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!