This is a work in progress/idea dump right now - any suggestions/improvements - please do leave a comment ;)
I have quite a few sites that are using drupal. I know I could go the route of a multisite install (I've done it before) - but I found that the requirement of upgrading all sites simultaneously was a bit too hard. However - upgrading everything by hand for each site is also poor.
I've a set of shell/perl scripts to make life easier - but - this really does feel like a version control issue to me.
Googling gave me the above link at versioncontrolblog. This is a nice start. But - I can't see how to make it work for many sites. You'd end up with a drupal-and-modules dir per site.
So - here's my initial thoughts. For the following -
/usr/local/drupal is where I'm storing things,
/srv/www/<sitename>/htdocs is where sites have their document root.
We have three types of release to retrieve - drupal itself, modules and themes.
This is a git repo that contains the latest release of drupal major version X. When drupal releases a new version (e.g. drupal 6.9 -> drupal 6.10) then I just extract that on top of the correct major version, git add where necessary then commit. I also tag the result with the major and minor version.
For each module that I need (for any site)- this is a git repo (per module) that contains the latest release of the module for drupal major version X. When a module releases a new version (e.g. drupal X.x-1.1 -> drupal X.x-1.2) then I just extract that on top, git add where necessary then commit. I also tag the result with the major and minor version.
For each theme that I need (for any site) - this is a git repo (per theme) that contains the latest release of the theme for drupal major version X. When a theme releases a new version (e.g. drupal X.x-1.1 -> drupal X.x-1.2) then I just extract that on top, git add where necessary then commit. I also tag the result with the major and minor version.
The idea of the themes and modules directories here is that I should only need to work with one copy of each despite having more than one site. However I don't want every site to update to a newer module at the same time. So now let's see how this gets used for the actual sites.
Each site is created by cloning the drupalX release repo. I then configure the settings.php (and add it), and add sites/all/themes and sites/all/modules directories (and add them). You may also choose to add sites/default/files to the git ignore list - depends if you want uploads and cached js/css/theme files to be git controlled over time.
Updates to drupal can then be handled with git pull.
To add a theme or module I can then run e..g
git submodule add /usr/local/drupal/modulesX/pathauto sites/all/modules/pathauto
Remember git submodule init/update if pulling a repo with submodules.
If a module is updated in the /usr/local/drupal/modulesX then where it is used should also be updated
cd /srv/www/<site>/htdocs/sites/all/modules/<modulename> git remote update git merge origin/master
If a module is no longer wanted - edit .gitmodules and .git/config - remove all references. Then git rm --cached <path to module>. Finally - remove the module itself.