Multiple Drupal Installations. PART II

A Multiple Drupal Installation architecture will separate completely one site from another. Most of the time this is not the best solution, especially if the sites should be integrated one with the other, Usually a better solution is to have multiple sites in one Drupal installation, but we’ll evaluate this architecture because there are situations when this could be the best way of implementing a solution for a client.
In general implementing multiple related sites in multiple Drupal installations could pose a big problem, but it could be a requirement in some cases. The client could for example, want one site for their office in the United States and one site for their office in Europe; with each site managed separately. In this case, having multiple Drupal installations could be the best option, even if they are hosted on the same servers. The scenario would look something like this:
When we are talking of multiple Drupal installations we are not necessarily talking of multiple environments. Both installations could reside in the same Apache and MySQL boxes. The scenario is very similar; we would just need to configure apache’s virtual hosts properly.
The scenario in this case, is just a little bit different:
The benefits of having multiple Drupal installations for serving the sites
  • Sites can escalate independently
  • Sites can be managed independently
  • Sites can be installed in different environments
  • The drawbacks of this design are obvious:
1. It won't be easy to share content and users between the sites Since we are not sharing the database, we have to be aware that you cannot access content from the other site directly, except by implementing a sharing mechanism like RSS feeds, web services or similar.
And since users in one site are completely different than users in the other, implementing a shared one time login would be a pain. You cannot have shared users, except by implementing your own custom solution, which could be difficult to implement and also difficult to maintain.
2. More difficult to maintain
In general, having multiple installations to maintain is a drawback: when you fix bugs, run updates, change any setting, etc. — if you want them to be applied in all the sites — you would have to do it manually one by one.
The roles, permissions and users also have to be maintained separately.