LudicrousDB (based on Automattic's HyperDB) configuration file that will split multisite per 100 sites based on site ID

db-config.php

<?php

/**
 * LudicrousDB configuration file
 *
 * This file should be copied to ABSPATH/db-config.php and modified to suit your
 * database environment. This file comes with a basic configuration by default.
 *
 * See README.md for documentation.
 * 
 * LudicrousDB on Github: https://github.com/stuttter/ludicrousdb
 */

// Exit if accessed directly
defined( 'ABSPATH' ) || exit;

/**
 * charset (string)
 * This sets the default character set. Since WordPress 4.2, the suggested
 * setting is "utf8mb4". We strongly recommend not downgrading to utf8,
 * using latin1, or sticking to the default: utf8mb4.
 *
 * Default: utf8mb4
 */
$wpdb->charset = 'utf8mb4';

/**
 * collate (string)
 * This sets the default column collation. For best results, investigate which
 * collation is recommended for your specific character set.
 *
 * Default: utf8mb4_unicode_520_ci
 */
$wpdb->collate = 'utf8mb4_unicode_520_ci';

/**
 * 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;

/**
 * recheck_timeout (float)
 * The amount of time to wait before trying again to ping mysql server.
 *
 * Default: 0.1 (Seconds)
 */
$wpdb->recheck_timeout = 0.1;

/**
 * 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;

/**
 * allow_bail (bool)
 * This determines whether to use mysql connect or mysql connect has failed and to bail loading the rest of WordPress
 * Default: false
 */
$wpdb->allow_bail = 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;

/**
 * The cache group that is used to store TCP responsiveness.
 * Default: ludicrousdb
 */
$wpdb->cache_group = 'ludicrousdb';

/**
 * $callback is a callable function or method. It will be called with two
 * arguments and expected to compute a dataset or return null.
 * $dataset = $callback($table, &$wpdb);
 *
 * Callbacks are executed in the order in which they are registered until one
 * of them returns something other than null. Anything evaluating to false will
 * cause the query to be aborted.
 */
$wpdb->add_callback( 'commercioo_saas_resolve_with_dataset' );

/**
 * Resolve the database name to use.
 * This will return a dataset string between `sites_100` to `sites_9999000`
 * 
 * @param    object    $query
 * @param    object    $wpdb
 * @return   string    $dataset
 */
function commercioo_saas_resolve_with_dataset( $query, $wpdb ) {
	$dataset         = null;
	$prefix_matches  = array();
	$site_id_matches = array();

	// 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		
		preg_match("/^{$wpdb->base_prefix}\d+_/i", $wpdb->table, $prefix_matches);
		$site_table_prefix = $prefix_matches[0];		

		// pull out any numerical matches here
		preg_match("/\d+/i", $site_table_prefix, $site_id_matches);
		$raw_site_id       = $site_id_matches[0];
		$site_id           = ( int ) $raw_site_id;
		$previous_limit    = 0;
		
		/**
		 * Iterate per 100
		 * The limit is 9999000, so it 10 million minus 1 sites
		 */
		for ( $i = 1; $i < 9999; $i++ ) { 
			$sites_limit = $i * 100;
			
			if ( $site_id > $previous_limit && $site_id <= $sites_limit ) {
				$dataset = 'sites_' . $sites_limit;
				break;
			}
			
			// set previous limit
			$previous_limit = $sites_limit;
		}

	}

	return $dataset;
}

/**
 * This is a multisite per 100 sites setting, that using iterate per 100 sites per database
 * The main site will be on the default database, then the child sites will be on `sites_100`, `sites_200`, etc dataset
 * This database sets is expandable, so it can be only 2 databases at first and then we can continue add to it later when it's needed
 */
$wpdb->add_database( array(
	'host'     => sprintf( "%s:%d", DB_HOST, 10053 ), // host:port
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => DB_NAME,
	'write'    => 1,
	'read'     => 1,
	'dataset'  => 'global'
) );

$wpdb->add_database( array(
	'host'     => sprintf( "%s:%d", DB_HOST, 10053 ), // host:port
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => 'comm_sites_100',
	'write'    => 1,
	'read'     => 1,
	'dataset'  => 'sites_100'
) );

$wpdb->add_database( array(
	'host'     => sprintf( "%s:%d", DB_HOST, 10053 ), // host:port
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => 'comm_sites_200',
	'write'    => 1,
	'read'     => 1,
	'dataset'  => 'sites_200',
) );

$wpdb->add_database( array(
	'host'     => sprintf( "%s:%d", DB_HOST, 10053 ), // host:port
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => 'comm_sites_300',
	'write'    => 1,
	'read'     => 1,
	'dataset'  => 'sites_300',
) );