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ZenUML

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

Mermaid can render sequence diagrams with ZenUML. Note that ZenUML uses a different syntax than the original Sequence Diagram in mermaid.

Code:
mermaid
zenuml
    title Demo
    Alice->John: Hello John, how are you?
    John->Alice: Great!
    Alice->John: See you later!

Syntax

Participants

The participants can be defined implicitly as in the first example on this page. The participants or actors 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 actor's order of appearance by doing the following:

Code:
mermaid
zenuml
    title Declare participant (optional)
    Bob
    Alice
    Alice->Bob: Hi Bob
    Bob->Alice: Hi Alice

Annotators

If you specifically want to use symbols instead of just rectangles with text you can do so by using the annotator syntax to declare participants as per below.

Code:
mermaid
zenuml
    title Annotators
    @Actor Alice
    @Database Bob
    Alice->Bob: Hi Bob
    Bob->Alice: Hi Alice

Here are the available annotators: img.png

Aliases

The participants can have a convenient identifier and a descriptive label.

Code:
mermaid
zenuml
    title Aliases
    A as Alice
    J as John
    A->J: Hello John, how are you?
    J->A: Great!

Messages

Messages can be one of:

  1. Sync message
  2. Async message
  3. Creation message
  4. Reply message

Sync message

You can think of a sync (blocking) method in a programming language.

Code:
mermaid
zenuml
    title Sync message
    A.SyncMessage
    A.SyncMessage(with, parameters) {
      B.nestedSyncMessage()
    }

Async message

You can think of an async (non-blocking) method in a programming language. Fire an event and forget about it.

Code:
mermaid
zenuml
    title Async message
    Alice->Bob: How are you?

Creation message

We use new keyword to create an object.

Code:
mermaid
zenuml
    new A1
    new A2(with, parameters)

Reply message

There are three ways to express a reply message:

Code:
mermaid
zenuml
    // 1. assign a variable from a sync message.
    a = A.SyncMessage()

    // 1.1. optionally give the variable a type
    SomeType a = A.SyncMessage()

    // 2. use return keyword
    A.SyncMessage() {
    return result
    }

    // 3. use @return or @reply annotator on an async message
    @return
    A->B: result

The third way @return is rarely used, but it is useful when you want to return to one level up.

Code:
mermaid
zenuml
    title Reply message
    Client->A.method() {
      B.method() {
        if(condition) {
          return x1
          // return early
          @return
          A->Client: x11
        }
      }
      return x2
    }

Nesting

Sync messages and Creation messages are naturally nestable with {}.

Code:
mermaid
zenuml
    A.method() {
      B.nested_sync_method()
      B->C: nested async message
    }

Comments

It is possible to add comments to a sequence diagram with // comment syntax. Comments will be rendered above the messages or fragments. Comments on other places are ignored. Markdown is supported.

See the example below:

Code:
mermaid
zenuml
    // a comment on a participant will not be rendered
    BookService
    // a comment on a message.
    // **Markdown** is supported.
    BookService.getBook()

Loops

It is possible to express loops in a ZenUML diagram. This is done by any of the following notations:

  1. while
  2. for
  3. forEach, foreach
  4. loop
zenuml
while(condition) {
    ...statements...
}

See the example below:

Code:
mermaid
zenuml
    Alice->John: Hello John, how are you?
    while(true) {
      John->Alice: Great!
    }

Alt

It is possible to express alternative paths in a sequence diagram. This is done by the notation

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

See the example below:

Code:
mermaid
zenuml
    Alice->Bob: Hello Bob, how are you?
    if(is_sick) {
      Bob->Alice: Not so good :(
    } else {
      Bob->Alice: Feeling fresh like a daisy
    }

Opt

It is possible to render an opt fragment. This is done by the notation

zenuml
opt {
  ...statements...
}

See the example below:

Code:
mermaid
zenuml
    Alice->Bob: Hello Bob, how are you?
    Bob->Alice: Not so good :(
    opt {
      Bob->Alice: Thanks for asking
    }

Parallel

It is possible to show actions that are happening in parallel.

This is done by the notation

zenuml
par {
  statement1
  statement2
  statement3
}

See the example below:

Code:
mermaid
zenuml
    par {
        Alice->Bob: Hello guys!
        Alice->John: Hello guys!
    }

Try/Catch/Finally (Break)

It is possible to indicate a stop of the sequence within the flow (usually used to model exceptions).

This is done by the notation

try {
  ...statements...
} catch {
  ...statements...
} finally {
  ...statements...
}

See the example below:

Code:
mermaid
zenuml
    try {
      Consumer->API: Book something
      API->BookingService: Start booking process
    } catch {
      API->Consumer: show failure
    } finally {
      API->BookingService: rollback status
    }

Integrating with your library/website.

Zenuml uses the experimental lazy loading & async rendering features which could change in the future.

You can use this method to add mermaid including the zenuml diagram to a web page:

html
<script type="module">
  import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid@10/dist/mermaid.esm.min.mjs';
  import zenuml from 'https://cdn.jsdelivr.net/npm/@mermaid-js/mermaid-zenuml@0.1.0/dist/mermaid-zenuml.esm.min.mjs';
  await mermaid.registerExternalDiagrams([zenuml]);
</script>