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My GemStone application integrate very well with a Java Form application.
Flow (user is already logged in seaside):
1- User click an image to fill a Form.
2- A redirect take place to a Java Application with an uuid param in the url and the value of _s and _k also as parameters).
3- Java aplication take the uuid param call a Seaside Rest-Service (_s and _k are stored as hidden in Java form).
4- Rest Service answer an XML.
5- Java Form show some values in the form (from the Rest response).
6- User fill the rest of the form.
7- User click on "Send" button in Java Form.
8- Java Form store the form in mySQL.
9- Java Form do a Post to another Seside Rest Service.
10- Seside POST Service answer a HTML code that is a redirect to original page in 1 (using _s and _k hidden values in the form).
11. Java form take the response of 10 (post service) and load it in the browser.
12. A redirect take place to the original seaside page (step 1) with the same _s and _k. And the seaside page is updated with some Java Form values.
This is working pretty well. :)
But if instead of doing a redirect i use an iframe then in step 12 the seaside page is rendered twice.
One with the original values of step 1 (with outdated data) and other with the correct page.