HyperDB multisite example

db-config.php

<?php

$wpdb->add_database( array(
	'host'     => 'global.db.example.com',
	'user'     => 'globaluser',
	'password' => 'globalpassword',
	'name'     => 'globaldb',
) );

$wpdb->add_database( array(
	'host'     => 'blog.db.example.com',
	'user'     => 'bloguser',
	'password' => 'blogpassword',
	'name'     => 'blogdb',
	'dataset'  => 'blog',
) );

$wpdb->add_callback( 'my_db_callback' );

// Multisite blog tables are "{$base_prefix}{$blog_id}_*"
function my_db_callback( $query, $wpdb ) {
	if ( preg_match("/^{$wpdb->base_prefix}\d+_/i", $wpdb->table) ) {
		return 'blog';
	}
}