Types & 8 Key Advantages of API Testing

August 10, 2023Shreya Bose
What is API Testing? Types & Advantages of API Testing cover

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APIs facilitate communication and both-way data trafficking between disparate software systems. API testing validates that these APIs are functional, reliable, secure, and perform optimally. Before going for the types and advantages of API testing, let’s first understand API testing and why it matters.
This comprehensive guide will teach you all about API testing. Besides, we’d uncover one of the best automated API testing tools available.

What is API Testing?

API testing checks whether the business logic of software works as expected and whether the output is in order.

Instead of the mouse-keyboard-voice inputs required for most software tests, API tests require testers to trigger calls to said API, receive output, and monitor if it is up to par. By nature, API tests are fundamentally different from GUI tests. They do not focus on an application’s aesthetics, usability or intuitiveness. They prioritize analyzing and evaluating the business logic behind the system architecture.

API Testing

Why does API Testing matter?

APIs connect software components. These components could be modules within the same software system or third-party applications your software must integrate with to provide certain functions.

For example, let’s say you offer social media sharing features on an eCommerce site. Every time someone tries to share a post, you need APIs to trigger a connection with said social media site.

APIs are also required for non-user facing tasks, like connecting with internal databases to authenticate login credentials.

API testing ensures that the right APIs work accurately to accomplish these functions. Another fact to note is that since APIs facilitate data exchange between different software systems, they actually travel in and out of each software’s security ecosystem.

That means APIs are more vulnerable to external attacks, hacking, and other malicious activities by hackers and their ilk. Often, hackers launch fake API calls and use their responses to reverse engineer access to sensitive business data (that the API must access to accomplish its task).

API tests are also required to ensure that the API’s security mechanisms don’t allow such unauthorized access. This requires authentication mechanisms and other methods to prevent API abuse. API tests must be written and executed to check their efficacy in various real-world circumstances – injection attacks, DDoS attacks, data exposure, web scraping attempts, etc.

Types of API Testing

You can run different API tests to check your API’s functionality, performance, and security. Some of the common types of API testing include:

Functional Testing

Functional tests check if the API works as expected within specific hardware and software conditions. It requires checking API functioning within the software codebase and monitoring how it works in the real world.

Validation Testing

Validation tests check if the software matches business requirements. API tests must match the expected results as defined in the test plan. They should be validated to accomplish every task required to provide software users with the functions they want. This is closely related to User Acceptance Testing.

UI/Visual Testing

This checks that the UI associated with API appears visually as it should (to developers, not end-users) in terms of the position of elements and code.

Security Testing

Security tests check that the API endpoints are secure from attackers. It validates that all APIs in the application ecosystem are protected against threats like unauthorized access, data leakage, fuzzy inputs, injection vulnerabilities, parameter interference, etc.

Security tests are particularly important because APIs generally have free access to sensitive data of both business and technical significance. Inadequate security testing could lead to:

  • Unauthorized alterations of website and app UI (text, images, design elements)
  • Customer data leakage
  • Lawsuits (if the software is found to have neglected its data protection obligations).

Load Testing

Load tests check how API performs when bombarded with a large amount of traffic i.e. API calls. By emulating real-world traffic, including intermittent spikes and dips, load tests verify the APIs’ durability to certain traffic numbers.

This is important to ensure that no API suffers memory leaks or any defects that may put its functionality or even user data at risk.

Runtime and Error Detection

These tests are meant to highlight any monitoring, errors, resource leaks, execution anomalies, or defects that appear when the API works.

Penetration Testing

These tests simulate cyber attacks against a system to highlight any exploitable vulnerabilities. Penetrate tests usually help uncover further ideas for augmenting the API’s security protections.

Usually, APIs are exposed to improper inputs that open them to injection attacks (among other threats). The insights derived from such tests are best for refining security policies and developing patches to update the security of existing software versions (if any).

