Common API Testing Interview Questions for 2023 - IQCode

API Testing: An Overview

API testing is a type of software testing that verifies and validates API (Application Programming Interface) functionality, reliability, security, and performance. It enables the quick and efficient testing of business logic and highlights defects or bugs that may exist in the application architecture. API testing is a crucial component in software development and assists in identifying issues at an early stage of software development. In this article, we will discuss some commonly asked interview questions for both novice and experienced professionals in the API testing field.

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1. What is API Testing?

Different Types of API Testing

API testing can be categorized into the following types:

  • Unit testing: This type of testing is performed on individual functions or methods of the API. It ensures that each function is working properly and returning the correct output.
  • Functional testing: It tests the integrated functionality of the API. It ensures that the API is working as expected and allows testing of requests and responses.
  • Load testing: This type of testing checks the performance of an API by subjecting it to a heavy load. It ensures that the API is capable of handling a large number of users without performance issues.
  • Security testing: Security testing validates the security measures of the API and ensures it is well protected against attacks like SQL injections, cross-site scripting (XSS), and cross-site request forgery (CSRF).
  • Penetration testing: This type of testing is designed to find vulnerabilities in the API. It helps identify security flaws and areas that need improvement.

Performing all these types of testing can help ensure that an API is reliable, performant, and secure.

Protocols that can be Tested using API Testing

API testing can be performed to test a variety of protocols, including:

HTTP and HTTPS protocols (for web services)
JMS (Java Message Service)
SOAP and REST protocols (for web services)
SMTP (Simple Mail Transfer Protocol)
FTP (File Transfer Protocol)

And others depending on the system being tested.

Commonly Used Tools for API Testing

During API testing, there are various tools you can use to ensure that the API functions as expected and meets all requirements. Some of the most commonly used tools for API testing include:

  • Postman
  • SoapUI
  • JMeter
  • Katalon Studio
  • REST-assured

Each of these tools has its own features and benefits, so it's important to evaluate them carefully to determine which one is best suited for your specific testing needs.

Differences between API Testing and Unit Testing

When it comes to software testing, two terms that are often used are API testing and unit testing. While they may sound similar, they are two fundamentally different types of testing. Here are the main differences between them:

API testing involves verifying the functionality and performance of the application programming interface (API). This type of testing is focused on the behavior of the API and the interfaces that it provides to the application. It verifies that the API functions correctly when called, returns the expected results, and handles failure cases.

Unit testing, on the other hand, is focused on individual code units or components, typically at the function or method level. It verifies that each component of the code is working as expected and that it integrates correctly with other components of the system. Unit testing is typically performed by developers during the coding phase to catch bugs early in the development process.

In summary, API testing and unit testing are two different approaches to software testing, each with its own goals and objectives. While they may overlap in some areas, they are both essential for ensuring the quality and reliability of software.

Advantages of API Testing

API testing has several advantages, including:

  • Early detection of defects: API testing can help identify issues early in the development cycle, allowing developers to fix bugs before they become major problems. This can save time and money in the long run.

  • Improved quality: By testing APIs, developers can ensure that their code is functioning as expected and delivering the expected results. This can improve the quality of the product and increase customer satisfaction.

  • Increased test coverage: With API testing, developers can cover more test cases and scenarios than with traditional UI testing. This can help ensure that all possible use cases are covered and that the application is functioning as intended.

  • More efficient testing: Since API testing focuses on the functionality of the core application logic, it can be more efficient than UI testing. This means that developers can test more thoroughly in less time.

// Example code for API testing using Postman
// Ensure to have a valid API key and endpoint
const apiKey = 'your_api_key';
const endpoint = '';

// Set up POST request with request body
pm.request.url = `${endpoint}/users`;
pm.request.method = 'POST';
pm.request.headers.add({key: 'Authorization', value: `Bearer ${apiKey}`});
pm.request.body.raw = JSON.stringify({name: 'John Doe', age: 30});

// Send request and test response
pm.sendRequest((err, res) => {
  pm.test('Response code is 201', () => {

  pm.test('User is created with correct details', () => {
    pm.expect(res.json().name).to.equal('John Doe');

Approach for API Testing

When testing an API, the following approach is generally followed: 1. Define the scope of testing, including the API endpoints and methods to be tested. 2. Prepare the test data and environment. 3. Write test cases to cover the expected functionality as well as negative scenarios. 4. Execute the test cases and record the results. 5. Analyze the test results and identify any failures or issues. 6. Debug and fix any issues found during testing. 7. Retest to ensure that the issues have been resolved. 8. Automate the API tests for regression testing and continuous integration.

By following this approach, testers can ensure that the API performs as expected and that it meets the requirements and specifications.

What needs to be verified in API testing?

