Functional testing and non-functional testing are two essential approaches to ensuring the quality and reliability of software systems. While both aim to validate the software, they focus on different aspects and employ distinct methodologies. Here's a comprehensive comparison between the two:

Functional Testing:

Functional testing verifies whether the software functions as intended and meets its specified functional requirements. It examines individual functions or features of the software to ensure they produce the correct outputs for given inputs.

The primary goal of functional testing is to validate the behavior of the software application according to the functional specifications provided by stakeholders.

Types Functional Testing:

1. Unit Testing: Tests individual units or components of the software in isolation.
2. Integration Testing: Tests the interaction between integrated components or modules.
3. System Testing: Tests the entire system as a whole to verify that it meets requirements.
4. User Acceptance Testing: Validates whether the software meets user requirements and is ready for release.

 Functional Testing
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        |         |            |
   Unit Testing Integration   System
                              Testing
                                |
                            User Acceptance
                              Testing

Example: Consider a banking application's "Transfer Funds" feature. Functional testing would verify that users can initiate transfers from one account to another, the correct amounts are deducted from the sender's account and added to the recipient's, and appropriate error messages are displayed for invalid inputs or insufficient funds.

Process:

1. Test Planning: Define test objectives, scope, and strategies based on functional requirements.
2. Test Design: Create test cases covering various scenarios, including normal and boundary cases.
3. Test Execution: Execute test cases, observe results, and document any deviations from expected behavior.
4. Defect Reporting: Report identified issues with detailed descriptions and steps to reproduce.
5. Regression Testing: Re-run functional tests to ensure fixes haven't caused new issues.

Non-Functional Testing:

Non-functional testing evaluates the aspects of a software system beyond its functional requirements, focusing on attributes such as performance, reliability, usability, security, and compatibility.

The primary goal of non-functional testing is to assess how well the software performs under specific conditions and to ensure it meets quality criteria related to its operation.

Types Non-Functional Testing:

1. Performance Testing: Assess system responsiveness, scalability, and resource usage under various loads.
2. Reliability Testing: Verify the system's ability to perform consistently and accurately over time.
3. Usability Testing: Evaluate the system's user-friendliness, accessibility, and intuitiveness.
4. Security Testing: Identify vulnerabilities and weaknesses in the system's security measures.
5. Compatibility Testing: Check the system's compatibility with different environments, devices, and operating systems.

Non-Functional Testing
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   |   |   |   |   |
Performance  Reliability  Usability  Security  Compatibility
   |           |            |          |           |
Load      Error Handling    UI      Penetration   Cross-
Testing                     Testing    Testing    Platform
                                               Testing

Example: In performance testing, a web application undergoes load testing to assess its response time and scalability under heavy user traffic. By simulating thousands of concurrent users accessing the application, testers can identify performance bottlenecks and optimize system resources accordingly.

Process:

1. Test Planning: Define objectives, scope, and strategies based on non-functional requirements.
2. Test Design: Create test cases covering various non-functional aspects, such as load scenarios for performance testing or security vulnerabilities for security testing.
3. Test Execution: Execute test cases and measure system behavior against non-functional criteria.
4. Analysis and Reporting: Analyze test results and report findings, including performance metrics or security vulnerabilities.
5. Optimization: Implement optimizations or fixes based on test findings to improve non-functional aspects of the software.

Difference between Functional Testing and Non-Functional Testing

1. Functional testing focuses on validating what the software does, ensuring it meets specified functional requirements. Non-functional testing focuses on how well the software performs, assessing attributes such as performance, reliability, usability, security, and compatibility.

2. Functional testing validates individual functions or features of the software, whereas non-functional testing assesses broader aspects related to the software's operation and quality.

3. Functional testing aims to ensure that the software behaves as expected according to functional specifications. Non-functional testing aims to assess and improve the software's performance, reliability, usability, security, and compatibility.

4. Functional testing employs techniques such as unit testing, integration testing, system testing, and acceptance testing. Non-functional testing employs techniques such as performance testing, reliability testing, usability testing, security testing, and compatibility testing.

5. while functional testing ensures that the software functions correctly according to specified requirements, non-functional testing evaluates the software's performance, reliability, usability, security, and compatibility to ensure it meets quality standards and user expectations. Both types of testing are essential for delivering high-quality software products.