I am trying to build a CMS I can use to host multiple sites. I know I'm going to end up reinventing the wheel a million times with this project, so I'm thinking about extending an existing open source Ruby on Rails CMS to meet my needs.

One of those needs is to be able to run multiple sites, while using only one code-base. That way, when there's an update I want to make, I can update it in one place, and the change is reflected on all of the sites. I think that this will be able to scale by running multiple instances of the application.

I think that I can use the domain/subdomain to determine which data to display. For example, someone goes to subdomain1.mysite.com and the application looks in the database for the content for subdomain1.

The problem I see is with most pre-built CMS solutions, they are only designed to host one site, including the one I want to use. So the database is structured to work with one site. However, I had the idea that I could overcome this by "creating a new database" for each site, then specifying which database to connect to based on the domain/subdomain as I mentioned above.

I'm thinking of hosting this on Heroku, so I'm wondering what my options for this might be. I'm not very familiar with Amazon S3, or Amazon SimpleDB, but I feel like there's some sort of "cloud database" that would make this solution a lot more realistic, than creating a new MySQL database for each site.

What do you think? Am I thinking about this the wrong way? What advice do you have to offer in this area?