Fuzz Testing

These tests introduce invalid, unpredictable, and misshapen input parameters into a system to check how it responds. Doing so exposes the software’s propensity towards crashes or information leakage in the event of unexpected inputs. It also lays bare any gaps with regard to security performance or overall functionality.

Advantages of API Testing

API testing has several benefits, including faster testing, better coverage, and lower costs. Let’s look at the significant advantages of API testing:

1.Quicker Release

API tests take a lot less time than GUI tests, because you don’t have a UI to verify and monitor. No need to check design, text, button placement, etc. All that matters is if the API is getting its specified job done.

Consequently, API tests are completed faster, which means testers can focus on more layered, complicated tests that take more time and effort to create and run.

2.Better Test Coverage

When you run API tests, you are testing the software down to its more granular level, way beneath the UI. API testing verifies the validity of the backend, of the communication channels between requisite databases for performing specific tasks.

This translates to far greater test coverage. When you’re testing an application down to its brass tacks, you’re bound to find more bugs, issues and anomalies that would have otherwise negatively affected end-user experience. The result? Happier users and better quality software.

3.Easier To Shift Left

As mentioned before, API tests require far less time than GUI tests. That means QAs can design and run API tests fast and early on in the pipeline, even before the GUI is ready. This contributes to the shift-left approach to testing, which has proved to be a superior methodology. Run API tests fast, get quick results and resolve issues early on, before you can start testing the actual UI.

4.Lower Maintenance To Do

Generally, the API layers do not have to be changed or updated too often. This translates to fewer regressions due to new merges or releases. API updates generally happen due to significant business logic changes – which do not occur too often.

All the above factors mean that testers don’t have to spend as much time maintaining API tests or reworking them to accommodate changes.

5.Faster Bug Fixes

APIs work fast. You insert the requisite input (the API call), bringing back the necessary information i.e., it connects to a third-party site or service.
So testers can know immediately if tests are passing or failing. Once again, this enables earlier detection, diagnosis, and resolution of problems, further facilitating the shift-left approach.

6.Reduced Testing Costs

API tests provide greater coverage, easy maintainability, and return fast results. Obviously, all this adds up to a much lower cost of overall testing. It frees up time, money and resources for other project necessities.

7.Universal Language Understanding

During API tests, data exchange occurs through XML and JSON formats. That means all programming languages can be used to write API tests, be it JavaScript, Java, Python, Ruby, PHP, etc.

8.Greater Test Stability

The interface of an API is far more stable, compared to the GUI of the application. The GUI is dynamic, and must change periodically to provide features required by end-users and/or business stakeholders.

On the other hand, APIs change less frequently and usually come with fairly detailed documentation. All changes reflect in said documentation, which gives QA engineers a solid reference with which to adjust test cases.

Consequently, you get more stable tests that are also easier to maintain.

How to Test API?

API testing includes multiple types of tests to check API functionality and performance.
  • Functional testing: Test your API’s functionality to ensure it returns the expected results for different inputs.
  • Stress test: Test your API under various load conditions to ensure it can handle expected traffic levels.
  • Security test: Test your APIs for vulnerabilities and ensure compliance with security standards.
  • UI tests: Test your API through the UI to ensure it works correctly when end users access it.
  • Integration test: Test integrations between different APIs and API components to ensure they work well together.
  • Interoperability test: Test your APIs on different platforms and systems to ensure they work correctly in all environments. 

Challenges of API Testing

API testing presents unique challenges that make ensuring API quality and reliability difficult.
  • Limited documentation: API documentation can be limited, making it difficult for testers to understand the API’s work.
  • Complex data format: APIs often use complex data formats that are difficult to parse and validate.
  • Authentication and Authorization: Testing APIs that require authentication and authorization can be difficult, as testers must ensure these mechanisms are properly implemented.
  • Changes in API functionality: Changes to API functionality can impact how you test, and testers should adjust their testing approach.
  • Environment dependent: APIs can depend on specific environments or configurations, making testing in different environments difficult.
  • Test script maintenance: API test scripts can be complex because APIs change frequently, and test scripts must be updated to maintain accuracy. 

