System Sequence Diagrams - SSD - Applying UML and Patterns

Oct 25, 2026

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Slides outline how a system sequence diagram (SSD) represents input/output events for a single use-case scenario, treating the system as a black-box. It explains that SSDs are derived from use cases, focus on main success scenarios and key alternatives, and are later used as input for object design. The material then introduces system operations and operation contracts, including template elements and a worked example for enterItem(), culminating in an SSD example for the Process Sale scenario.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages17

Reading Time43 min

AccessFree to download

System Sequence Diagrams

What an SSD shows

System events and system operations

Operation contract template

Operation contract for enterItem()

Example SSD for Process Sale Scenario

Dr. Michael Eichberg Software Engineering Department of Computer Science Technische Universität Darmstadt Software Engineering System Sequence Diagrams The following slides make extensive use of material from: Applying UML and Patterns, 3rd Edition; Craig Larman; Prentice Hall A system sequence diagram (SSD) illustrates input and output events. System Sequence Diagram | 2 • An SSD shows – for one particular scenario of a use case – • the events that external actors generate, • their order, and • inter-system events • The system is treated as a black-box • SSDs are derived from use cases; SSDs are often drawn for the main success scenarios of each use case and frequent or complex alternative scenarios • SSDs are used as input for object design System Events and System Operations Object-oriented Design | 3 • System operations are the operations that the system as a blackbox component offers in its public interface. These are high-level operations triggered by an external input event / system event generated by an external actor • During system behavior analysis, system operations are assigned to a conceptual class System The system operations are shown in the system sequence diagram (SSD) . Object-oriented Design | 4 • To provide more analysis detail on the effect of the system operations implied use cases,(System) Operation Contracts may be considered Process Sale Scenario | loop [more items]
| | | --- | --- | :System | makeNewSale
enterItem(itemId, quantity)

description, price, total | | --- | | endSale
total with taxes | | | :Cashier Operation Contract Template Object-oriented Design | 5 Operation: Name of the operation and parameters . Cross References: Use cases this operation can occur with. Preconditions: Noteworthy / non-trivial assumptions about the system or objects in the domain model before execution of the operation. Postconditions: The state of the objects in the domain model after completion of the operation. Domain model changes include: ▶ instances created, ▶ associations formed or broken,▶ attributes changed.[Postconditions should be stated in the Helpful when assigning responsibilities to classes (More details will follow). Operation Contract for “enterItem()” Object-oriented Design | 6 Operation: enterItem(itemId: ItemId, quantity: Integer) Cross References: Use Cases: Process Sale Preconditions: There is a sale underway. Postconditions: ▶ A SalesLineItem instance (SLI) was created. (instance creation) ▶ SLI was associated with the current Sale. (association formed) ▶ SLI was associated with a ProductDescription, based on itemId match. (association formed) Example of an SSD for the Process Sale Scenario System Sequence Diagram | 7 Use Case: Process Sale Scenario-Main Success Story 1. Cashier starts new sale 2. Cashier enters item identiﬁer 3. System records sale line item and presents item description, price and running total Steps 2 and 3 are repeated until all items are processed. 4. System presents total with taxes calculated 5. Cashier tells Customer the total and asks for payment 6. Customer pays and System handles payment Example of an SSD for the Process Sale Scenario System Sequence Diagram | 8 SSDs are drawn using UML’s sequence diagram notation. The name of each event should state the intention (e.g.“enterItem(itemId)” vs.“scan(itemId)”) . Message Order :System | makeNewSale
enterItem(itemId, quantity)

description, price, total | | --- | | endSale
total with taxes | | makePayment (amount)
change due, receipt | | | :Cashier Communication Partners Process Sale Scenario Example of an SSD for the Process Sale Scenario System Sequence Diagram | 9 SSDs are drawn using UML’s sequence diagram notation. The name of each event should state the intention (e.g.“enterItem(itemId)” vs.“scan(itemId)”) . Process S

FAQs

What does a system sequence diagram (SSD) show?

An SSD shows, for one use-case scenario, the events external actors generate, their order, and inter-system events. The system is treated as a black-box.

How are SSDs derived and what are they used for?

SSDs are derived from use cases and are often drawn for main success scenarios and frequent or complex alternative scenarios. They are used as input for object design.

What is included in an operation contract for a system operation?

An operation contract includes the operation name and parameters, cross references to use cases, preconditions before execution, and postconditions describing domain model changes such as created instances and formed or broken associations.

Oct 25, 2026

Upload by SANS