Storing and accessing user-specific data across multiple domains

Question

I've recently been developing a userscript to display a sidebar on all the websites we develop, to tell us things like whether the site is meant to be AA, browser support etc etc. This is for internal use only. I am limited to using Classic ASP and jQuery / JavaScript.

My problems are as follows:

• There is a law coming in soon which means it is best that clients do not know about any cookies being used by the script.
• The database team would not be happy with us creating a new database (or extending the main one) just for this script.
• It has recently come to my attention that the maximum number of cookies per domain is around 50 for most browsers; bearing in mind that I was thinking of storing data on a per-site, per-server or for all servers, I am almost certain this would eventually take it over 50 cookies for this script alone.
• I have heard that there are 'mini database' facilities in HTML 5 - however most of our websites are on HTML 4 so this is not an option.
• If we were to go down the database route, we would not be able to use one single database - it would have to be one database per server. I reckon that syncing would be very difficult, even using one page to sync all servers using many iFrames (I believe, although not 100% sure, that if a user navigates away from a page prematurely, any iFrames terminate their request so data may not be updated - but feel free to correct me if I'm wrong).
• Data needs to be stored permanently.

We have a set up as follows (very simplified representation):

• a.b.com (server 1, own domain)
• b.b.com (server 2, own domain)
• c.b.com (server 3, own domain)
• d.b.com (server 4, own domain)
• jobs.a.com (URL mask)
• careers.b.com (URL mask)
• e.m.com (server 1, own domain)
• f.m.com (server 2, own domain)
• g.m.com (server 3, own domain)
• h.m.com (server 4, own domain)

Based on the above, does anyone have any suggestions for the best way to be able to sync data and store as much as we need on a per-user basis? Note that this, ideally, needs to work with GreaseMonkey on Firefox, Chrome, Opera and Trixie on IE (6-9). IE 6-7 are not absolute musts, but are preferable.

Answer 1

If it needs to be permanent and you do not want the clients to be able to see that you store info then client side storage is out of the question. It has to be stored server side..

Ao what you need is:

1. A way of uniquely identifying users cross domain.
2. A server side storage of some sort

The first one you could do using a third party cookie (like google and facebook do to track you on multiple domains). On each site include a link (0x0px iframe, small imgage or something else) that calls a service on a tracking server. The first time the user visits a site, set a long lived cookie. The cookie will be send to the tracking server every time thus uniquely identifying your user. Third party cookies must be enabled on the browser for this to work though.

The second one will have to be a database or file of some sort. I'd say if you cannot use a datebase create a service on one of the machines (the tracking server) and let that one write some info to a disk. This is a poor mans solution as files are hard to query.