1. Configuration

LudicrousDB can manage connections to a large number of databases. Queries are distributed to appropriate servers by mapping table names to datasets.

A dataset is defined as a group of tables that are located in the same database. There may be similarly-named databases containing different tables on different servers. There may also be many replicas of a database on different servers. The term "dataset" removes any ambiguity. Consider a dataset as a group of tables that can be mirrored on many servers.

Configuring LudicrousDB involves defining databases and datasets. Defining a database involves specifying the server connection details, the dataset it contains, and its capabilities and priorities for reading and writing. Defining a dataset involves specifying its exact table names or registering one or more callback functions that translate table names to datasets.

Sample 1: Default

This is the most basic way to add a server to LudicrousDB using only the required parameters: host, user, password, name. This adds the DB defined in wp-config.php as a read/write server for the 'global' dataset. (Every table is in 'global' by default.)

$wpdb->add_database( array(
	'host'     => DB_HOST,     // If port is other than 3306, use host:port.
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => DB_NAME,
) );

This adds the same server again, only this time it is configured as a replica. The last three parameters are set to the defaults but are shown for clarity.

$wpdb->add_database( array(
	'host'     => DB_HOST,     // If port is other than 3306, use host:port.
	'user'     => DB_USER,
	'password' => DB_PASSWORD,
	'name'     => DB_NAME,
	'write'    => 0,
	'read'     => 1,
	'dataset'  => 'global',
	'timeout'  => 0.2,
) );

Sample 2: Partition

This example shows a setup where the multisite blog tables have been separated from the global dataset.

$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';
	}
}

Functions

add_database()

$wpdb->add_database( $database );

$database is an associative array with these parameters:

host          (required) Hostname with optional :port. Default port is 3306.
user          (required) MySQL user name.
password      (required) MySQL user password.
name          (required) MySQL database name.
read          (optional) Whether server is readable. Default is 1 (readable).
                         Also used to assign preference. See "Network topology".
write         (optional) Whether server is writable. Default is 1 (writable).
                         Also used to assign preference in multi-primary mode.
dataset       (optional) Name of dataset. Default is 'global'.
timeout       (optional) Seconds to wait for TCP responsiveness. Default is 0.2
lag_threshold (optional) The minimum lag on a replica in seconds before we consider it lagged.
                         Set null to disable. When not set, the value of $wpdb->default_lag_threshold is used.

add_table()

$wpdb->add_table( $dataset, $table );

$dataset and $table are strings.

add_callback()

$wpdb->add_callback( $callback, $callback_group = 'dataset' );

$callback is a callable function or method. $callback_group is the group of callbacks, this $callback belongs to.

Callbacks are executed in the order in which they are registered until one of them returns something other than null.

The default $callback_group is 'dataset'. Callback in this group will be called with two arguments and expected to compute a dataset or return null.

$dataset = $callback($table, &$wpdb);

Anything evaluating to false will cause the query to be aborted.

For more complex setups, the callback may be used to overwrite properties of $wpdb or variables within LudicrousDB::connect_db(). If a callback returns an array, LudicrousDB will extract the array. It should be an associative array and it should include a $dataset value corresponding to a database added with $wpdb->add_database(). It may also include $server, which will be extracted to overwrite the parameters of each randomly selected database server prior to connection. This allows you to dynamically vary parameters such as the host, user, password, database name, lag_threshold and TCP check timeout.