settings.php database comments

db-slaves-01.txt

For each database, you may optionally specify multiple "target" databases.
A target database allows Drupal to try to send certain queries to a
different database if it can but fall back to the default connection if not.
That is useful for master/slave replication, as Drupal may try to connect
to a slave server when appropriate and if one is not available will simply
fall back to the single master server.

The general format for the $databases array is as follows:
@code
$databases['default']['default'] = $info_array;
$databases['default']['slave'][] = $info_array;
$databases['default']['slave'][] = $info_array;
$databases['extra']['default'] = $info_array;
@endcode

In the above example, $info_array is an array of settings described above.
The first line sets a "default" database that has one master database
(the second level default).  The second and third lines create an array
of potential slave databases.  Drupal will select one at random for a given
request as needed.  The fourth line creates a new database with a name of
"extra".