UML & Modeling • Sequence DiagramsEasy⏱️ ~2 min
What Are Sequence Diagrams?
Definition
Sequence Diagrams are a type of UML (Unified Modeling Language) interaction diagram that shows how objects interact with each other in a particular scenario, focusing on the order and timing of messages exchanged between participants.
What Problem Does It Solve?
When designing complex object interactions, it becomes difficult to understand the flow of control and data between objects. Sequence diagrams solve this by:
- First, visualizing the temporal ordering of method calls between objects
- Second, identifying responsibilities and message flows in a specific use case
- Third, documenting API (Application Programming Interface) interactions and protocols
- Fourth, catching design flaws before implementation by seeing the entire flow
Interview Tip: Sequence diagrams answer "how" objects collaborate for a specific scenario, while class diagrams answer "what" the structure is. Use sequence diagrams to explain parking lot checkout flow or elevator request handling.
When to Use Sequence Diagrams
Use sequence diagrams when you need to: design a specific workflow (like "user reserves book"), explain complex interactions during interviews, validate that your class design supports required operations, or document the order of operations for asynchronous flows. However, if you only need to show static structure, use class diagrams instead. If you need to show state transitions within one object, use state diagrams instead.
💡 Key Takeaways
✓Shows temporal ordering of messages between objects in a specific scenario
✓Focuses on behavior and interaction flow, not static structure
✓Reads top-to-bottom showing time progression
✓Essential for explaining complex workflows in LLD interviews
✓Complements class diagrams by showing dynamic behavior
📌 Examples
1Parking lot: Customer enters, gets ticket, parks, exits, pays fee
2Library system: User searches book, reserves, checks out, returns
3Elevator: User presses button, controller assigns elevator, elevator moves, doors open