WordPress HyperDB setting for multisite with 3 datasets: global, even_sites, odd_sites

db-config.php

<?php

/**
 * HyperDB configuration file
 *
 * This file should be installed at ABSPATH/db-config.php
 *
 * $wpdb is an instance of the hyperdb class which extends the wpdb class.
 *
 * See readme.txt for documentation.
 */

/** Variable settings **/

/**
 * save_queries (bool)
 * This is useful for debugging. Queries are saved in $wpdb->queries. It is not
 * a constant because you might want to use it momentarily.
 * Default: false
 */
$wpdb->save_queries = false;

/**
 * persistent (bool)
 * This determines whether to use mysql_connect or mysql_pconnect. The effects
 * of this setting may vary and should be carefully tested.
 * Default: false
 */
$wpdb->persistent = false;

/**
 * max_connections (int)
 * This is the number of mysql connections to keep open. Increase if you expect
 * to reuse a lot of connections to different servers. This is ignored if you
 * enable persistent connections.
 * Default: 10
 */
$wpdb->max_connections = 10;

/**
 * check_tcp_responsiveness
 * Enables checking TCP responsiveness by fsockopen prior to mysql_connect or
 * mysql_pconnect. This was added because PHP's mysql functions do not provide
 * a variable timeout setting. Disabling it may improve average performance by
 * a very tiny margin but lose protection against connections failing slowly.
 * Default: true
 */
$wpdb->check_tcp_responsiveness = true;

/**
 * This is a multisite even-odd setting
 * using 2 different databases to save the even and odd sites number
 */
$wpdb->add_database( array(
	'host'     => sprintf( "%s:%d", DB_HOST, 10047 ), // host:port
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => DB_NAME,
	'dataset'  => 'global'
) );

$wpdb->add_database( array(
	'host'     => sprintf( "%s:%d", DB_HOST, 10047 ), // host:port
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => 'multisite_site_even',
	'dataset'  => 'even_sites'
) );

$wpdb->add_database( array(
	'host'     => sprintf( "%s:%d", DB_HOST, 10047 ), // host:port
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => 'multisite_site_odd',
	'dataset'  => 'odd_sites',
) );

$wpdb->add_callback( 'resolve_with_dataset' );
function resolve_with_dataset( $query, $wpdb ) {
	// Multisite blog tables are "{$base_prefix}{$blog_id}_*"
	if ( preg_match("/^{$wpdb->base_prefix}\d+_/i", $wpdb->table) ) {
		// break down into just wp_86_ prefix
		$prefix_matches = array();
		preg_match("/^{$wpdb->base_prefix}\d+_/i", $wpdb->table, $prefix_matches);
		$site_table_prefix = $prefix_matches[0];
		$site_id_matches = array();

		// pull out any numerical matches here
		preg_match("/\d+/i", $site_table_prefix, $site_id_matches);
		$site_id = $site_id_matches[0];
		
		if ( ( ( int ) $site_id % 2 ) == 0) {
		  	return 'even_sites';
		} 
		else {
		  	return 'odd_sites';
		}
	} 
    else {
		return null;
	}
}