The Agile Modeling (AM) Method

UML State Machine Diagrams: Diagramming Guidelines

UML state machine diagrams, formerly called state chart diagrams in UML 1, depict the dynamic behavior of an entity based on its response to events, showing how the entity reacts to various events depending on the current state that it is in. Create a UML state machine diagram to explore the nature of a:

  • Explore the complex behavior of a class, actor, subsystem, or component.
  • Model real-time systems.

There are guidelines for:

  1. General issues
  2. State
  3. Substates
  4. Transitions and Actions
  5. Guards

1. General

Figure 1. A UML state machine Diagram for a seminar during enrollment.

  1. Create A state machine When Behavior Differs Based on State. A Seminar object is fairly complex, reacting to events such a enrolling a student differently depending on its current state, as you see depicted in Figure 1.
  2. Place The Initial State In The Top-Left Corner
  3. Place The Final State In The Bottom-Right Corner

2. State Guidelines

A state is a stage in the behaviour pattern of an entity. States are represented by the values of the attributes of an entity. For example, in Figure 1 a seminar is in the Open For Enrollment state when it has been flagged as open and there are seats available to be filled.

  1. State Names Should be Simple but Descriptive
  2. Question “Black Hole” States. A black hole state is one that has transitions into it but none out.
  3. Question “Miracle” States. A miracle state is one that has transitions out of it but none into it.


3. Substate Modeling Guidelines

Figure 2. The complete lifecycle of a Seminar.

Figure 3. A top-level state machine for Seminar.


  1. Model Substates For Targeted Complexity. Figure 2models the entire lifecycle of a Seminar, depicting Figure 1as a collection of substates of a new Enrollment composite state, also called a superstate.
  2. Aggregate Common Substate Transitions
  3. Create a Hierarchy of State Machines for Very Complex Entities. Figure 3represents the top-level view andFigure 1depicts a more detailed view.
  4. Top-Level State Machines Always Have Initial and Final States

4. Transitions and Actions

A transition is a progression from one state to another and will be triggered by an event that is either internal or external to the entity being modeled. For a class, transitions are typically the result of the invocation of an operation that causes an important change in state, although it is important to understand that not all method invocations will result in transitions. An action is something, in the case of a class it is an operation, that is invoked by/on the entity being modeled.


  1. Name Software Actions Using Implementation Language Naming Conventions
  2. Name Actor Actions Using Prose
  3. Indicate Entry Actions Only When Applicable For All Entry Transitions
  4. Indicate Exit Actions Only When Applicable For All Exit Transitions
  5. Model Recursive Transitions Only When You Want to Exit and Re-Enter the State
  6. Name Transition Events in Past Tense
  7. Place Transition Labels Near The Source State
  8. Place Transitions Labels Based on Transition Direction. To make it easier to identify which label goes with a transition, place transition labels according to the following heuristics:
  • Above transition lines going left-to-right
  • Below transition lines going right-to-left
  • Right of transition lines going down
  • Left of transition lines going up


5. Guards

A guard is a condition that must be true in order to traverse a transition.

  1. Guards Should Not Overlap. The guards on similar transitions leaving a state must be consistent with one another. For example guards such as x <0, x=0, and x>0 are consistent whereas guard such as x <= 0 and x>= 0 are not consistent because they overlap.
  2. Introduce Junctions to Visually Localize Guards .In Figure 2you see that there are two transitions from Being Taught as the result of the student dropped event, whereas there is only one in Figure 3– the transitions are combined into a single one that leads to a junction point (the filled in circle).
  3. Guards Need Not Form a Complete Set
  4. Name Guards Consistently

Recommended Reading

Modeling style: Elements of UML 2.0 Style The Elements of UML 2.0 Style describes a collection of standards, conventions, and guidelines for creating effective UML diagrams. They are sound, proven strategies that lead to diagrams that are easier to understand and work with. These conventions exist as a collection of simple, concise guidelines that if applied consistently, represent an important first step in increasing your productivity as a modeler. This book is oriented towards intermediate to advanced UML modelers. Although there are numerous examples throughout the book it would not be a good way to learn the UML (instead, consider The Object Primer). The book is 188 pages long and is conveniently pocket-sized so it’s easy to carry around.
Choose Your WoW! 2nd Edition This book, Choose Your WoW! A Disciplined Agile Approach to Optimizing Your Way of Working (WoW) – Second Edition, is an indispensable guide for agile coaches and practitioners. It overviews key aspects of the Disciplined Agile® (DA™) tool kit. Hundreds of organizations around the world have already benefited from DA, which is the only comprehensive tool kit available for guidance on building high-performance agile teams and optimizing your WoW. As a hybrid of the leading agile, lean, and traditional approaches, DA provides hundreds of strategies to help you make better decisions within your agile teams, balancing self-organization with the realities and constraints of your unique enterprise context.
The Object Primer 3rd Edition: Agile Model Driven Development (AMDD) with UML 2 The Object Primer 3rd Edition: Agile Model Driven Development with UML 2 is an important reference book for agile modelers, describing how to develop 35 types of agile models including all 13 UML 2 diagrams. Furthermore, this book describes the fundamental programming and testing techniques for successful agile solution delivery. The book also shows how to move from your agile models to source code, how to succeed at implementation techniques such as refactoring and test-driven development(TDD). The Object Primer also includes a chapter overviewing the critical database development techniques (database refactoringobject/relational mappinglegacy analysis, and database access coding) from my award-winning Agile Database Techniques book.