Testing is a crucial part of the software development life cycle. It helps ensure that software works as intended and meets user expectations. There are two main types of software testing - API testing and UI testing. Understanding the differences between doing API tests and UI testing can help developers choose the right testing strategy to build robust, failure-resistant software.

What is API Testing?

API stands for Application Programming Interface. APIs allow different software systems to communicate with each other by exposing some functionality. For example, the Twitter API allows other applications to connect with Twitter and get data like tweets, users, trends etc.

API testing is the process of testing API endpoints directly, with automated tests focusing on input and output of APIs. The aim of API testing is to validate that the APIs meet their design specifications and perform as intended.

Some examples of what API testing covers:

  • Functional Testing - Checking that API business logic and endpoints work as expected.
  • Load Testing - Testing API performance under different user loads.
  • Security Testing - Testing authentication, permissions, SSL configurations etc.
  • Reliability Testing - Testing API robustness under various conditions.
  • Usability Testing - Testing API usability from a developer's perspective.

API testing is performed at the backend of a software without having a UI frontend. Test cases directly call the API endpoints and validate the responses.

What is UI Testing?

UI stands for User Interface. UI testing focuses on testing the graphical user interface of an application from an end user's perspective. The aim ui tests is to ensure that the UI allows users to effectively use the software and behaves as designed.

Some examples of what UI testing covers:

  • Functional Testing - Validating UI elements like buttons, menus, forms etc. work correctly.
  • Usability Testing - Testing how easy/intuitive the UI is for users.
  • Localization Testing - Testing UI text for different languages and locale.
  • Cross-browser Testing - Checking UI consistency across browsers.
  • Visual Testing - Testing UI appearance across devices.
  • Accessibility Testing - Testing UI compliance to accessibility standards.

UI testing process replicates real user interactions like clicking buttons, entering data in forms, navigation etc. It validates that the UI responds correctly to user actions.

Key Differences Between API and UI Testing

API Testing UI Testing

Tests backend application logic

Tests frontend UI

Focuses on input/output of API endpoints

Focuses on look, feel and behavior of UI

Validates functionality and design of APIs

Validates functionality and design of UI

Tests APIs independently without UI

Tests UI as an end user would

Automated using API testing tools

Automated using UI testing tools

Requires API level access

Requires application front end access


API Testing vs UI Testing: Which is More Important?

API testing and UI testing address different aspects of the user interface testing an application. Both API testing and UI testing are equally important for comprehensive testing.

Here are some factors that determine which kind of test scenarios of testing should be emphasized:

  • For customer facing applications like web apps, mobile apps, priority should be given to UI testing as the UI is the main mode of interaction for users.
  • For backend centric systems like microservices architectures, API testing takes precedence as APIs power communication between services.
  • During initial development phases, API testing helps validate functionality early before UI is built.
  • For migrating or upgrading APIs, rigorous API testing is required to prevent regressions.
  • When revamping UI, increased UI testing ensures new UI does not impact user workflows.
  • Applications that handle sensitive or critical data require more API testing to prevent security issues and data leaks.
  • Latency sensitive systems like trading apps need more API testing to avoid performance bottlenecks.
  • Customer facing apps require more visual, usability and localization testing to polish UI appearance and behavior.

So in summary, the testing strategy should consider the context, priorities and type of system being tested. A balanced mix of both manual API testing and UI testing is ideal for most software projects.

API vs UI Testing: Would Both Have Prevented the Epic Software Failure at Level 3?

Level 3 Communications was one of the largest telecommunication companies that filed for bankruptcy in 2014, after an failed effort to integrate and upgrade its billing systems led to turmoil. Customers were billed incorrectly, couldn't view or pay bills online and service was disrupted. So what went wrong and could API or UI testing have prevented this failure?

What Caused the Epic Failure at Level 3?

Level 3 had multiple billing systems for various services they acquired over the years. To simplify operations, they decided to consolidate all these into one unified billing system.

Here are some key pain points and limitations that led to the massive failure:

  • Integrating multiple legacy systems with inconsistent data models proved complex.
  • New consolidated system was not tested rigorously before going live.
  • Little API testing was done to check if interfaces between old and new systems work correctly.
  • Insufficient UI testing of new self-service portal led to customer facing issues.
  • Lack of load testing resulted in billing system crashing frequently under high traffic.
  • Failure recovery procedures were inadequate to rollback or restore services.
  • Customer support teams were not trained properly on new systems.

