Question:

The <a href="http://www.grails.org/doc/latest/guide/6.%20The%20Web%20Layer.html#6.2%20Groovy%20Server%20Pages" rel="nofollow">grails manual</a> shows the following example:

<g:set var="now" value="${new Date()}" scope="request" />

and also indicates by default variables defined by the set are page scope (out of the page, request, flash, session, and application choices). I'm wondering what the difference between page and request scope is, and what an example use of the difference might be.

Also, with the flash scope, the manual indicates: "Grails supports the concept of flash scope as a temporary store for attributes which need to be available for this request and the next request only. Afterwards the attributes are cleared. This is useful for setting a message directly before redirection."

It isn't immediately apparent to me how redirection relates to "this request and the next request", since the example of redirection they give is redirecting from one controller action to another, which doesn't respond in two pages/http responses being sent to the client?

Hopefully those two questions make sense -- i.e. high level difference between page and request scope, and how redirecting between actions is useful for flash scope?

A redirect(controller: "foo", action:"bar") equals a new request (in the context of a servlet at least). Which is why you need flash to be a sort of 'two requests scope', the action you get sent to treats your redirection as a new request. You can explicitly avoid this by using chain().

As for the difference between the page and request scope, my understanding is that the page scope is more or less the model a given view / render process operates on whereas the request is for the entire request cycle. Meaning that whatever you pass off to the view in an action return (or the stuff you put in model: [] of a render()) is the 'page scope'.

As for the manual example I have no clue why they would show any scoping at all in a view g:set operation, setting variables in the view should generally be avoided anyways (separation of concerns and all that jazz).