In API testing, the following things must be verified:

1. The request and response objects are properly formatted.

2. The API returns the expected response for various input parameters.

3. The API handles errors and exceptions gracefully.

4. The response time of the API is within acceptable limits.

5. The API documentation accurately reflects the behavior of the API.

6. The API can handle different types of data.

7. The API properly handles authentication and authorization.

8. The API integrates with relevant third-party APIs.

9. The API is secure and protected against potential threats.

In summary, API testing requires comprehensive tests to ensure that the API meets the functional, performance, security, and integration requirements.

Best Practices for Writing Test Cases

Writing clear, concise, and comprehensive test cases is crucial for effective software testing. Here are some best practices that you should follow when writing test cases:

1. Make sure that each test case is objective and verifies one specific requirement or functionality. 2. Use proper naming conventions and avoid using abbreviations or vague terms. 3. Include a description of the test case that clearly explains its purpose and expected outcome. 4. Provide all necessary steps to execute the test case and reproduce the test environment. 5. Use simple and easy-to-understand language, and avoid technical jargon. 6. Use a consistent format for all test cases to maintain consistency and clarity. 7. Prioritize test cases based on their criticality and impact on the system. 8. Re-review and update test cases regularly to ensure their relevance and effectiveness. 9. Use a test management tool to organize and track the test cases and their execution status.

Understanding Black Box Testing

Black box testing is a type of software testing where the tester has no internal knowledge of the system being tested. The tester only interacts with the user interface and evaluates the system's functionalities, inputs, and outputs, without any knowledge of its internal workings. The goal of black box testing is to identify any errors, defects, or inconsistencies in the system's functionality from an end-user's perspective.

Defining Test Data

Before writing and running tests, it's important to define the test data that will be used. Test data should be representative of the expected use cases and scenarios for the application. This includes both valid and invalid data, as well as edge cases.

// Example test data for a login form
const validCredentials = {
  username: "johndoe",
  password: "password123",

const invalidUsername = {
  username: "",
  password: "password123",

const invalidPassword = {
  username: "johndoe",
  password: "",

In this example, we have defined test data for a login form. We have a set of valid credentials, as well as invalid data for both the username and password fields. This will allow us to test various scenarios and ensure that the application handles them appropriately.

Defining Test Coverage

Test coverage refers to the extent to which a software application has been tested. It is measured by identifying the source code coverage, requirements coverage and risk coverage of the testing process. Code coverage involves determining which lines or statements of code have been executed during testing. Requirements coverage ensures that all of the application's requirements have been tested. Risk coverage involves identifying and testing the most critical and high-risk areas of the application. Defining test coverage is important for ensuring that all aspects of the application are thoroughly tested and potential issues are identified before release.

Is Coding Knowledge Required for Performing API Testing?

No, it is not always necessary for an API tester to have coding knowledge to perform API testing. However, having basic knowledge of coding and programming concepts can be helpful in understanding the functionality of an API and writing test scripts. It also allows the tester to identify and report potential issues more efficiently. Ultimately, the level of coding knowledge required for API testing depends on the complexity of the API being tested and the specific requirements of the testing project.

What is the process for reviewing API specifications?

To review API specifications, follow these steps:

1. Gather all relevant documents
2. Review the documents for completeness and accuracy
3. Verify that the API follows industry standards and best practices
4. Identify any potential design or implementation issues
5. Provide feedback and suggestions for improvement
6. Iterate on the review process until all issues are resolved
7. Approve the API specification for implementation

Understanding Latency in API Testing

Latency in API testing refers to the time delay between the sending of a request to an API and the receipt of a response from it. This delay can cause problems in real-time application environments where responses need to be received quickly. In API testing, measuring and optimizing latency is important to ensure that the API meets its performance requirements. By using tools such as load testing, endpoint monitoring, and diagnostics, testers can identify and remedy latency issues in APIs.

Understanding Throughput in Performance Testing

Throughput in performance testing refers to the number of units of data that can be processed by a system in a given amount of time. It is often used as a measure of a system's efficiency and capacity to handle a high volume of transactions or requests. In other words, throughput is a metric that helps to determine how much work a system can perform in a given period. It is typically measured in units of data per unit of time, such as bytes per second or requests per minute. In performance testing, measuring throughput can help identify potential bottlenecks in a system and optimize its performance.

API Functional Testing Interview Questions for Experienced

One important aspect of API testing is documentation. Here's how to document API functionality and the tools available for the job:

// Begin code

1. Start by identifying the purpose of the API and its intended audience.
2. Write clear and concise documentation that explains the API's functionality, inputs, and outputs.
3. Use examples and sample code whenever possible to illustrate how the API works.
4. Include information on error handling and status codes.
5. Document security and encryption measures, if applicable.
6. Use a documentation tool that supports API-specific features, such as Swagger, RAML, or Apiary.
7. Make the documentation accessible and easy to navigate.

