SeqDiag

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SeqDiag draws sequence diagrams from arrows. An arrow is a message, a returned arrow is a reply, and nesting is inferred from the order.

Mermaid and PlantUML both do more, but SeqDiag's source is shorter than either.

#What it is good at

  • Simple call sequences where the syntax should stay out of the way.

  • Generated sequence diagrams from a log or trace.

  • Documentation where the sequence is short and unlikely to grow.

#What it draws

Element

Syntax

Message

browser -> server [label = "GET /"];

Reply

browser <-- server [label = "200 OK"];

Round trip

a => b [label = "call", return = "result"];

Note

... anything ...; with the label option

#A worked example

A request, a database call and the replies:

SeqDiag

The source, which you can paste into a new diagram and edit:

seqdiag {
  browser  -> web    [label = "GET /orders"];
  web      -> api    [label = "getOrders(userId)"];
  api      -> db     [label = "SELECT ..."];
  api     <-- db     [label = "rows"];
  web     <-- api    [label = "orders JSON"];
  browser <-- web    [label = "200 OK"];
}

#A few things that catch people out

  • Participants appear in the order they are first mentioned; mention them in the order you want.

  • The => form draws a call and its return in one line, which keeps long sequences short.

  • For activations, loops and alt blocks, use Mermaid or PlantUML.

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