UML Sequence Diagram Tutorial

UML sequence diagrams are used to document the interactions between the components of a system. Time is represented in the vertical direction showing the sequence of interactions of the header elements, which are displayed horizontally at the top of the diagram. Although UML sequence diagrams are used to describe object-oriented software systems, they are also extremely useful as system engineering tools to design system architectures, in business process engineering as process flow diagrams, as message sequence charts and call flows for telecom/wireless system design, and for protocol stack design and analysis.

What is a UML sequence diagram?

UML sequence diagrams are used to represent or model the flow of messages, events and actions between the objects or components of a system. They are used primarily to design, document and validate the architecture, interfaces and logic of the system by describing the sequence of actions that need to be performed to complete a task or scenario. UML sequence diagrams are useful design tools because they provide a dynamic view of the system behavior which can be difficult to extract from static diagrams or specifications.

Sequence Diagram Drawing Elements

This tutorial describes the basic drawing elements used in sequence diagrams and when they are used. These are the diagram elements that are supported by the Sequence Diagram Editor tool, some are not part of the UML specification and may not be supported by other UML tools.

Sequence Diagram Header Elements

The header portion of the sequence diagram represents the components or objects of the system being modeled and are laid out horizontally at the top of the diagram. See an example sequence diagram here.

Actor	Represents an external person or entity that interacts with the system
Object	Represents an object in the system or one of its components
Unit	Represents a subsystem, component, unit, or other logical entity in the system (may or may not be implemented by objects)
Separator	Represents an interface or boundary between subsystems, components or units (e.g., air interface, Internet, network)
Group	Groups related header elements into subsystems or components

Sequence Diagram Body Elements

Action	Represents an action taken by an actor, object or unit
Asynchronous Message	An asynchronous message between header elements
Block	A block representing a loop or conditional for a particular header element
Call Message	A call (procedure) message between header elements
Create Message	A "create" message that creates a header element (represented by lifeline going from dashed to solid pattern)
Destroy Element	Represents the destruction of a header element
Destroy Message	Represents the destruction of a header element as a result of a call from another element
Diagram Link	Represents a portion of a diagram being treated as a functional block. Similar to a procedure or function call that abstracts functionality or details not shown at this level. Can optionally be linked to another diagram for elaboration.
Else Block	Represents an "else" block portion of a diagram block
Flow Note	Documentation note that is automatically formatted to flow after previous elements
Free Note	Documentation note that is free-flowing and can be placed anywhere in the diagram (can also be anchored relative to a flow element)
Message	A simple message between header elements
Page Break	A page break in the diagram
Return Message	A return message between header elements
Scenario Start	Start of a scenario (set of alternatives)
Scenario Case	Start of an alternative or case in a scenario
Scenario End	End of a scenario
State	A state change for a header element
Steady State	A steady state in the system
Timer Start	Start of a timer for a particular header element
Timer Stop	Stop of a timer for a particular header element
Timer Expiration	Expiration of a timer for a particular header element