// End code

By following these steps and utilizing the right tools, you can create thorough and effective documentation for an API's functionality.

The main difference between API testing and UI testing

When it comes to testing applications, there are different types of tests that can be performed. Two of the most important types are API testing and UI testing. The main difference between them is that API testing involves testing the functionality of an application's API, while UI testing involves testing the user interface of an application.

API testing is focused on making requests to an API endpoint and then verifying that the response is correct. This means that the API is tested without any user interface, as the focus is on the data that is returned by the API. API testing can be done at any point in the development phase, as it checks the functionality that is to be carried out by the API.

On the other hand, UI testing involves testing the user interface of an application. This means that testers are concerned with how the application looks and functions from the user's perspective. UI testing is typically done towards the end of the development phase, after most of the functionality has been implemented, and focuses on the user experience.

In summary, the main difference between API testing and UI testing is that API testing is focused on the functionality of an application's API, while UI testing is focused on the application's user interface.

Major Challenges in API Testing

When it comes to API testing, there are several challenges that testers might face. Here are some of the major blockers:

  • Lack of Documentation: APIs that have poor or incomplete documentation can make testing difficult, as testers might not know the expected behavior of the API.
  • Complexity: APIs can be complex, with multiple endpoints to test. This can make it difficult for testers to ensure that all aspects of the API are functioning correctly.
  • Time Constraints: Testing an API can be time-consuming, especially if the API is large or complex. This can lead to testers rushing through the testing process, which can result in missed bugs and issues.
  • Data Security: APIs often deal with sensitive data, such as personal customer information. Ensuring that this data is secure can be a challenge for testers.
  • Integration: APIs may need to be integrated with other systems and APIs, which can create additional challenges for testers.

To overcome these challenges, it's important to have a clear understanding of the API being tested, to prioritize testing efforts based on the most critical parts of the API, and to use testing tools and frameworks that can help streamline the testing process.

Principles to Follow for API Testing

API testing is an essential part of software testing, and it requires following fundamental principles to achieve success:

1. Test for all possible inputs: Testing should cover all possible inputs, including edge cases and invalid inputs.

2. Test for responses and error codes: It's essential to check responses and error codes to ensure the API responds correctly.

3. Test for security: API testing should be performed to identify any security vulnerabilities.

4. Test for performance: Test API performance under different conditions to ensure it meets requirements.

5. Test for reliability: The API should be consistent in providing accurate responses over time.

6. Test for compatibility: APIs should be compatible with different systems and software.

7. Test for scalability: It's crucial to test API scalability to determine its ability to handle increased traffic.

8. Test for documentation: API documentation should be accurate, up-to-date, and easy to use for developers.

Different Types of Bugs Found in API Testing

During API testing, the following types of bugs may be identified:

1. Functional Bugs:

API functional bugs can occur in case of incorrect data formats and parameters, inappropriate error handling, incorrect data validation, and incorrect HTTP response codes.

2. Performance Bugs:

Performance bugs in APIs include issues with latency, throughput, and API response time. Poor performance can negatively impact user experience and scalability.

3. Security Bugs:

APIs may be vulnerable to security risks such as SQL injection, malware, cross-site scripting, and electronic eavesdropping. Security bugs must be identified early on and mitigated to avoid damage.

4. Authorization Bugs:

Authorization bugs can arise when APIs do not properly implement user authentication and access control. This can lead to unauthorized access to sensitive user data.

5. Error Handling Bugs:

Error handling bugs can lead to erroneous error messages, missing error messages, or incomplete error messages, resulting in difficulty troubleshooting and identifying issues.

6. Compliance Bugs:

If APIs do not comply with industry standards, protocols, and regulations, their functioning may be affected. This can result in issues such as non-compliance with GDPR, PCI, and WCAG 2.0 guidelines.

It is vital to identify, document, and fix all bugs as soon as possible to provide users with a seamless and secure API experience.

Understanding Runscope

Runscope is an API performance testing tool that helps developers and operations teams monitor and validate API endpoints. It provides a simple, collaborative environment for teams to create and run API tests, as well as real-time insights into API performance and reliability.

Why is Caching Mechanism Important?

Caching mechanism is important because it can significantly improve the performance and speed of an application. By storing frequently used data in a cache, the application can access the data quickly, without having to retrieve it from a slower data source, such as a database or file system.

Caching can also reduce the workload on servers, as fewer requests need to be processed and served. This can result in lower server costs and better scalability.

Overall, caching is a powerful optimization technique that can greatly enhance the user experience and efficiency of an application.

