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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.

Code Block
A->B: Event
A.method() {
  if(x) {
    C.doSomething
  }
}
Zenuml sequence macro
uuide81b78c9-ceaf-4e5b-ab09-a87d597f857b
updatedAt2021-02-16T11:45:59Z
A->B: Event
A.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:

Code Block
B
A
A.method()
A->B: Event
Zenuml sequence macro
param2
uuida1abacc4-481c-4e4e-9e46-29e892a867ad
Participant type (Annotation)
You can change the shape of the participant representation with annotations.
 Code Block
@Actor A
@Database B
@Boundary C
@Control D
@Entity E
@EC2 F
@ECS G
@RDS H
@S3 I
@IAM J
@Lambda K
 Image AddedImage AddedImage AddedImage Added
Stereotype
It is possible to add stereotypes to participants using << and >>.
 Code Block
<<Callable>> B
<<Service>> A
A.method()
A->B: Event
 Zenuml sequence macro
uuid00fa2199-9f9b-4132-8a0d-2187df19f0fa
updatedAt2021-02-16T11:34:32Z
<<Callable>> B
<<Service>> A
A.method()
A->B: Event
Starter
 Status
colourBlue
titleadvanced
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.
 Code Block
<<Callable>> B
<<Service>> A
@Starter(User)
A.method()
A->B: Event
 Zenuml sequence macro
uuid100349a5-ae3c-42eb-b440-c5adab28c2df
updatedAt2021-02-16T11:44:21Z
<<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.
 Info
See Unified Modeling Language v2.5.1, section 17.4.4.1.
Message type
DSL
Line and arrowhead (Spec)
Asynchronous
A->B: Asynchronous message
solid line with open arrowhead 
Synchronous
A.method()
filled arrowhead
Reply
reply = A.method(), or
reply = method()
dashed line with either an open or filled arrowhead
* ZenUML renderer use open arrowhead.
Object creation
new ClassName()
dashed line with an open arrowhead
Object deletion
 Status
colourRed
titleNOT SUPPORTED YET
Must end in a DestructionOccurrenceSpecification
Lost
 Status
colourRed
titleNOT SUPPORTED YET
A small black circle at the arrow end of the message
Found
 Status
colourRed
titleNOT SUPPORTED YET
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:
 Code Block
while(condition) {}
for(enumerator) {}
forEach(enumerator) {}
See the example below:
 Code Block
loop("Every minute") {
  Alice->Bob: Great!
}
 Zenuml sequence macro
uuidb826eb57-3f43-44b0-a88d-c852034e9db7
updatedAt2021-02-16T11:46:38Z
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:
 Code Block
if (condition1) {
  ...
} else if (condition2) {
  ...
} else {
  ...
}
 Code Block
if (x) {
  A.m1()
} else if (y) {
  A.m2()
} else {
  A.m3()
}
 Zenuml sequence macro
uuid2e636d8c-99a7-425a-aa12-a760a8632634
updatedAt2020-09-26T02:52:18Z
if (x) {
  A.m1()
} else if (y) {
  A.m2()
} else {
  A.m3()
}