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Vaadin 7 alpha 2 does not provide push support. An API for push implementations and a default implementation (possibly as an add-on) are planned in the alpha 4 timeframe, and have not been designed in detail.
You can see the initial ideas about this on this page, but note that it mostly tells we need to evaluate the different options (including websockets) and fall-backs (for browsers that do not support the primary approach).