The Importance of Automated API Testing

Automated API testing is highly useful because it allows for efficient and reliable testing of API functionalities. With automated testing, problems in the API can be quickly identified and resolved, thereby saving time and effort. Additionally, automated testing allows for greater test coverage, meaning that a wider range of test cases can be executed to ensure the API is working as expected. This helps guarantee the quality and performance of the API, leading to improved customer satisfaction and a better user experience.

Understanding Input Injection

Input injection refers to a type of attack where an attacker can input malicious code or data into an application or system through input fields, such as login forms or search bars. This can happen when an application fails to properly validate or sanitize user inputs, allowing the attacker to manipulate and potentially exploit the system. Input injection can lead to various security vulnerabilities, including data breaches, theft, and manipulation. Proper input validation and sanitization practices can mitigate the risk of input injection attacks.

Understanding the API Test Environment

The API test environment refers to a simulated environment used to test API functionalities and performance before deploying the API to a live production environment. It allows developers to verify that the API works as intended, avoids potential issues, and ensures that the API meets the required standards. The test environment typically involves creating mock data and running scripts to test various scenarios to ensure that the API can handle different requests and provide accurate responses. This step is critical in ensuring that the API is reliable and functional before it goes live.

Can an API be hacked during testing?

When testing an API, it is possible to identify security vulnerabilities or weaknesses that could potentially be exploited by hackers. However, the purpose of testing an API is to identify and fix these issues before they can be exploited. It is important to thoroughly test an API for security and ensure that all necessary measures are in place to prevent unauthorized access and protect sensitive data.

How to Test API Security?

When it comes to API security testing, there are several steps to take to ensure that the API is secure from potential threats and attacks. Here are some tips to follow:

1. Authentication and Authorization Testing: Test whether the authentication and authorization mechanisms are implemented correctly and check if tokens or credentials are secure.

2. Data Validation Testing: Check whether data validation is properly implemented, including input validation, output validation, and sanitization techniques to avoid security risks such as SQL injection, cross-site scripting (XSS), and other types of injection attacks.

3. Encryption Testing: Verify whether the API uses encryption properly to protect confidential data.

4. Injection Testing: Test for injection vulnerabilities such as SQL or NoSQL injection.

5. Error Handling Testing: Test whether error messages are properly handled to avoid exposing sensitive information.

6. Performance Testing: Conduct performance testing to detect issues such as denial of service (DoS) attacks or large volume requests.

7. Code Review: Conduct a detailed review of the code to identify possible security weaknesses and vulnerabilities.

By following these steps and ensuring that all potential vulnerabilities are detected and addressed, you can help ensure the security of your API.

What is the Big Bang approach in testing?

The Big Bang approach is a software testing method where all the parts of an application are tested altogether at once instead of testing individual modules separately. In this approach, testers do not follow a structured testing process and jump directly into testing the whole application. The Big Bang approach is mostly used in small applications with minimum or no complexity, and it is not recommended for larger applications with complex functionalities. This approach can save time but can also lead to missing crucial defects in the application.

Performing API Load Testing

API load testing involves simulating high traffic to the API to test its performance under pressure. Here are the steps to perform API load testing:

  1. Identify the important APIs to be tested.
  2. Create a test plan with a list of tests to be performed and expected outcomes.
  3. Use a load testing tool such as Apache JMeter or Gatling to create test scenarios simulating high traffic.
  4. Execute the test scripts with various user loads to find the breaking point of the API.
  5. Analyze the results to find bottlenecks in the system and identify areas for improvement.
  6. Repeat the load testing after implementing optimizations and verify if the performance has improved.


// Identify important APIs to be tested
const importantAPIs = ['api/example1', 'api/example2', 'api/example3'];

// Create a test plan
const testPlan = {
  'api/example1': {
    tests: [
      { userLoad: 100, expectedOutcome: 'success' },
      { userLoad: 500, expectedOutcome: 'success' },
      { userLoad: 1000, expectedOutcome: 'failure' }
  'api/example2': {
    tests: [
      { userLoad: 100, expectedOutcome: 'success' },
      { userLoad: 500, expectedOutcome: 'success' },
      { userLoad: 1000, expectedOutcome: 'failure' }
  'api/example3': {
    tests: [
      { userLoad: 100, expectedOutcome: 'success' },
      { userLoad: 500, expectedOutcome: 'success' },
      { userLoad: 1000, expectedOutcome: 'failure' }

// Use a load testing tool to simulate high traffic
const loadTester = new LoadTester('');
for (let api of importantAPIs) {
  for (let test of testPlan[api].tests) {
    loadTester.addTest(api, test.userLoad);

// Execute the test scripts
const results = loadTester.runTests();

// Analyze results to identify bottlenecks and areas for improvement
const failures = results.filter(res => res.outcome === 'failure');
if (failures.length > 0) {
  console.log(`API failed under ${failures[0].userLoad} user load on ${failures[0].api}`);
} else {
  console.log('API was able to handle load successfully');

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