This article provides an overview of UML sequence diagrams, a type of interaction diagram used in software engineering to model the flow of messages between objects within a system. It explores the notation, strengths, and limitations of these diagrams, highlighting their utility in visualizing dynamic system behavior while also acknowledging their constraints in representing parallel processes.
Alexander S. Ricciardi
January 24, 2025
Unified Modeling Language (UML) sequence diagrams illustrate the sequence of messages in an interaction (IBM, 2021). They are a form of interaction diagram used in Software Engineering (SE) to model the interactions between objects in a single use case. This post provides an overview of the UML sequence diagrams and their strengths and limitations in software development.
UML Sequence Diagrams Overview
UML sequence diagrams provide a dynamic illustration of object interactions within a system. In SE, they are usually based on UML class diagrams and are used primarily to show the interactions between objects (classes) in the chronological order in which those interactions occur. In the problem space, they are used to model the behavior of systems from the actors’ (users) perspective (Unhelkar, 2018). In solution space, the diagrams are more detailed and used to illustrate objects' interactions and their messages in detail such as the sequence of those messages, a parameter list within messages, and the return values of the messages. The figure below illustrates the different elements that can be found in a UML sequence diagram.
Figure 1
UML Sequence Diagram Notation
Note: The figure illustrates the different elements that can be found in UML sequence diagrams. Made with the Lucidchart app.
The table below describes some of the main notation elements that can be found in UML sequence diagrams.
Table 1
UML Sequence Diagram Notation Description
Note: The table describes some of the main notation elements that can be found in UML. A stereotyped object or class is an object with a specific purpose or functionality defined by a stereotype (e.g. boundary, control, and entity). The data was collected from several sources (Visual-Paradigm n.d.; Unhelkar, 2018; rmb1905, 2009).
Strengths and Limitations of UML Sequence Diagrams
UML sequence diagrams are excellent for visualizing the dynamic behavior of a system, particularly the interactions between objects or components over time. They can be used for analyzing and designing object interactions within a system, they are especially helpful in identifying missing elements in class diagrams and they can be used for documentation. See Table for a description of diagram strengths and limitations. For example, when designing a shopping cart system, they are especially useful at showing in detail the user (actor) interacting with the shopping cart and checkout system (objects). A sequence diagram can clearly depict the sequence of messages, for instance, addItem, viewCart, initiateCheckout, processPayment, etc., that is the exchanges between the user, the shopping cart, and the checkout system.
However, sequence diagrams also have their limitations, they are not well-suited for representing highly concurrent or parallel processes. Even though UML 2 implemented the parallel frame element, the dynamic nature of the diagrams makes it difficult to depict all the concurrent and parallel interactions that may exist in a complex system. In other words, trying to illustrate many concurrent and parallel interactions in a single diagram would make it extremely large and cluttered. Ultimately, it may require the generation of several additional sequence diagrams. Activity diagrams are better suited for such scenarios as they support parallel flows by using fork and join nodes.
Table 2
Strengths and Limitations of UML Sequence Diagrams
Note: The table describes the strengths and limitations of UML sequence diagrams. The data is from “Chapter-12 Interaction Modeling with Sequence Diagrams” by Unhelkar (2018).
To summarize UML sequence diagrams are a useful tool for illustrating dynamic interactions of objects within a system. In SE, they are usually based on UML class diagrams and they are used in the problem space to model the behavior of systems from the user perspective, in solution space, the diagrams are more detailed and used to illustrate objects' interactions such as the sequence of the message, a parameter list within messages, and the return values of the messages. They help identify missing elements, detail object interactions, and model object deletion. However, the diagrams have limitations such as representing single flows or threads, they do not illustrate or model well parallelism, conditional logic, or loops, and using a single sequence diagram for an entire complex system can lead to confusion and errors. Nonetheless, they are an indispensable and powerful tool in the software engineer's toolkit, they are essential for designing, analyzing, and documenting interactions within a system.
References:
IBM (2021, March 5). Sequence diagrams. Rational software modeler. IBM Documentation. https://www.ibm.com/docs/en/rsm/7.5.0?topic=uml-sequence-diagrams
rmb1905. (2009, March 10). 10.09_Fragments - parallels [Video]. YouTube. https://www.youtube.com/watch?v=mtVVf1mhYKk
UML Diagrams Org. (n.d.). UML sequence diagrams. UML Diagrams Org. https://www.uml-diagrams.org/sequence-diagrams.html#execution
Unhelkar, B. (2018). Chapter 12 — Interaction modeling with sequence diagrams. Software engineering with UML. CRC Press. ISBN 9781138297432
Visual Paradigm (n.d.). Sequence diagram. Visual Paradigm. https://www.visual-paradigm.com/VPGallery/diagrams/Sequence.html