Session Tracking Servlet

I have 2 applicatio开发者_开发百科ns(EARs) . In App1 I have a few jsps and at a point I want to save the details of a person I have a button which has a the javascript to save the details. But now I am asked to authenticate the user with another app App2. So I send a callback url from App1 so that once the user is successfully authenticated I can come back to the same page on App1.

Question is I will loose all the details that App1 was holding in its session before the control was given over to App2 since the session App2 uses is different from App1.

This is almost on the same lines as we do any transaction on a website like Amazon.com where the user chooses to pay using paypal and pay pal returns the user back to the same page on Amazon

Can someone let me know how to come back to the App1 page with all its session details in place.

Another condition is App1 and App2 are deployed on different boxes

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Pass a session identifier along in the callback URL.