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.

A->B: Event
A->B.method() {
  if(x) {
    C.doSomething
  }
}
A->B: Event
A->B.method() {
  if(x) {
    C.doSomething
  }
}

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:

B
A
A.method()
A->B: Event












Participant group
We can group the participants with group keyword.












group GroupName {
  A
  B
}
C








group GroupName {
  A
  B
}
C












Participant type (Annotation)
We can change the shape of the participant representation with annotations.












@Actor A
@Database B
@Boundary C
@Control D
@Entity E
@EC2 F
@ECS G
@RDS H
@S3 I
@IAM J
@Lambda K




















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












<<Callable>> B
<<Service>> A
A.method()
A->B: Event








<<Callable>> B
<<Service>> A
A.method()
A->B: Event












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.












<<Callable>> B
<<Service>> A
@Starter(User)
A.method()
A->B: Event








<<Callable>> B
<<Service>> A
@Starter(User)
A.method()
A->B: Event












Messages
A message is shown as a line from the sender MessageEnd to the receiver MessageEnd.
See Unified Modeling Language v2.5.1, section 17.4.4.1.
Message type
DSL
Line and arrowhead (Spec)
Async
A->B: Async message
solid line with open arrowhead 
A->B: Async Message
Sync
A.method()
filled arrowhead
A->B.method
Reply
  1. ret = A.method
  2. return ret
  3. @return
dashed line with either an open or filled arrowhead
* ZenUML renderer use open arrowhead.
// 1
ret = A.method()
B.method() {
  // 2
  return ret
  // 3
  @return
  B->A: ret
}
Object creation
new ClassName()
dashed line with an open arrowhead
new A()
Object deletion
 
Must end in a DestructionOccurrenceSpecification
Lost
  
A small black circle at the arrow end of the message
Found
  
A small black cirle at the starting end of the message
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:












loop("Every minute") {
  Alice->Bob: Great!
}








loop("Every minute") {
  Alice->Bob: Great!
}














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 {
  ...
}












if (x) {
  A.m1()
} else if (y) {
  A.m2()
} else {
  A.m3()
}








if (x) {
  A.m1()
} else if (y) {
  A.m2()
} else {
  A.m3()
}