Sequence Diagram
A Sequence diagram is an interaction diagram that shows how processes operate with one another and in what order.
sequenceDiagram
Alice->>John: Hello John, how are you?
John-->>Alice: Great!
A note on nodes, the word "end" could potentially break the diagram. If unavoidable, one must use parentheses(), quotation marks "", or brackets {},[], to enclose the word "end". i.e : (end), [end], {end}.
Syntax
Participants
The participants can be defined implicitly as in the first example on this page. The participants or actors are rendered in order of appearance in the diagram source text. Sometimes you might want to show the participants in a different order than how they appear in the first message. It is possible to specify the actor's order of appearance by doing the following:
sequenceDiagram
participant John
participant Alice
Alice->>John: Hello John, how are you?
John-->>Alice: Great!
Aliases
The actor can have a convenient identifier and a descriptive label.
sequenceDiagram
participant A as Alice
participant J as John
A->>J: Hello John, how are you?
J->>A: Great!
Messages
Messages can be of two displayed either solid or with a dotted line.
[Actor][Arrow][Actor]:Message text
There are six types of arrows currently supported:
| Type | Description |
|---|---|
| -> | Solid line without arrow |
| --> | Dotted line without arrow |
| ->> | Solid line with arrowhead |
| -->> | Dotted line with arrowhead |
| -x | Solid line with a cross at the end (async) |
| --x | Dotted line with a cross at the end (async) |
Activations
It is possible to activate and deactivate an actor. (de)activation can be dedicated declarations:
sequenceDiagram
Alice->>John: Hello John, how are you?
activate John
John-->>Alice: Great!
deactivate John
There is also a shortcut notation by appending +/- suffix to the message arrow:
sequenceDiagram
Alice->>+John: Hello John, how are you?
John-->>-Alice: Great!
Activations can be stacked for same actor:
sequenceDiagram
Alice->>+John: Hello John, how are you?
Alice->>+John: John, can you hear me?
John-->>-Alice: Hi Alice, I can hear you!
John-->>-Alice: I feel great!
Notes
It is possible to add notes to a sequence diagram. This is done by the notation Note [ right of | left of | over ][actor]: Text in note content
See the example below:
sequenceDiagram
participant John
Note right of John: Text in note
It is also possible to create notes spanning two participants:
sequenceDiagram
Alice->John: Hello John, how are you?
Note over Alice,John: A typical interaction
Loops
It is possible to express loops in a sequence diagram. This is done by the notation
loop Loop text
... statements ...
end
See the example below:
sequenceDiagram
Alice->John: Hello John, how are you?
loop Every minute
John-->Alice: Great!
end
Alt
It is possible to express alternative paths in a sequence diagram. This is done by the notation
alt Describing text
... statements ...
else
... statements ...
end
or if there is sequence that is optional (if without else).
opt Describing text
... statements ...
end
See the example below:
sequenceDiagram
Alice->>Bob: Hello Bob, how are you?
alt is sick
Bob->>Alice: Not so good :(
else is well
Bob->>Alice: Feeling fresh like a daisy
end
opt Extra response
Bob->>Alice: Thanks for asking
end
Parallel
It is possible to show actions that are happening in parallel.
This is done by the notation
par [Action 1]
... statements ...
and [Action 2]
... statements ...
and [Action N]
... statements ...
end
See the example below:
sequenceDiagram
par Alice to Bob
Alice->>Bob: Hello guys!
and Alice to John
Alice->>John: Hello guys!
end
Bob-->>Alice: Hi Alice!
John-->>Alice: Hi Alice!
It is also possible to nest parallel blocks.
sequenceDiagram
par Alice to Bob
Alice->>Bob: Go help John
and Alice to John
Alice->>John: I want this done today
par John to Charlie
John->>Charlie: Can we do this today?
and John to Diana
John->>Diana: Can you help us today?
end
end
Background Highlighting
It is possible to highlight flows by providing colored background rects. This is done by the notation
The colors are defined using rgb and rgba syntax.
rect rgb(0, 255, 0)
... content ...
end
rect rgba(0, 0, 255, .1)
... content ...
end
See the examples below:
sequenceDiagram
participant Alice
participant John
rect rgb(191, 223, 255)
note right of Alice: Alice calls John.
Alice->>+John: Hello John, how are you?
rect rgb(200, 150, 255)
Alice->>+John: John, can you hear me?
John-->>-Alice: Hi Alice, I can hear you!
end
John-->>-Alice: I feel great!
end
Alice ->>+ John: Did you want to go to the game tonight?
John -->>- Alice: Yeah! See you there.
Comments
Comments can be entered within a sequence diagram, which will be ignored by the parser. Comments need to be on their own line, and must be prefaced with %% (double percent signs). Any text after the start of the comment to the next newline will be treated as a comment, including any diagram syntax
sequenceDiagram
Alice->>John: Hello John, how are you?
%% this is a comment
John-->>Alice: Great!
Sequence Numbers
It can also be be turned on via the diagram code as in the diagram:
sequenceDiagram
autonumber
Alice->>John: Hello John, how are you?
loop Healthcheck
John->>John: Fight against hypochondria
end
Note right of John: Rational thoughts!
John-->>Alice: Great!
John->>Bob: How about you?
Bob-->>John: Jolly good!