Drupal: Multi-site and Signing-in once

With Drupal a developer has many different choices in order to help him create a multi-site installation.

First of all, we should clarify what needs we have. We might need different modules in each site, but we might, also, need different themes, different names and nothing else. We might need to create affiliate sites with one common sign-in. All this can be done with drupal.

The drupal way (without using any modules) is to create different folders under the sites folder. The installation already contains sites/all and sites/default. By creating folders below:

1. sites/all/modules
2. sites/all/themes
3. sites/default/modules
4. sites/default/themes
5. sites/example
6. sites/example/modules
7. sites/example/themes

we allow drupal to associate different (sub)domains with the same installation but with different modules and enabled.

Under sites/all/modules and sites/all/themes are placed the modules and themes that will be available to all (sub)domains. In contrary, under sites/default/modules and sites/default/themes drupal recognises modules and themes available for installation only in default installation. The same goes for example as well.

If we create subdomains, and not different domains, if we want to allow users to sign-in once for all subdomains, we must set the $cookie_domain variable in settings.php to .example.com. Be careful at the first dot!

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Another way to create something like above, is to use the module DomainAccess. It comes with many other modules and has the ability to associate the sites/default folder to different (sub)domain names. All it does is create different tables for each (sub)domain in the same database whenever the administrator chooses.

For example, let's say we want to create www.example.com and sub.example.com. We want both to have their content available to both sites but the permissions for each site to be different. All we have to do is copy the permissions table from the primary site www.example.com for sub.example.com to find it. And this is done of course from the modules' drupal administration page. No programming skills required.

Another possibility is that we have different webservers, with different drupal installations, associated with different domains, installed in different databases, using the same database server(ex. 192.xxx.x.1 and MySQL). Even in this case, a user can sign-in only once to all sites. Using the SingleSignOn module including the SSO controller and SSO client modules. This module can be used even with different domains. It needs a primary domain to be set, like DomainAccess does, but the session id stored in database by the controller domain, is recognised by all client domains.

What will be used for each project is something totally chosen by the developer. The table below tries to simplify things a bit:

	Users sign-in once for all sites	Users sign-in for every different site
Domain with one or more subdomain(s) (only one drupal installation) or (only one webserver installation)	drupal way(no modules) use $cookie_domain Domain Access module use $cookie_domain	drupal way(no modules) don't use $cookie_domain Domain Access module don't use $cookie_domain
Domain with one or more subdomain(s) (many drupal installations)	drupal way(no modules) use $cookie_domain Domain Access module use $cookie_domain	drupal way(no modules) don't use $cookie_domain Domain Access module don't use $cookie_domain
One or more domains with zero or more subdomains (only one drupal installation) or (only one webserver installation)	drupal way(no modules) use SSO modules Domain Access module use SSO modules	drupal way(no modules) don't use SSO modules Domain Access module don't use SSO modules
One or more domains with zero or more subdomains (many webservers) and/or (many drupal installations)	use SSO modules in each installation	don't use SSO modules at all
One or more domains with zero or more subdomains (many webservers) and/or (many drupal installations)	-	-