Best Practices of API Testing

To ensure the quality and reliability of your APIs, it’s essential to follow best practices when conducting API tests.
  • Test early and often: Start testing your APIs as early as possible in the development cycle and frequently test throughout development to catch problems early.
  • Use automated testing: Use automated testing to improve efficiency and reduce the risk of human error.
  • Use realistic data: Use accurate test data to ensure your API works as expected under real-world conditions.
  • Monitor API performance: Monitor API performance to identify bottlenecks and other performance issues.
  • Use a mock service: Use mock services to simulate APIs that aren’t yet available or to test APIs in isolation from other components.
  • Collaboration across teams: Collaboration between developers, testers, and other stakeholders helps ensure that APIs are thoroughly tested and meet all needs. 

Best Automated API Testing Tool

Use Testsigma’s intuitive platform to shift your tests left. Automate API tests right for the application design phase, and integrate them flawlessly into your end-to-end functional testing pipeline. Here’s a quick glimpse of what you get when you sign up for Testsigma to run API testing:

  • No coding or setting up environments. You just sign up, start executing automated API tests and validate their responses via the platform’s UI.
  • You can automate and customize all API call sequences to align with your application’s workflow. This cuts down on the emergence of unexpected errors.
  • Run API tests and functional tests with a single platform.

Testsigma also offers specific features to scale automated API testing:

  • Craft API tests without any test scripts. Just enter the endpoint, select the API method, add header content, calibrate status code and trigger the test.
  • Test APIs via multiple REST API methods (GET, PUT, POST, DELETE, HEAD). You can either create separate tests per method or develop a single integrated test.
  • If API tests must be triggered for numerous datasets, you don’t have to make script changes for each. Just store parameters as data profiles on the platform.
  • Set the meticulousness with which you compare actual responses with expected responses. You can choose between Strict, Lenient, Non-extensible, Strict order, or JSON schema.
  • Bring closer control and flexibility to your tests by letting the platform automatically acquire runtime data from API responses that you can reuse in upcoming tests.


Comprehensive API tests are essential to ensuring the quality of any software, inside and out. Given how important they are to the flawless functioning of modern applications, APIs must work as expected under different real-world circumstances.

Testsigma makes API testing exceptionally simple, without compromising an ounce on speed, accuracy and efficacy. Our cloud-based platform strives to provide an all-in-one experience – allowing testers to run API tests as easily as functional and GUI tests. No coding, no scripts, no muss, no fuss.

Frequently Asked Questions

Why is API Testing faster?

APIs don’t have complex interfaces like an application’s GUI. You don’t have to set up tests to validate design or any aspect except the API’s function – is it responding correctly to API calls? API tests are easier to create and complete faster, which makes API testing faster than other GUI tests.

Why is API testing better than UI testing?

In terms of faster test completion and test maintainability, API testing fares “better” than UI testing. As mentioned above, API tests take less time to complete. The tests are also easier to maintain since APIs don’t have to be upgraded as frequently as end-user-facing features.

Finally, API tests are more stable because of the same reason – they don’t have to be changed to match upgrades as often as the GUI. This also means that you have to run fewer regression tests for APIs.

What API is most frequently used?

The REST(representational state transfer) architecture is the most frequently used approach to creating and deploying highly functional APIs. REST APIs are the most popular ones used in software development and testing.

Which API is the fastest?

While there is no single definitive answer to this question, here are a few API frameworks that generally tend to offer speed and performance:

  • Express.js + Node.js
  • FastAPI
  • Phoenix + Elixir
  • Fiber
  • Fastify
  • .NET Core

Suggested Readings

Automated API testing tool 

10 API Testing Tools You Can’t Live Without In 2023  

API Testing Checklist and Best Practices for 2022 

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