This has been plaguing my mind for a while now, but I don't really know any database experts, so I'll go ahead and ask you guys here. I'm a member of Bodybuilding.com, and they've seemed to bridge their vBulletin database with a custom social networking-like profile system called BodySpace, which has several unique features like creating your own blog (could be using Wordpress for that, I think they are actually), automated progress charts, etc. Every time you post a blog, or update your progress, or post; you get 'BodyPoints' in your profile. So this BodyPoints system is working universally. My question is this: Say something crazy were to happen and Google or some other web company took over the Internet by buying out all the major websites (YouTube already, Facebook, eBay, Paypal, etc), and then wanted to create a universal login, so that a user could just log in once, and be logged in to all of these websites with unique information for each one, but a universal profile. 1) If someone were to create these concepts from scratch. For example: if I have a business that's an online social networking community like Facebook, but I also have several other online businesses like a variety shop such as Amazon, an auction system like eBay, etc; but I also want to create our own unique payment processor like Paypal; bridging all these together plus others, can this even be done? If so, what would be the difficulty level for a staff of database experts to do so? Would each website have their own unique database with user account entries, just 'linked' to the other databases and the appropriate user accounts, or would there be one central universal database to handle the accounts? How difficult would it to be to add/modify/drop fields if you have a large number of user accounts (in the millions)? 2) Say if I wanted to implement a points system like BodyPoints. Where a user would get points for buying something from the shop, sending money using the Paypal-like payment processor, bidding on auctions, whatever else; and the social networking community would act as a 'universal' profile, that would record/display on request the transactions and points earned. This could probably be done? How difficult do you think it would be to implement this? 3) Could you give me some insight/general info as to how the process would be to bridge all of them? 4) What database type would be the typical for a big bridging project like this? Crazy inquiry, I know, but I, like a lot of the webmasters here, have crazy dreams to own big websites one day; and I believe that universal login is very key to establish brand loyalty and have each website market the other. I know some of the questions are rather vague and not easy to estimate, I'm not much of a database expert so I wasn't able to narrow it down. Sorry about that. Thanks!
Generally you would do this on an application level, and use one of the databases for the user authentication. In your example, you would use the users table from the vBulletin database to hold the user information, and have all of your applications access that database to authorize a user. This way, the independant functions would all remain without any modification.