What is the difference between UML Use Case and Sequence Diagram, and when is each used?

UML Use Case and Sequence Diagram serve different purposes and are used at different stages of design.

• Use Case Diagram describes the functional requirements of the system from the perspective of users (actors). It shows what actions (use cases) are available to users and how they interact with the system. This provides a high-level overview of functionality.

• Sequence Diagram shows the interaction between objects or components of the system over time. It details the order of method calls, message exchanges, and events that occur during the execution of a specific use case or process.

When to use:

• Use Case Diagram — in the early stages of requirements analysis to understand what the system should do and who its users are.
• Sequence Diagram — during the design phase to detail the interaction logic of components for implementing a specific scenario.

Example: A Use Case might show that a user can "Place an Order," while a Sequence Diagram describes how message exchanges occur between the interface, server, and database during the order placement.