9 Types of API Testing

Denny Lesmana
3 min readJan 28, 2024

What is API testing?

API testing is a black box test evaluating APIs either independently or as part of the integration testing to ensure that an API meets the following:

  • Functionality
  • Security
  • Performance
  • Reliability

Why Is API Testing Important?

  • Efficient back-end testing: API testing can help identify and fix remaining bugs at the unit/server or level, which can severely delay the product’s release, requiring extensive code rewrites.
  • Early testing: Developers can start testing APIs early in the SDLC before the user interface is consumer-ready. This lets developers catch and fix bugs at the server layer before they are visible at the UI layer, preventing them from becoming more severe.
  • Infrastructure for microservices: Many companies are using microservices in their applications, making API testing increasingly crucial for ensuring that all parts of the software are working properly.
  • Enabling agile software development: API testing is integral to agile software development, where instant feedback is necessary to the process flow. In agile environments, unit and API tests are usually better than GUI (graphical user interface) tests because they are more efficient and easier to maintain. GUI testing often requires extensive reworking to keep up with the rapid changes of an agile environment.

9 Types of API Testing

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  • Smoke Testing
    This is done after API development is complete. Simply validate if the APIs are working and nothing breaks.
  • Functional Testing
    This creates a test plan based on the functional requirements and compares the results with the expected results.

Define different scenarios for each function.

Automate testing for functions that you test repeatedly.

Test periodically.

  • Integration Testing
    This test combines several API calls to perform end-to-end tests. The intra-service communications and data transmissions are tested.
  • Regression Testing
    This test ensures that bug fixes or new features shouldn’t break the existing behaviors of APIs.
  • Load Testing
    This tests applications’ performance by simulating different loads. Then we can calculate the capacity of the application.

Begin testing your API from the lowest level functions and build it up from there.

Build one test for each API.

Define performance measures and goals for your API.

  • Stress Testing
    We deliberately create high loads to the APIs and test if the APIs can function normally.
  • Security Testing
    This tests the APIs against all possible external threats.

Analyze 3rd party dependency for vulnerabilities.

Use data encryption to increase API security.

Set multi-factor authentication requirements for making API calls.

  • UI Testing
    This tests the UI interactions with the APIs to make sure the data can be displayed properly.
  • Fuzz Testing
    This injects invalid or unexpected input data into the API and tries to crash the API. In this way, it identifies the API vulnerabilities.

Follow the fuzz testing phases.

Choose between dumb fuzz & smart fuzz strategy.

Choose or combine behavioral and coverage-guided fuzz testing types.

Choose the right fuzz testing tools



Denny Lesmana

Senior Full Stack Engineer @ Ajaib | Tech & Investment Enthusiast | twitter: https://twitter.com/Denny_lesmanaa