Session state consistency - how does APEX keep session state across pages

Question

I would like to know how the APEX develper tool keeps session state from being overwritten across mutiltple pages in the same Session. Because this is difficult to explain I'll use an example from withing the APEX dev tool
For Example:
1. I navigate to the page (4150) thet allows me to edit my Page 880 [Window 1]
2. I now do a Cntrl^N to Open a New Window in the same session (this opens on the same Page 880 in the new window) [Window 2].
3. In Window 1, I click an item to update ie I want to put a comment on the Unconditional Branch so click this to go to the Update page. I see the F4000_P4313_ID is passed in the URL (ie the context, UID of the Branch).
4. In Window 2, I change the page to 881 - ie this is overwriting my Session State variables for page 880 with page 881.
5. I click an item in Page 881 to update ie I want to put a comment on the Unconditional Branch so click this to go to the Update page. I see the F4000_P4313_ID is passed in the URL (ie the context, teh UID of the Branch).
6. In Window 1 (the Page 880), I add a comment to the Branch and 'Apply' changes. On returning to the Page the context has changed from 880 to 881 (as expected as Window 2 had overwritten the session state). Slightly confusing for the user as they did go from Page 880 not 881 but ok
What intrigues me is how the Edit Branch page applied the update to the correct branch [good !]. Why weren't the Branch Session state variables (ie the Branch for Page 880) overwritten with the Branch Session state variables for the Branch from Page 881 ?
Both navigations go to the same Page passing the UID of the branch, hence I would of thought the last navigation is the one that would be persisting (ie from Page 881 Window) ?
Any tips on preventing users updating the wrong record because of the Session State being overwritten would be most welcome.
Look forward to hearing from you.

Answer 1

Let me take a shot at explaining this.
Session state is stored in Oracle database tables. When a APEX page is rendered, those tables are read and the HTML is sent to the browser. When that page is submitted, all the HTML form inputs on the page are saved back into session state over-writing any existing values for that session
So, in your example, since Page 4313 has the hidden page item (the "GUID" of the branch), it doesn't matter if any other window has the same page open with some other GUID. Each page is a self-contained "unit" which has all the information necessary to properly process that page when it is submitted.
Think of it this way...in your example, when you have windows 1 and 2 open (window 2 was opened after window 1), go back to Window 1 and click the browser's Reload/Refresh button. Instead of refreshing the same page (branch 9000000), you will find that it will load branch 8000000! This is because session state is read from the database and when Window 2 was rendered, it has set F4000_P4313_ID in session state to 8000000.
Hope this helps.