> For the complete documentation index, see [llms.txt](https://zenuml.gitbook.io/language-reference/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://zenuml.gitbook.io/language-reference/methods/uml-sequence-diagrams.md).

# UML Sequence Diagrams

**Sequence diagram** is one kind of `interaction diagram`. It focuses on the `message` interchange between a number of `lifelines`.

In [ZenUml](https://github.com/zenuml/language-reference/tree/021e55b7c7f2fd3c875a54d061e915817c0e25d6/www.zenuml.com), we use a simple syntax for the main elements.

## Lifeline

**Lifeline** is a `named element`. Each `lifeline` represents **only one** interacting entity. ![](https://865943-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LLCCL8zqZmFm5BFvX_c%2F-LLCCO4605kf2zdmvIdc%2F-LLCCQdA8sEd03ZWT7Js%2Flifeline.JPG?generation=1535672434190111\&alt=media)

### How do popular tools define a `lifeline`?

All the tools allows implicit definition of `lifeline` and that is the default behavior and recommended way in [ZenUml](https://github.com/zenuml/language-reference/tree/021e55b7c7f2fd3c875a54d061e915817c0e25d6/www.zenuml.com). However, most of them support explicit `lifeline` definition. This is particularly useful when you want to have the `lifeline` in an order different from when it appears in the text.

To generate a `lifeline` like the above, here are the different way in some popular tools.

#### [ZenUml](https://github.com/zenuml/language-reference/tree/021e55b7c7f2fd3c875a54d061e915817c0e25d6/www.zenuml.com) (To be supported)

`EventBus` [Test it on the site](https://github.com/zenuml/language-reference/tree/021e55b7c7f2fd3c875a54d061e915817c0e25d6/www.zenuml.com)

At the beginning of the text, each line with one This syntax can only be used at the beginning of the code. Each line for **one** `lifeline`.

#### [websequencediagrams](https://github.com/zenuml/language-reference/tree/021e55b7c7f2fd3c875a54d061e915817c0e25d6/www.websequencediagrams.com)

`participants EventBus`

#### JUMLY

```
@found "You"
```

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