In summary, Level 3 failed to realize the enormity of the change and did not test the integrated systems thoroughly before launch.

Could API Testing Have Helped?

More rigorous API testing could have mitigated some of the integration issues:

  • Testing data exchange between the legacy systems and new billing system could have surfaced inconsistencies earlier.
  • Thorough functionality testing of the underlying billing, provisioning and payment APIs could have reduced malfunctions.
  • Testing APIs under load would have ensured they don't crash under high usage.
  • Security testing would have detected vulnerabilities that led to data leaks.
  • Proper test data setup would have uncovered data migration errors beforehand.
  • Robust API testing strategy would have improved overall stability of the integrated systems.

Could Better UI Testing Have Helped?

Improved UI and automated testing, of the customer portal and internal tools could have averted customer facing failures:

  • User acceptance testing across various usage scenarios would have ensured customer workflows function correctly.
  • Testing across browsers, devices and locations would have detected compatibility issues.
  • Visual UI testing could have identified display issues like broken images, misaligned elements etc.
  • Localization testing of the portal in all required languages would have reduced confusion.
  • Security testing could have prevented vulnerabilities like customer data leaks.
  • Thorough accessibility testing would have ensured compliance with disability laws.
  • Comprehensive test coverage of the new portal functionalities would have reduced customer impact.

The poor API and UI testing rigour contributed significantly to Level 3's billing system meltdown. Extensive testing of the interfaces and full-stack functionality could have reduced overall technical and business impact.

Key Takeaways - Integrating API and UI Testing

Level 3's story exemplifies that complexity often leads to inadequately tested systems prone to failure. So how can organizations avoid such issues?

Here are some key lessons on integrating API and UI testing:

  • Adopt API first testing - With microservices and distributed systems, start testing APIs first before UI is ready. This builds a solid foundation.
  • Design for testability - Architect systems to enable easy testing of any component in isolation. Make APIs and UI test-friendly.
  • Build a test automation pipeline - Automate API, UI and integration testing to enable frequent regression testing.
  • Test early, test often - Start testing from initial stages of development and keep testing throughout lifecycle.
  • Focus on customer experience - Test real world usage scenarios from a user's perspective early.
  • Perform security testing - Rigorously test for vulnerabilities during integration to avoid exploits.
  • Test failure handling - Validate robustness by testing system behavior under failures, outages etc.
  • Invest in performance testing - Prevent performance bottlenecks by load testing APIs and UI.
  • Lean on cloud testing - Leverage on-demand cloud testing platforms to enable flexibility in testing.

By combining API and UI with automated testing tools, organizations can deliver robust software that meets customer expectations even as complexity increases.


API testing and UI testing are complementary disciplines that jointly improve software quality and user experience. API testing focuses on validating application logic and backend functionality. UI testing replicates user workflows and verifies front end behavior.

To prevent epic failures like Level 3's billing system meltdown, companies need to emphasize both API and UI testing especially during integrations. Testing the entire system including interfaces early and often is the best way to de-risk complex changes. With cloud platforms making testing economical and more flexible, the ROI of rigorous test automation continues to grow.

As complexity explodes with technologies like cloud, IoT, AI and microservices, testing will prove pivotal in building resilient software that exceeds customer expectations. Organizations that skimp on testing do so at their own peril. Companies like Worksoft provide automated testing solutions to enable comprehensive and economical test coverage for complex business applications.

FAQs on API and UI Testing

What is the main difference between UI and API testing?

The main difference is that UI testing validates the graphical front end of an application while API testing focuses on testing backend APIs.

Which testing type finds more defects?

Usually UI testing finds more visual and usability defects while API testing finds more functional defects. Both testing tools and types are essential for comprehensive coverage.

Is API or UI testing more difficult?

API testing generally requires technical expertise to send requests and write test cases. UI testing has lower barrier to entry than api testing tools but can get complex for advanced UI flows.

Which testing should we start first?

It's good to start API testing in the initial stages of development as the backend gets built. UI testing can begin as stable UI becomes available.

Can API and UI testing be automated?

Yes, both API testing and UI testing can be effectively automated using commercial and open source test automation tools and frameworks.

1 Federal Communications Commission, Level 3 Nationwide Outage Report, March 2018