Sequence Diagrams: When to Use and Trade-offs

When to Use Sequence Diagrams
Sequence diagrams are valuable in specific situations but not universally applicable. Use them when:
Complex Interactions: When objects collaborate through multiple method calls. Example: Payment processing involving Order, PaymentGateway, Inventory, and NotificationService
Temporal Dependencies: When order of operations matters. Example: In elevator system, requestFloor() must happen before moveElevator(), which must precede openDoors()
Interview Communication: When explaining "how your design handles X scenario" during machine coding rounds. Example: "Let me draw how book reservation works..."
API Documentation: When documenting protocols between components. Example: Authentication flow between client, server, and database
When NOT to Use Sequence Diagrams
Wrong Context
Using sequence diagram to show static class relationships and inheritance hierarchies
→
Right Tool
Use class diagrams instead for structure. Use sequence diagrams only for behavior
First, don't use for simple CRUD (Create, Read, Update, Delete) operations. If interaction is just user → service → database, verbal explanation suffices
Second, avoid for showing all possible paths. Focus on primary scenario plus one error case, not exhaustive coverage
Third, skip for internal object logic. If method has complex internal algorithm, use pseudocode or activity diagram instead
Fourth, don't draw during initial brainstorming. Start with class diagram for structure, add sequence diagrams later for key flows
Alternatives and When They're Better
Diagram TypeUse WhenExample
Class DiagramNeed to show structure, relationships, attributesParking lot class hierarchy
State DiagramSingle object with complex state transitionsElevator states: Idle, Moving, DoorsOpen
Activity DiagramBusiness process with parallel flows and decisionsOrder fulfillment with branches
Component DiagramHigh-level module dependenciesService layer communicating with data layer
Interview Tip: In 45-minute machine coding rounds, draw sequence diagrams for 1-2 critical flows maximum. Spend time on class design first. If interviewer asks "how does checkout work," then sketch sequence diagram on the side.
Trade-offs to Discuss
Benefit: Validates that your class design actually supports required operations. You might discover missing methods or wrong responsibilities. Cost: Time-consuming to draw. In interviews, balance detail with speed.
Benefit: Makes temporal dependencies explicit, catching race conditions and ordering bugs. Cost: Can become cluttered with too many participants. Limit to 4-6 objects per diagram, otherwise split into multiple scenarios.
Benefit: Excellent communication tool for explaining complex interactions. Cost: Requires audience familiarity with UML notation, but if interviewer doesn't know UML, explain verbally instead of forcing diagram.

Diagram Type	Use When	Example
Class Diagram	Need to show structure, relationships, attributes	Parking lot class hierarchy
State Diagram	Single object with complex state transitions	Elevator states: Idle, Moving, DoorsOpen
Activity Diagram	Business process with parallel flows and decisions	Order fulfillment with branches
Component Diagram	High-level module dependencies	Service layer communicating with data layer

💡 Key Takeaways

✓Use for complex multi-object interactions with temporal dependencies

✓Skip for simple CRUD or static structure (use class diagrams)

✓Limit to 4-6 participants per diagram to avoid clutter

✓Draw 1-2 critical flows in interviews, not exhaustive coverage

✓State diagrams better for single-object state machines

📌 Examples

1Good use: Payment processing with Order, Gateway, Inventory coordination

2Bad use: Showing Vehicle inheritance hierarchy (use class diagram)

3Good use: Explaining elevator request handling to interviewer

4Overkill: Drawing sequence for simple getter/setter calls

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