Architecture :: How To Save The Current Location Of Each Logged-in User

Let me tell you what I need to implement on my company .NET application and I will be more than glad to hear your advise how to do it:

For every user loged-in to the company website, We want to know on which page he is right now. It means, If user X is now on page "1.html", I need to save this information somewhere. If user X after 30-60 seconds going to page "2.html" I want to update it fast where he is now surfing. It means I want to be able to tell at REAL TIME, on each page, Which users are right now observing the page. This is the problem, now I'm looking for the right solution.

If some day my company website will have 400,000 user log-in at the same time, I am going to save to every one of them where he is surfing right now. Maybe I exaggerated, but we do have 45,000 users at the same time on the company web site and the number is growing. I think big so the solution for this problem will be good for a lot of users.

So, my data object for every user should keep this information somewhere:

1. User code (User Number).
2. Which page user is now observing (Page Number).

This object is going to update every 30-60 seconds by using AJAX, so it should be updated fast. It is a lot of reading/writing data. If a user want to see which other users are now on the same page as he his, he will be able to see it, just by going through the whole array of objects.

I don't need this information to be saved in DB, because it doesn't mean anything. It is temporary and changing A LOT. So, I think the right place to do it is using RAM at the the company server, using the Caching Data option. What do you think? Am I wrong?

If I am going to have a data table with lots of records and every record is going to be update every 30-60 seconds. Where should I save all this table data? I am using SQL Server 2008 to save important data as needed, but this data is not important to save, just need to use it for my purpose. Where should I save this data? I think saving it in DB is a bad idea, because of the dynamic update of records every few seconds. I am thinking about 100,000 users who update this table every few seconds. It probably going to kill my DB, so DB is not an option.

What is the right way to implement this kind of thing?

You may think of it like every page is "a chat room" and I want to know who enter now the room (the page), it doesn't have to be completely accurate in time, but approximately.