Concept of API Testing

  • By Chetna Malagi
  • May 3, 2024
  • Software Development
Concept of API Testing

Concept of API Testing

API testing is a type of software testing that focuses on verifying and validating the functionality, performance, reliability, and security of Application Programming Interfaces (APIs). APIs act as a communication bridge between different software systems, allowing them to interact and exchange data. In this blog, we will discuss the Concept of API Testing, its types, and many more.

  • What is an API?

 An API (Application Programming Interface) is a set of rules, protocols, and tools that allows different software applications to communicate with each other. It defines how software components should interact.

  • Why API Testing?

  • Early Detection of Issues: API testing can be done at an early stage of development, even before the GUI is ready.
  • Cost-Effectiveness: Since API testing is typically automated, it can be performed quickly and repeatedly at a lower cost compared to manual testing.
  • Improved Test Coverage: API testing allows for comprehensive testing of business logic and data exchanges between systems.
  • Performance Testing: APIs are often crucial for the performance of applications, and API testing helps in evaluating performance under various conditions.

  • Types of API Testing:

  • Functional Testing: This type of testing focuses on testing the functionality of APIs, including inputs, outputs, and behavior under different conditions.
  • Unit Testing: Testing individual units or components of an API in isolation to ensure they function as expected.
  • Integration Testing: Verifying that APIs work correctly when integrated with other systems or components.
  • Load Testing: Assessing the performance of APIs under expected load conditions.
  • Security Testing: Ensuring that APIs are secure from vulnerabilities and threats.

  • Tools for API Testing:

  • Postman: A popular tool for API testing that allows you to create and run automated tests.
  • Swagger/OpenAPI: Tools for defining, building, and testing APIs using the OpenAPI Specification.
  • SoapUI: An open-source tool for functional and load testing of SOAP and RESTful APIs.
  • JMeter: Often used for load testing of APIs to measure performance under various loads.
  • RestAssured: A Java library for testing RESTful APIs.

  • API Testing Process:

  • Test Planning: Identifying the scope, objectives, and requirements of API testing.
  • Test Design: Creating test cases and scenarios based on functional requirements and use cases.
  • Test Execution: Running test cases and collecting results.
  • Defect Reporting: Documenting and reporting any issues encountered during testing.
  • Test Maintenance: Updating and maintaining test cases as the API and application evolve.


API Testing Using Postman Tool

Postman is a popular API testing tool that simplifies the process of testing APIs by providing a user-friendly interface. Here’s a brief overview of API testing using Postman:

  • Understanding APIs: Before testing, it’s important to understand the APIs you’re working with. Know their endpoints, request methods (GET, POST, PUT, DELETE, etc.), request parameters, headers, authentication methods, and expected responses.
  • Installation: First, download and install Postman from their website or use the Chrome extension.
  • Create a Collection: In Postman, a collection is a group of saved requests. You can organize your API tests into collections for better management. Create a new collection for your API tests.
  • Create Requests: Inside your collection, create requests for each API endpoint you want to test. Specify the request method (GET, POST, etc.), URL, request headers, parameters, body (if applicable), and authentication details.
  • Send Requests: Once you’ve configured your request, you can send it to the API endpoint by clicking the “Send” button. Postman will then display the response from the server, including the status code, headers, and body.
  • Test Response: After receiving the response, you can verify that it matches the expected behavior. Check the status code to ensure it’s correct (e.g., 200 for success, 404 for not found), inspect the response body for expected data, and validate headers.
  • Assertions: Postman allows you to add assertions to your tests to validate the response automatically. Assertions can check various aspects of the response, such as status code, response body content, headers, etc. If an assertion fails, the test will be marked as failed.
  • Scripting: Postman also supports scripting using JavaScript. You can write pre-request scripts or post-response scripts to automate certain tasks or validations. This is useful for dynamic testing scenarios.
  • Environment Variables: Postman lets you define environment variables that can be used across requests. This is handy for managing different environments (e.g., development, staging, production) or for storing authentication tokens.
  • Run Tests: You can run individual requests, entire collections, or even multiple collections in a specific order. Postman provides detailed test results, including which tests passed or failed and any assertions that didn’t meet expectations.
  • Save and Share: Once you’ve created and tested your requests, you can save your collection and share it with your team. Postman also supports version control integrations like Git.
  • Monitor: Postman offers a monitoring feature that allows you to schedule and run your API tests automatically at specified intervals. This helps ensure the ongoing reliability of your APIs.


Let’s consider a simple example of testing a hypothetical RESTful API for a weather service. In this example, we’ll test the endpoint for retrieving the current weather for a specific city.


Create a Collection: Open Postman and create a new collection named “Weather API Testing”.

  • Create a Request:
  • Click on “New” to create a new request.
  • Name the request as “Get Current Weather”.
  • Set the request method to GET.
  • Enter the URL of the API endpoint, for example:
  • Add any required query parameters, such as city or zip code.
  • Add headers if needed, such as Content-Type or Authorization.
  • Send the Request:
  • Click on the “Send” button to send the request to the API endpoint.
  • Postman will display the response from the server.
  • Test the Response:
  • Check the status code of the response to ensure it’s correct (e.g., 200 for success).
  • Inspect the response body to verify that it contains the expected data (e.g., current temperature, weather conditions).
  • Validate any headers if necessary.
  • Add Assertions (Optional):
  • To automatically validate the response, you can add assertions:
  • For status code: The status code is 200.
  • For response body: The response body contains the key “temperature”.
  • You can add more assertions based on your API’s response structure and expected data.
  • Run Tests:
  • Click on the “Run” button to execute the request and any associated tests.
  • Postman will display the test results, indicating whether each test passed or failed.
  • Save and Share:
  • Once you’re satisfied with the request and test setup, save the request to the collection.
  • You can then share the collection with your team or save it for future reference.
  • Environment Variables (Optional):
  • If you’re testing against different environments (e.g., development, staging, production), you can define environment variables to easily switch between them.

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