Domain model diagrams using PlantUML

Recreating the diagrams described in Ch. 1 ‘Document your domain’ and Ch. 2 ‘Enhance your domain model’ of Creating software with modern diagramming by Ashley Peacock, using PlantUML

See Software design diagrams using PlantUML for the other posts in the series.

These diagrams are generated using the -Smonochrome=true and -Sshadowing=false flags.

Entities in a domain model and how they are related (class diagram)

Association: loose relation, exist independently, neither owns the other. example: title – genre

Composition: the “child” entity cannot exist independently of the “parent”. examples: title – season, season – episode, title – review

Aggregate: can exist independently, but one is the “parent” (owner of the relationship). example: title – actor

@startuml
Title -- Genre

Title *-- Season
Title *-- Review
Season *-- Episode

Title o-- Actor
@enduml

Generalisation: represents inheritance (subtypes). example: title ∈ {tv show, film, short}

@startuml
"TV show" --|> Title
Short --|> Title
Film --|> Title
@enduml

Descriptions of the relationships can be added as labels on the vertices:

@startuml
Title -- Genre: "is associated with"

Title *-- Season: has
Title *-- Review: has
Season *-- Episode: contains
Season *-- Review: has
Episode *-- Review: has

Title o-- Actor: features

"TV show" --|> Title: implements
Short --|> Title: implements
Film --|> Title: implements

Viewer --> Title: watches
@enduml

Multiplicity of a relation (one-to-one, one-to-many, many-to-many, etc.) is described by specifying the cardinality on each end of the vertex. For example, each title (1) has zero or more seasons (0..*).

@startuml
Title "1..*" -- "1..*" Genre: "is associated with"

Title "1" *-- "0..*" Season: has
Title "1" *-- "0..*" Review: has
Season "1" *-- "1..*" Episode: contains
Season "1" *-- "0..*" Review: has
Episode "1" *-- "0..*" Review: has

Title "0..*" o-- "1..*" Actor: features

"TV show" --|> Title: implements
Short --|> Title: implements
Film --|> Title: implements

Viewer "0..*" --> "0..*" Title: watches
@enduml

Improving the visual style:

• add a title
• hack to increase spacing: add newlines in name or fields/methods on central entity

title Streamy Domain Model

' added to improve readability
class "\n\nTitle\n\n" as Title

/' alternative
class Title {
  \n
  --
  \n
}
'/

Note: Unlike in Mermaid, it isn’t possible to add a hyperlink to an entity in a class diagram, though it is possible on a Gantt diagram.