization mechanisms to ensure they are working as intended and prevent unauthorized access to protected resources. This means each user should only be accessing only their owned data (granular row level security).
  3. Penetration testing: Simulates a cyberattack to identify vulnerabilities of the API that a potential attacker could exploit. This testing is done by using specialized algorithms that look out for code injections and stop such requests before they can cause any harm to the server.

Conducting API testing at various stages of the API lifecycle, including the design stage, development phase, and post-deployment, is a best practice that provides several benefits.

By testing at the design stage, developers can ensure that the API meets business logic and requirements, which helps improve their overall design. During the development phase, testing can verify that the API generates the appropriate response to requests and adheres to quality standards, allowing you to identify and resolve issues at the initial stages.

Testing after deployment helps identify and fix issues that may have been missed during the API development stages.

What are the challenges in API testing?

While testing an API is an important part of the lifecycle, it can be quite time-consuming, especially if carried out manually. From complex APIs with numerous endpoints to time constraints and resource limitations, there are many factors that make testing a challenging exercise for organizations.

Here are some common challenges during API testing:

Lack of resources

First and foremost, testing can be resource-intensive, requiring specialized tools, skilled personnel, and a dedicated testing environment. Limited resources are a significant barrier to implementing a robust API testing strategy. Organizations need to weigh the cost of testing against the potential risks of untested APIs.

Data dependency

Isolated tests ideally focus on a single functionality of the API without affecting or relying on other parts of the system. However, not all APIs operate in a vacuum; some rely on specific data already existing within the system to function properly. As an example, let’s say updating a user profile needs a valid user ID. If the API test tries to update a profile without that ID, it’ll likely fail not because of the update functionality itself, but because of missing data.

Similarly, repeatable tests must produce consistent results every time they run. However, some APIs alter the data state within the system. Imagine an API for placing an order. A successful test might create an order, but that changes the data (an order now exists). Running the same test again without resetting the data (deleting the order) will cause it to fail since the API might not allow creating duplicates.

Handling asynchronous processes

Traditional API tests expect immediate responses. However, processes associate with asynchronous APIs, like sending emails or processing large datasets, take time to complete in the background. Testing these processes can be tricky—it’s not as simple as sending a request and expecting an answer right away. The challenge lies in ensuring the API initiates the process correctly and in figuring out how to verify the outcome later, potentially requiring additional steps to check the status or results of the asynchronous task.

The complexity of APIs

APIs can be complex with multiple endpoints, which often rely on each other to achieve a complete task. For instance, an order confirmation endpoint might depend on a successful payment processing endpoint. The complexity arises because testing every possible combination of these interdependent endpoints becomes overwhelming. It’s challenging to thoroughly cover all scenarios and interactions between them during the testing process and warrant the need for specialized tools and techniques.

Changing business requirements

As APIs grow and improve, they often introduce new features or change how they work. This evolution is great, but it complicates testing. Ideally, you want new versions of the API to work seamlessly with existing applications that rely on it. However, ensuring backward compatibility during testing means verifying that new functionalities work as expected while also making sure existing features haven’t broken due to the changes. In some cases, frequently updating or modifying APIs requires a complete overhaul of the testing strategy.

Security concerns

Testing your API with all kinds of valid and invalid inputs ensures that it doesn’t leak data. Security testing involves accurately mimicking an attack, which requires expertise to verify that an API is protected from security breaches and unauthorized access. You need developers with a deep understanding of the system itself and the potential attacker’s goals and tactics, especially as attackers constantly develop new methods to exploit vulnerabilities. It’s exceptionally challenging to design simulations that truly reflect the real-world scenario without knowing all the possible threats the API can face.

API testing is even more challenging with APIs having complex authentication and authorization mechanisms due to their multifaceted nature. These mechanisms often involve several components like tokens, certificates, user roles, and permissions. Testing needs to ensure all these parts work together seamlessly.

What are some API testing best practices?

  • Clearly outline what aspects of the API need testing. Is it the performance, functionality, or security that you need to test? With that, it’s best to identify and create detailed test cases covering positive, negative, and edge scenarios.
  • Simulate real-world usage patterns with high volumes of concurrent requests to identify performance bottlenecks.
  • Isolate the API for focused testing. Mocking allows the developers to concentrate on the functionality and performance of the API itself without the variability and unpredictability of external dependencies.
  • Develop automated tests to verify that the API adheres to its documented specifications (contracts). These tests should cover all endpoints, methods, request parameters, response formats, and status codes to ensure consistency and reduce the risk of breaking changes.
  • Automate API tests using tools like Astera to save time and resources on predictable tests.
  • To ensure compatibility, always test new versions of APIs against old versions. With that, it’s also a good practice to communicate and document deprecation plans for older API versions.
  • Thoroughly test the API for scenarios like token expiration to ensure that tokens are properly invalidated upon user logout or after a certain period of inactivity.
  • It’s always good practice to have a process that allows you to identify bugs before they turn into bigger problems. One of the best ways to do this is to integrate API testing into the API lifecycle and test early and frequently.

The need for automated API testing tools

You’re more likely to face challenges if you opt for manual API testing. According to the State of API report, 36.6% of developers write their tests in code, which is not only time-consuming but a rigid approach since you cannot easily incorporate changing requirements. The best way to mitigate challenges is through API test automation, which is the process of automating the testing of APIs to streamline and enhance the testing lifecycle. You can easily automate the manual aspects of API testing, such as writing code for each test and generating results, using API testing tools.

Automated API testing provides rapid feedback to developers, essential in agile and DevOps environments where continuous integration and continuous deployment (CI/CD) pipelines necessitate quick iteration. It significantly reduces the time required for repetitive and extensive testing tasks, allowing testers to focus on more complex and exploratory testing activities. You can also cover edge cases that are easy to miss with manual testing, covering more ground in less time.

Although there is an initial investment in setting up automation, it proves cost-effective in the long run by reducing the manual effort required, particularly for large-scale projects with frequent updates. Automated testing also allows for early detection of issues through shift-left testing, integrating tests early in the development cycle to catch defects sooner and reducing the overall cost of fixing bugs.

Test your APIs in just a few clicks with Astera

The best way to automate API testing is to use a no-code API management platform.

Astera offers a unified and complete API management solution with advanced, easy-to-use automated testing features. Every feature you need to build, test, and manage your APIs is provided in a single, intuitive, integrated, no-code environment.

Astera’s approach to building APIs emphasizes continuous testing and validation.

API testing with Astera API Management Solution

API Testing with Astera

The tool has the following powerful testing capabilities:

  • Instant data preview: Fix API errors promptly by continuously testing and validating the design at every step of the development process using the instant data preview feature to make sure all your APIs are running properly.
  • Post-deployment test flows: Run automated and fully configured test flows after deployment to inspect your API performance in the target environment.
  • Documentation for external testing: Automatically generate Open API documentation that can be used and shared to call APIs from various external testing tools.

Nothing is easier than using an intuitive, drag-and-drop interface to test your APIs. With no-code testing, you can focus on what really matters: delivering high-quality, reliable APIs to your users.

Want to build a reliable and robust API? Try Astera API Management today! Sign up for a free 14-day trial and see how it simplifies and streamlines the process of building, publishing, testing, and monitoring APIs. Alternatively, schedule a call for a demo with one of our experts to discuss your use case.

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