Gaetan, excellent demo. You covered 3 technologies that work wonderfully together: Play! 2.0, WebSockets and Canvas. Fortunately the usage of each was all straight forward but you married them together into a creative demo app that didn't require black-magic in any of the implementations.

I've never worked with Canvas before but was surprised/pleased to see how it feels a lot like working with Java2D when rendering a 2D context (strokes, lines, joins, etc.) -- I felt the same about WebSockets, definitely had a "It is that easy to use them? Nice!"

Scala on the otherhand... I'll happily concede that I am possibly a lost-cause when it comes to Scala (14 years of Java will do that do you, and C/C++ before that). When you went to that Hub impl on the server side, a small tear rolled down my cheek caused by brain-pain trying to feel my way through that Scala code (nothing wrong with your code, I just don't like the way Scala looks/feels).

Thanks again for putting this all together and sharing.