In JSF, there is a viewstate associated with each page, which is passed back and forth with submits etc.

I know that viewstate is calculated using the states of the various controls on the page, and that you can store it either client side or server side.

The question is: how is this value used? Is it used to validate the values sent at submit, to ensure that the same request is not sent twice?

Also, how is it calculated - I realise that richfaces may be calculated differently from myfaces, but an idea would be nice.

Thanks.