The DRYest approach would be common cookbooks with individual roles per client. The roles could override unique client attributes, things like apache, tomcat and mysql tuning parameters...or ports that apache runs on. Since each role also has it's own run_list it would allow you to account for the different groupings of software each client may also have (ie some clients use apache only, no tomcat).
Unfortunately, I'm not sure that will work. Each of our "clients" is probably larger than the average chef user's complete infrastructure in size and complexity. Imagine several large web companies with a single operations group...
For example, client "A" may have 4 different application "stacks":
- apache -> tomcat -> mysql
- varnish -> apache -> NFS mounts
- apache + wsgi python app -> memcache + mysql
- varnish -> proprietary app server -> who knows what
and also have development, reference, and productions environments for each of those.
Clients B and C may have some of the same components, but in a completely different layout.
Granted, we have been standardizing more, but this is the unfortunate place we find ourselves in now.
Our first deployments will be for some of the more "standardized stacks" we have, so we have some time to figure it out, I hope.
--Brian