I have a social networking site programmed from the ground up in PHP/MySQL using 1 Database.
I would like to add a Forum to the site and have looked at the different PHP Forum Classes available at phpclasses.org and the open-source solutions available at sourceforge.net.
I haven't decided which solution to use (and I'm open to suggestions), but I was wondering if I should install the forum within the current (and only) Database of my site as new Tables or as a completely new Database with its own independent Tables?
In other words, when is it a good idea to forgo the main DB of your site and create a new DB for a new feature?
There's no "one size fits all" answer to that. But in general, I try to compartmentalize functionality as much as possible in my database structure and would most likely create a separate DB for the foums (and have on my sites). There is also the issue of security (using open source software to access a proprietary DB sounds like worrying I could do without).
There are also times when you would want/need to keep everything in the same table (e.g., a hosting limitatiion that only allows 1 DB).
Unless you have a specific reason to do so, I'd say separate DB. Especially with open source board software, any security breaches in the board, which is a pretty high profile target for hacking, could potentially affect your other databases. Make sure it logs in to mySQL with a different U and P too.
Or you could have a table with a name that conflicts with your other DBs.