Running multiple Drupal installations on one URL: is it advisable? My employer is considering replacing our fairly static website with Drupal, mainly so we can more easily implement a few ideas we've had, and also as a way of future-proofing it. It's something of an umbrella organization in that it has a few different customer-facing departments which aren't really related to each other, apart from falling under the same organization. 

So, broadly speaking, each department has pretty specific needs - certain types of defined users, types of nodes; some require their own little forums, basic shopping carts, etc. My first instinct is to treat each department as its own Drupal installation with its own specific configuration of modules and keep things separate in that way. That said, there might need to be some functionality that spreads across the entire site. For example, one or two more senior members of staff might need to be able to log in and edit content across a few of the departments.

Is it possible to do this? To run different installations at domainname.org/dept1, domainname.org/dept2, domainname.org/dept3, etc. More importantly, is it advisable? What issues would I be likely to face if I were to go ahead with a setup like this?
Not only is it not a problem but it's actually a quite common practice!

Check out this link for some tips: multisite howtos

The only suggestion I have is to give each site a subdomain rather than a subfolder, in my experience this works a lot better:


dept1.domainname.org
dept2.domainname.org
dept3.domainname.org


(with all 3 hosts pointing to the same directory)

Setup

To set this up, just add a separate configuration folder under sites for each one:


/sites/dept1.domainname.org/setings.php // dept1 production
/sites/dept1.stagingdomain.org/setings.php // dept1 staging
/sites/dept1.domainname.local/setings.php // dept1 development
and so on...


Also, you should have separate files directories for each site and you can place site-specific (custom) modules on a modules subdirectory under the respective site's settings folder, to keep the /sites/all/modules directory uncluttered!