IT

UML tools

• Astah Community is freeware (limited relatively to the full version) and is one of my favorite tools to draw UML diagrams. See my Astah Community tips article.

Sequence Diagrams

Good articles:

• Best for a quick overview: A Quick Introduction to UML Sequence Diagrams (check also: Gallery of UML sequence diagrams) (pdf)
• A more verbose article: UML basics: The sequence diagram (pdf)

Sequence diagrams display what messages are sent between a system's objects as well as the order in which they occur.
The vertical axis (top down) is the time and the horizontal axis the object (instances of classes) or roles that the messages are sent to.

• Frame: the frame element was added to the notation in UML 2 diagrams and is used as a basis for many other diagram elements. The frame element is optional in UML diagrams and one of its usages is to define a graphical boundary of a diagram.
  The frame contains:
  • a label on the top left corner: this should follow the UML naming specification of "Diagram Type Diagram Name" (ex.: sd MyLabel - "sd" for "sequence diagram" followed by the name of your diagram);
  • the actual UML diagram within its body (the enclosing rectangle);

• Lifelines: represent either roles or object instances that participate in the sequence being modeled.
  Lifeline standard naming is "Instance Name : Class Name".But any of the parts can be omitted, ex.:
  
  • "student:User";
  • just "student" if at design time we dont have a type defined yet;
  • just the type "User" if we dont want to go into much detail;