QA Testing

Understanding API: A Complete Guide to Various Tools and Testing

Understanding API  

API testing stands out as a challenging aspect of software and QA testing due to the intricate nature of APIs. These interfaces often rely on protocols and standards uncommon in other testing scenarios.

API Meaning
An Application Programming Interface (API) acts as a bridge, enabling two applications to communicate seamlessly, sans user intervention. In simpler terms, an API is a code facilitating access and execution between distinct software components.

Decoding API Testing
API testing serves the purpose of validating not only the business logic but also aspects like performance and security within an application. Unlike unit testing, which delves into individual components, or user interface (UI) testing, which assesses visual presentation, API testing zeroes in on the application's core functionality.

API testing provides a valuable advantage by allowing various teams, including developer operations, quality assurance, and development, to initiate testing on an application's essential functions before the user interface is finalized. This proactive approach helps unearth errors and weaknesses early in the development cycle, a more cost-effective solution compared to addressing them in later stages, which could necessitate extensive code rewriting and lead to substantial release delays.

Types of API testing:
Below are examples of types of API testing.

Functional Testing:
Focuses on verifying that the API functions according to its specifications. It involves testing the input and output of API calls, as well as the functionality of the API in different scenarios.

Load Testing:
Evaluates an API's performance under varying levels of load, ensuring it can handle expected usage volumes. This type of testing helps identify potential bottlenecks and performance issues.

Security Testing:
Ensures that the API is secure from unauthorized access, data breaches, and other security threats. It involves testing for vulnerabilities such as SQL injection, cross-site scripting, and authentication issues.

Reliability Testing:
Validates the API's reliability and stability over an extended period. It assesses how well the API performs under normal and stressful conditions to identify any potential reliability issues.

Interoperability Testing:
Checks if the API can interact seamlessly with other software components, including different operating systems, databases, and web servers. It ensures compatibility and smooth communication between diverse systems.

Usability Testing:
Assesses the ease of use and user-friendliness of the API, including the clarity of documentation, error messages, and the overall developer experience. Usability testing aims to enhance the API's usability and accessibility for developers.

Penetration Testing:
Identifies security vulnerabilities by simulating real-world attacks on the API to ensure robust protection against potential threats.

Runtime and Error Detection Testing:
Monitors the API during execution to detect and address errors, memory leaks, and other runtime issues, enhancing the overall reliability of the API.

Concurrency Testing:
Evaluates how well an API handles multiple simultaneous requests or transactions. It helps identify issues related to data consistency, resource contention, and the overall performance of the API under concurrent usage.

Fuzz Testing:
Involves sending random or unexpected data as inputs to the API to uncover vulnerabilities or unexpected behaviors. Fuzz testing is particularly useful for discovering security flaws and ensuring that the API can handle unexpected data gracefully.

Protocol Testing:
Verifies that the API adheres to specified communication protocols, such as HTTP, HTTPS, or other application-layer protocols. It ensures that the data exchange between the client and server follows the expected standards.

Scalability Testing:
Assesses an API's ability to handle an increasing amount of load or data volume. Scalability testing helps determine the API's capacity to scale up or down based on varying levels of demand, ensuring optimal performance under different workloads.

Mutation Testing:
Involves introducing small changes or "mutations" to the source code of the API to assess the effectiveness of the test suite. The goal is to identify weaknesses in the test coverage by checking if the tests can detect and respond to the introduced mutations.

Compliance Testing:
Ensures that the API complies with industry standards, regulations, or internal organizational guidelines. This type of testing is crucial for APIs that need to adhere to specific compliance requirements, such as those in the healthcare or financial sectors.

Various tools are available for API testing, each serving specific purposes in ensuring the robustness and functionality of APIs. Below are those tools,
- Postman, recognized as a top-tier API testing tool,
- RESTAssured, renowned for its automation testing capabilities,
- Swagger, acknowledged for its excellence in designing and documenting REST APIs.
- Ready API
- Katalon Platform.


Key Highlights:

Provides a comprehensive set of assertions, granting users flexibility in manipulating received data, creating local variables, and implementing loops for repeated test execution.
Allows the creation of collections of integration tests to verify that APIs perform as expected.
Facilitates code reuse across multiple environments, enhancing efficiency in test development.

Why Use Postman:


Collections and environments can be easily imported or exported, streamlining file sharing. Direct links can also be utilized for efficient collection sharing.

Use of Collections:
Postman enables the creation of collections for API calls, allowing for the organization of test suites through subfolders and multiple requests.

Automation Testing:
Leveraging the Collection Runner or Newman, tests can be executed iteratively, optimizing time for repetitive test scenarios.

The Postman console aids in checking retrieved data, simplifying the debugging process.

