Use Case Diagram

Let's have few examples :

Note that you can disable the shadowing using the skinparam shadowing false command.

Usecases

Use cases are enclosed using between parentheses (because two parentheses looks like an oval).

You can also use the usecase keyword to define a usecase. And you can define an alias, using the as keyword. This alias will be used latter, when defining relations.

@startuml

(First usecase)
(Another usecase) as (UC2)  
usecase UC3
usecase (Last\nusecase) as UC4

@enduml
usecase diagram

Actors

Actor are enclosed using between two points.

You can also use the actor keyword to define an actor. And you can define an alias, using the as keyword. This alias will be used latter, when defining relations.

We will see later that the actor definitions are optional.

@startuml

:First Actor:
:Another\nactor: as Men2  
actor Men3
actor :Last actor: as Men4

@enduml
usecase diagram with actor

Usecases description

If you want to have description on several lines, you can use quotes.

You can also use the following separators: --..==__. And you can put titles within the separators.

@startuml

usecase UC1 as "You can use
several lines to define your usecase.
You can also use separators.
--
Several separators are possible.
==
And you can add titles:
..Conclusion..
This allows large description."

@enduml
long description for usecase

Basic example

To link actors and use cases, the arrow --> is used.

The more dashes "-" in the arrow, the longer the arrow. You can add a label on the arrow, by adding a ":" character in the arrow definition.

In this example, you see that User has not been defined before, and is used as an actor.

@startuml

User -> (Start)
User --> (Use the application) : A small label

:Main Admin: ---> (Use the application) : This is\nyet another\nlabel

@enduml
relation between actor and usecase

Extension

If one actor/use case extends another one, you can use the symbol <|-- (which stands for ).
@startuml
:Main Admin: as Admin
(Use the application) as (Use)

User <|-- Admin
(Start) <|-- (Use)

@enduml
usecase generalization

Using notes

You can use the note left of , note right of , note top of , note bottom of keywords to define notes related to a single object.

A note can be also define alone with the note keywords, then linked to other objects using the .. symbol.

@startuml
:Main Admin: as Admin
(Use the application) as (Use)

User -> (Start)
User --> (Use)

Admin ---> (Use)

note right of Admin : This is an example.

note right of (Use)
  A note can also
  be on several lines
end note

note "This note is connected\nto several objects." as N2
(Start) .. N2
N2 .. (Use)
@enduml
notes on usecase

Stereotypes

You can add stereotypes while defining actors and use cases using " << " and " >> ".
@startuml
User << Human >>
:Main Database: as MySql << Application >>
(Start) << One Shot >>
(Use the application) as (Use) << Main >>

User -> (Start)
User --> (Use)

MySql --> (Use)

@enduml
stereotype in usecase diagram

Changing arrows direction

By default, links between classes have two dashes -- and are vertically oriented. It is possible to use horizontal link by putting a single dash (or dot) like this:
@startuml
:user: --> (Use case 1)
:user: -> (Use case 2)
@enduml
default direction in usecase diagram
You can also change directions by reversing the link:
@startuml
(Use case 1) <.. :user:
(Use case 2) <- :user:
@enduml
changing direction in usecase diagram
It is also possible to change arrow direction by adding left, right, up or down keywords inside the arrow:
@startuml
:user: -left-> (dummyLeft) 
:user: -right-> (dummyRight) 
:user: -up-> (dummyUp)
:user: -down-> (dummyDown)
@enduml
specify direction in usecase diagram
You can shorten the arrow by using only the first character of the direction (for example, -d- instead of -down-) or the two first characters (-do-).

Please note that you should not abuse this functionality : Graphviz gives usually good results without tweaking.

Title the diagram

The title keywords is used to put a title.

You can use title and end title keywords for a longer title, as in sequence diagrams.

@startuml
title Simple <b>Usecase</b>\nwith one actor

"Use the application" as (Use)
User -> (Use)

@enduml
title in usecase diagram

Splitting diagrams

The newpage keywords to split your diagram into several pages or images.
@startuml
:actor1: --> (Usecase1)
newpage
:actor2: --> (Usecase2)
@enduml
splitting a large usecase diagram

Left to right direction

The general default behavior when building diagram is top to bottom.
@startuml
'default
top to bottom direction
user1 --> (Usecase 1)
user2 --> (Usecase 2)

@enduml
changing direction in usecase diagram top to bottom
You may change to left to right using the left to right direction command. The result is often better with this direction.
@startuml

left to right direction
user1 --> (Usecase 1)
user2 --> (Usecase 2)

@enduml
changing direction in usecase diagram left to right

Skinparam

You can use the skinparam command to change colors and fonts for the drawing.

You can use this command :

You can define specific color and fonts for stereotyped actors and usecases.
@startuml
skinparam handwritten true

skinparam usecase {
	BackgroundColor DarkSeaGreen
	BorderColor DarkSlateGray

	BackgroundColor<< Main >> YellowGreen
	BorderColor<< Main >> YellowGreen
	
	ArrowColor Olive
	ActorBorderColor black
	ActorFontName Courier

	ActorBackgroundColor<< Human >> Gold
}

User << Human >>
:Main Database: as MySql << Application >>
(Start) << One Shot >>
(Use the application) as (Use) << Main >>

User -> (Start)
User --> (Use)

MySql --> (Use)

@enduml
changing fonts and colors in usecase diagram

Complete example

@startuml
left to right direction
skinparam packageStyle rect
actor customer
actor clerk
rectangle checkout {
  customer -- (checkout)
  (checkout) .> (payment) : include
  (help) .> (checkout) : extends
  (checkout) -- clerk
}
@enduml
simple usecase diagram example