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15 UML Sequence Diagrams and Activity Diagrams
The Unified Modeling Language (UML) offers various diagramming techniques, two of which are com-
monly employed in requirements specification to model the dynamic aspects of a system: sequence
diagrams and activity diagrams. Both provide a graphical notation that can enhance a textual specification
(e.g., based on use cases). They are optional in processes like the UP; Hence, the requirements engineer
has to make a decision whether to include them within a specific project. Sequence diagrams are the UML
version of message sequence charts (MSCs), originally developed for the telecom industry. In requirements
engineering, system sequence diagrams (SSDs) model the system as a black box and focus on the events
generated by actors and how they are handled by the system. UML activity diagrams are data/control flow
graphs, similar to “old-fashioned” flow charts traditionally used in business process modeling. They can
enhance textual use cases descriptions by modeling all extensions and alternative scenarios in a single
diagram, which can help understanding a complex use case. Typically, these diagrams are added in later
iterations within a project. However, since they add more artifacts to the specification, they also add more
traceability challenges.
Learning Objectives
Learning Objective 1:
Learn how to enhance textual specifications with graphical representations based on UML sequence
and activity diagrams
Understand which diagram type to use, and when
Understand the traceability challenges when working with diagramming techniques
Understand the concept of text-based modeling for UML diagrams
Learning Objective 2:
Understand the difference between a sequence diagram and a system sequence diagram (SSD)
Learn the UML notation for sequence diagrams
Learn how to map use case scenarios to sequence diagrams
Understand how to model conditional messages and repetitions
Learning Objective 3:
Learn the UML notation for activity diagrams
Learn the use of partitions (swimlanes) for structuring activity diagrams
Learn the connections between use cases and activity diagrams
Understand how to model parallel activities, branches, repetitions, sub-activities, signals, and guard
conditions in activity diagrams
Learn guidelines for developing good activity diagrams
Notes and Further Reading
UML sequence diagrams for modeling the behaviour of a system, in particular based on scenarios, are
covered in [
, Chapter 13.1]. Pohl provides a similar introduction in [
, Chapter 11.5]. Larman has more
examples and a good description of the role of SSDs in the overall development process [
, Chapter 10].
For an introduction to UML activity diagrams, read [
, Chapter 28]. Pohl also briefly covers them in [
Chapter 11.6]. The UML guide [
] covers both sequence and activity diagrams, but note that it does not
specifically address their role in requirements engineering.
Modeling activities in the form of business processes or workflows is a much broader topic that we
cannot cover in the scope of this lecture. The Business Process Model and Notation (BPMN) is an example
for a graphical language used in this domain. While graphically similar to UML activity diagrams, it also
targets the execution of the modeled workflow, in particular based on the Business Process Execution
Language (BPEL). The open source framework Camunda (
) supports BPMN and
is widely used in industry, in particular in the finance and insurance domains. Not based on BPMN, but
very popular for modeling workflows in science and technology, in particular biotechnology, is the Taverna
Workbench (

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