Creation of Tests:
Test checkpoints, including verification of successful HTTP response status, can be seamlessly added to each Postman API call, ensuring thorough test coverage.

How to use Postman:

  1. New – This is where you will create a new request, collection, or environment.
  2. Import – This is used to import a collection or environment. There are options such as importing from a file, folder, link, or pasting the raw text.
  3. Runner – Automation tests can be executed through the Collection Runner. This will be discussed further in the next lesson.
  4. Open New – Open a new tab, Postman Window or Runner Window, by clicking this button.
  5. My Workspace – You can create a new workspace individually or as a team.
  6. Invite – Collaborate on a workspace by inviting team members.
  7. History – Past requests that you have sent will be displayed in History. This makes it easy to track actions that you have done.
  8. Collections – Organize your test suite by creating collections. Each collection may have subfolders and multiple requests. A request or folder can also be duplicated as well.
  9. Request tab – This displays the title of the request you are working on. By default, “Untitled Request” would be displayed for requests without titles.
  10. HTTP Request – Clicking this would display a dropdown list of different requests such as GET, POST, COPY, DELETE, etc. In Postman API testing, the most commonly used requests are GET and POST.
  11. Request URL – Also known as an endpoint, this is where you will identify the link to where the API will communicate with.
  12. Save – If there are changes to a request, clicking save is a must so that new changes will not be lost or overwritten.
  13. Params – This is where you will write parameters needed for a request, such as key values.
  14. Authorization – To access APIs, proper authorization is needed. It may be in the form of a username and password, bearer token, etc.
  15. Headers – You can set headers such as content type JSON depending on the organization's needs.
  16. Body – This is where one can customize details in a request commonly used in POST requests.
  17. Pre-request Script – These are scripts that will be executed before the request. Usually, pre-request scripts for the setting environment are used to ensure that tests will be run in the correct environment.
  18. Tests – These are scripts executed during the request. It is important to have tests as it sets up checkpoints to verify if the response status is ok if retrieved data is as expected and other tests.
What is a “GET” Request:

GET requests will not affect any data on the server. When you make the GET request on the server, then the server responds to the request. This means there is no creation, update, addition, or deletion of data on the server when you are making a GET request.

GET requests to contain all information inside the URL, and because of that, some people do not prefer to use GET requests while they are sending confidential data such as passwords. For example, if you search anything on Google, you are using a GET request because there is no sensitive information, and you are just requesting the page. You can try to search for something on Google; you will get the same search string in the URL.

What is a “Post” Request:

We use this method when additional information needs to be sent to the server inside the body of the request. In general, when we submit a POST request, we expect to have some changes on the server, such as updating, removing or inserting.

One of the best examples of using POST requests is the login page of Instagram or the login page of other sites; you send your personal information, such as the password, to the server. The server creates a new account with the same information and that account, and the information is added permanently to the Instagram server.

  • 201 with a location header pointing to the new resource.
  • 400 if the new item is not created.
What is a “Put” Request: A PUT request is used to pass data to the server for the creation or modification of a resource.
  • 204 for OK/SUCCESS (but no content).
  • 200 for OK with Content Body (Updated response).
  • 400 if the data sent was invalid. 

What is a “Delete” Request:
The DELETE method sends a request to the server to delete the request mentioned in the endpoint.

  • 200 (OK)
  • 204 (if there is no content for the record that we want to delete)
  • 202 (Accepted, deletion is not a single operation).
Stander status code for Response:
  • 201: For a successful request and data was created.
  • 204: For empty response.
  • 200: This code is used for a successful request.
  • 400: This is used for Bad Request. If you enter something wrong or you miss some required parameters, then the request will not be understood by the server, and you will get a 400-status code
  • 401: This is used for Unauthorized Access. If the request authentication fails or the user does not have permission for the requested operations, then you will get a 401 status code.
  • 503: And this code is used for Service Unavailable.
  • 403: This is for Forbidden or Access Denied.
  • 404: This will come if the Data is Not Found.
  • 405: This will come if the method is not allowed or if the requested method is not supported.
  • 500: This code is used for Internal Server Errors.

What are Assertions?

Assertions are used to verify if the actual and expected values have matched after the execution of a test. If they are not matching, the test shall fail, and we shall get the reason for failure from the output of the test.  

An assertion returns a Boolean value of either true or false. In Postman, we can take the help of the JavaScript Chai Assertion Library to add assertions to our tests. It is available in the Postman application automatically.


Application Programming Interface lets two applications interact with each other without user intervention. API testing includes the process of validating the business logic with regard to the performance and security of an application.

Functional testing, runtime and error detection testing, load testing, penetration, and validation testing. Some of the tools for this API testing are Postman, RESTAssured, Swagger, ReadyAPI, Katalon, etc. Each of these tools comes with its own limitations and benefits.


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