NutUml is an opensource, free UML building tool, base on Javascript and HTML5. You can edit UML description text on the web page and view the generated UML diagram in real time.
We use -> to indicate the message transfer between participants, without explicitly declaring participants.
You can also use --> to indicate a dashed line.
You can try to modify the content in the text box below, and then check the changes in the picture on the right
For more syntax descriptions and examples, please refer to Sequence
Nutuml provide confluence plugin Download