Ask HN: Creating a central database and file repository

I'm looking to create a central database and also a repository of images and files that users upload to each other across multiple domains.<p>Structure would be as follows.<p>Main site A: Hosts DB & files Site B: Accesses DB and files on site A Site C: Accesses DB and files on site A<p>Just wondering if anyone has any knowledge of setting something like this up or any guides people can point me towards. Looking to ensure consistency and speed.<p>E.g. User X is on domain A and is overwriting files being used by user Y on site B.<p>Thanks in advance.