A sequence diagram is an interaction diagram that shows how processes operate with one another and in what order.

Most developers would find the syntax fairly familiar. The following example demonstrates some basic syntaxes.

The above diagram is generated from the follow text:

// Optionally declare a participant (see Participant)
@Lambda BookLibService
// Sync message; <br>
// Use `A->B: message` for async messages
BookLibService.Borrow(id) {
  // Nested message
  User = Session.GetUser() {
    // Self message
    loadUserProfile()
  }
  // Alt fragment; also try `while`, `par` (see Fragments)
  if(User.isActive) {
    // Try/Catch/Finally fragment
    try {
      BookRepository.Update(id, onLoan, User)
      // Creation message with return message
      receipt = new Receipt(id, dueDate)
    } catch (BookNotFoundException) {
      ErrorService.onException(BookNotFoundException)
    } finally {
      Connection.close()
    }
  }
  // separte return message
  return receipt
}

Participants

The participants can be defined implicitly as in the first example on this page. The participants 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 participant’s order of appearance by doing the following:

Participant group

We can group the participants with group keyword.

Participant type (Annotation)

We can change the shape of the participant representation with annotations.

Stereotype

It is possible to add stereotypes to participants using << and >>.

Starter

By default, the “client” of the interaction is not shown in the diagram. However, you can specify a “client” with the @Starter keyword. Specifically, if the starter’s name is “User” or “Actor”, we will use a Stickman icon. @Starter must be put after you have declared all participants and before any messages.

Messages

A message is shown as a line from the sender MessageEnd to the receiver MessageEnd.

Loops

The loop operand will be repeated a number of times. This is expressed by the notation:

while(condition) {}
for(enumerator) {}
forEach(enumerator) {}

See the example below:

Alt

The alt operand represents a choice of behavior. At most one of the operands will be chosen. This is expressed by the notions:

if (condition1) {
  ...
} else if (condition2) {
  ...
} else {
  ...
}