This is a very insightful article on MediaTemple's efforts to build one of the first massive web hosting grid: Anatomy of MySQL on the GRID
This article explains a practical implementation of a technique outlined in the article "Sharing Drupal tables between databases using MySQL5 Views".
You have multiple (multisite) Drupal sites and you would like to manage the content for all of these sites through a single interface. Depending on the nature of a given piece of content, you may want the content published on one, several or all of your subsites, but you do not want to have to create copies of the same content for each site.
Taxonomy plus MySQL5 views. (NOTE: this solution will not work with versions of MySQL prior to 5.)
A while back, I came across a "duplicate entry" problem that resulted from a missing row in the drupal table 'users'.
It turns out that the users table is not the only one that requires information on "user 0" (aka "anonymous").
This week I came across another problem in which anonymous users get a "you are not authorized ..." message when trying to view any sort of content.
Drupal is as scalable as the applications it relies on. While there's nothing preventing Drupal itself from being scaled across any number of web servers, scalability of open-source databases such as MySQL and PostgreSQL is an ongoing issue. While these databases do support replication, that's not always enough. For a very small percentage of websites, daily user visits are counted not in hundreds or thousands but in hundreds of thousands.
You have a "master" database that contains data from multiple Drupal sites and you want to share it among them. Normally, you could use table prefixing to allow each of your sites to point to a single table. But what if you do not want content from one site to "bleed" across to the other sites? Let's say you have a network of Drupal sites sharing a user database. You want to share that user's information across your entire network of sites, but only make the information visible from those sites to which the user has subscribed. Or maybe you want to populate baz.com with users who meet some arbitrary criteria. There are lots of possibilities here, but nobody's paying me to write this, so let's get on with it.