#Sequence Diagram

A sequence diagram shows how participants exchange messages over time. kymo reads the Mermaid sequenceDiagram syntax.

This page works like a quickstart: as you scroll, the pane on the right shows the source and the preview for the section you're reading. Copy grabs the source; ▶ Open in editor loads it into editor.kymo.studio (pick mermaid in the diagram-type dropdown when starting from scratch).

Beyond the live preview, kymo converts sequence sources into a proper UML interaction model — ready to open in StarUML, Gaphor, or any XMI-consuming UML tool. See Exporting to UML tools.

sequenceDiagram
    participant A as Alice
    participant J as John
    A->>J: Hello John, how are you?
    J-->>A: Great!
    A-)J: See you later

▶ Try it in the editor

#Participants

Declare participants with participant, or actor for a stick-figure actor. The optional as alias sets the display label; the id is what you use in messages. Participants you don't declare are created automatically the first time a message references them, in order of appearance.

sequenceDiagram
    participant C as Client
    actor U as User
    U->>C: click

▶ Try it in the editor

#Messages

A message is Sender<arrow>Receiver: text. Each arrow maps to a UML message sort, so the distinction survives into the exported model:

Syntax	Line	UML message sort
A->>B: text	solid, filled head	synchronous call
A-->>B: text	dashed, filled head	reply
A->B: text	solid, open head	asynchronous signal
A-->B: text	dashed, open head	asynchronous signal
A-)B: text	solid, open head	asynchronous call
A--)B: text	dashed, open head	asynchronous call
A-xB: text	solid, × head	asynchronous call
A--xB: text	dashed, × head	asynchronous call

Self-messages (A->>A: think) are supported.

sequenceDiagram
    participant A
    participant B
    A->>B: sync call
    B-->>A: reply
    A->B: async signal
    A-)B: async call

▶ Try it in the editor

#Activations

Mark when a participant is actively processing with the + / - shorthand on the arrow, or with explicit statements:

Alice->>John: Hello John
activate John
John-->>Alice: Great!
deactivate John

+ activates the target after the message; - deactivates the source.

sequenceDiagram
    Alice->>+John: Hello John
    John-->>-Alice: Great!

▶ Try it in the editor

#Notes

Attach commentary to one participant or span several. Note left of X, Note right of X, and Note over X (or Note over X,Y) are all supported.

sequenceDiagram
    participant A
    participant B
    Note over A,B: handshake begins
    A->>B: SYN
    Note right of B: B is now listening
    B-->>A: SYN-ACK
    Note left of A: connection established

▶ Try it in the editor

#Fragments: loop, alt, opt, par

Combined fragments group messages under an operator, close with end, and nest arbitrarily:

Operator	Meaning
loop label … end	repetition
alt guard … else guard … end	mutually exclusive alternatives
opt guard … end	optional block
par label … and … end	parallel blocks

These map to UML combined fragments (loop, alt, opt, par) with the labels carried as guards.

sequenceDiagram
    participant C as Client
    participant S as Server
    alt is authorized
        C->>S: GET /data
        S-->>C: 200 OK
    else unauthorized
        S-->>C: 401 Unauthorized
    end
    opt retry on flake
        loop up to 3 times
            C->>S: retry
        end
    end
    par notify
        S->>C: event A
    and
        S->>C: event B
    end

▶ Try it in the editor

#Numbering

autonumber switches on sequential message numbering.

sequenceDiagram
    autonumber
    Alice->>John: Hello
    John-->>Alice: Hi

▶ Try it in the editor

#Exporting to UML tools

kymo's own pipeline turns the same source into a UML interaction model. With the Rust CLI (cargo install kymostudio) the output extension picks the target:

kymo seq.mmd seq.xmi          # OMG XMI 2.5.1 — portable UML interchange
kymo seq.mmd seq.mdj          # StarUML project, model + laid-out diagram
kymo seq.mmd seq.gaphor       # Gaphor project, model + laid-out diagram

• .xmi is the vendor-neutral model: import it into Enterprise Architect, Modelio, or any XMI 2.5 consumer.
• .mdj and .gaphor are native project files — open them directly in StarUML or Gaphor and the diagram is already drawn.

Status. Sequence previews on this page and in the editor use the Mermaid renderer; rendering sequence diagrams with kymo's own engine (SVG/PNG/PDF, like flowcharts) is on the roadmap.

#Differences from Mermaid

• Accepted and ignored (no error, no effect): box/end participant boxes, rect background highlights, title, links/link, create/destroy participant lifecycle, and autonumber format arguments.
• Not supported: the critical and break fragments.

#See also

• Flowchart — the other Mermaid diagram type kymo imports.
• BPMN — for multi-participant business processes with execution semantics.