Establishing an effective testing environment is crucial because it simulates the production environment in which the software will be used. This allows testers to identify issues that may not appear in a different environment and ensures that the software behaves as expected in real-world conditions.

Complexity is a software quality metric that is often used to identify the complexity of a software program. It measures how intricate or convoluted the code is, which can impact maintainability and understandability.

HTTP (Hypertext Transfer Protocol) is commonly used for making API requests on the web. It is a protocol that allows data to be transferred between a client and a server, making it a fundamental part of web-based API communication.

System testing ensures correct behavior in all intended environments by simulating real-world scenarios, which helps identify any issues or conflicts that might occur in different situations. This comprehensive testing approach checks how the software responds to a variety of conditions, ensuring robustness.

The implementation phase of the SDLC is where the system is actually built or purchased. It involves coding, development, and integration of the components to create the software system.

Continuous customer or stakeholder involvement in the Agile model can increase the need for quality assurance. Their frequent feedback and evolving requirements necessitate ongoing testing and validation to ensure that the product meets their expectations.

User-Centered Design is a graphical UI design approach that prioritizes usability. It focuses on creating interfaces that are intuitive and user-friendly, ensuring that users can easily interact with the system.

For an e-commerce application that requires multiple services like payment, user authentication, and product management, a microservices architecture is suitable. Microservices allow each component to be developed and scaled independently, making it easier to manage and maintain different parts of the application.

In software engineering, verification ensures that the product is built according to the specified requirements, making it "right." Validation ensures that the product meets the user's needs and is "built right." These two processes work together to ensure the software's quality and alignment with both requirements and user expectations.

In a Big Bang model, if a major flaw is discovered, the team might have to start from scratch, which can result in wasted effort. The model's lack of incremental development and detailed planning can make it challenging to adapt and rectify critical issues efficiently.