CS302: Software Engineering

Dynamic UML models

• While static models (use cases, class diagram, component diagram) are done for the system as a whole, dynamic models are done only for key components
• State diagram
  • Specifies behavior of a single object 
  • Diagram has states and transitions only 
• Sequence diagram
  • Shows details of one scenario and messages that flow between objects/organizations in that scenario over time
  • Heavily used in standards
• Activity diagram 
  • Shows flow of logic, data, messages
  • Diagram has activities, decisions, forks, joins (parallel) 
  • Replaces flow charts
• Communication diagram
  • Shows flow of messages as a graph
  • Used as variant of sequence diagram 
• Others, as needed

State diagram example

An object (account in this example) can be in only one state at any time

State diagram exercise

• Model the state transitions of a student's registration in this class: 
  • Preregistered
  • Registered
  • Listener
  • Dropped 
  • Complete, incomplete (not resolved), etc.
• Remember that an entity can only be in one state at any time. It cannot be in two or more states.

State diagram solution

Activity diagram 

• Shows flow of messages, logic, actions 
• This is at a much higher level of abstraction than flow charts
  • Flow charts show logic for single method (if statements, loops, etc.) 
  • Activity diagrams show flow among objects

Activity diagram example

Activity diagram exercise 

• Draw an activity diagram for getting an apartment. Example activities are: 
  • Find roommates
  • Find apartment
  • Sign apartment lease
  • Get electric service 
  • Get phone or cable TV service
  • Get gas or oil heat account set up 
  • Obtain furniture
  • Move in 
• (Use this as a simple model of setting up a warehouse…)
• Use activities, decision nodes, fork/join nodes

Activity diagram solution

Sequence diagram (optional)

• Objects or entities are diagrammed at the top
• Each object's life is represented by a vertical line from creation to destruction
• Messages or events are diagrammed from the sending object to the receiving object, in the order in which they occur
• Responses may or may not be diagrammed, depending on complexity/obviousness 
• These are sometimes called ‘swim lane' diagrams
  • Swim lanes can be used in activity diagrams as well

Sequence diagram example

UML Summary

• Use UML while writing scenarios and narratives as an initial requirements document 
  • Diagram use cases, then refine them into scenarios
  • Focus on completeness of use cases
• Use UML component diagrams to list all system elements
  • Focus on completeness, and use to set system boundaries
• Prepare the initial data model (next lecture)
  • Add operations/methods to the entities, after understanding the data, to create a class diagram
• Use UML state diagrams, sequence diagrams and activity diagrams to specify objects and processes 
  • Prepare these selectively for complex or interesting objects
• UML is becoming a 'universal' language: staff coming to a project know it, which sharply reduces learning curve
  • Developers and analysts can both understand it readily
  • Consultants/analysts use UML even for analysis-only projects (as well as writing requirements and modeling data)
  • Business process execution language (BPEL) in Web lectures is UML extension to directly create systems

Source: Massachusetts Institute of Technology, https://ocw.mit.edu/courses/1-264j-database-internet-and-systems-integration-technologies-fall-2013/resources/mit1_264jf13_lect_7/
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