Recent versions of Drupal have the oh-so-cool feature that allows you to host many websites off a single Drupal codebase. The coolest part about this is that you can share some tables accross multiple websites; which means you can do things like have a single username/password table accross all the websites. This can easily be done, as specified in the settings.php comments as:

*   $db_prefix = array( *     'default'   => 'main_', *     'users'     => 'shared_', *     'sessions'  => 'shared_', *     'role'      => 'shared_', *     'authmap'   => 'shared_', *     'sequences' => 'shared_', *   );

Now here’s an important thing to note: The first table you have to share is the sequences table. This is the table that handles all the id counters, so if you don’t share this one, something like this can happen:

[you shared only the users table]

1. User 1 signs up on Site A, gets user id#1
2. User 2 signs up on Site A, gets user id#2
3. User 3 signs up on Site B, gets user id#….? The correct answer is not 3!

This happens because you didn’t share sequences… Site B uses it’s own sequence generator to render a duplicate userid… which the user table would not accept, and this would go on till the Site B sequence catches up with the Site A sequence, and then things would be normal. The code quality in user.module helps protect the user table from data corruption, but you will have many signups disappear into thin air with a set up like this. Hence, all you need to do is share the sequences table along with the users… and you’re all set!

Btw, hello Planet people!