cdsalmons
commented
8 years ago By extending the database with a dropin, a global table can be added to refine what needs to actively sync
`/**
- Adds data center ip. *
- @global type $dc_ips
- @param type $ip
- @param type $dc */ function add_dc_ip( $ip, $dc ) { global $dc_ips; $dc_ips[$ip] = $dc; }
// setup the list of known global database tables add_global_table( 'blogs' ); add_global_table( 'blog_versions' ); add_global_table( 'registration_log' ); add_global_table( 'signups' ); add_global_table( 'site' ); add_global_table( 'sitecategories' ); add_global_table( 'sitemeta' ); add_global_table( 'usermeta' ); add_global_table( 'users' ); add_global_table( 'bp_activity_sitewide' ); add_global_table( 'bp_activity_user_activity' ); add_global_table( 'bp_activity_user_activity_cached' ); add_global_table( 'bp_friends' ); add_global_table( 'bp_groups' ); add_global_table( 'bp_groups_groupmeta' ); add_global_table( 'bp_groups_members' ); add_global_table( 'bp_groups_wire' ); add_global_table( 'bp_messages_messages' ); add_global_table( 'bp_messages_notices' ); add_global_table( 'bp_messages_notices' ); add_global_table( 'bp_messages_recipients' ); add_global_table( 'bp_messages_threads' ); add_global_table( 'bp_messages_threads' ); add_global_table( 'bp_notifications' ); add_global_table( 'bp_user_blogs' ); add_global_table( 'bp_user_blogs_blogmeta' ); add_global_table( 'bp_user_blogs_comments' ); add_global_table( 'bp_user_blogs_posts' ); add_global_table( 'bp_xprofile_data' ); add_global_table( 'bp_xprofile_fields' ); add_global_table( 'bp_xprofile_groups' ); add_global_table( 'bp_xprofile_wire' ); add_global_table( 'bp_activity' ); add_global_table( 'bp_activity_meta' );
require_once WP_CONTENT_DIR . '/db-config.php';
if ( !defined( 'DATACENTER' ) ) { foreach ( $dc_ips as $dc_ip => $dc ) { if ( substr( $_SERVER['SERVER_ADDR'], 0, strlen( $dc_ip ) ) == $dc_ip ) { define( 'DATACENTER', $dc ); break; } } }
if ( file_exists( WP_CONTENT_DIR . '/db-list.php' ) ) { require_once WP_CONTENT_DIR . '/db-list.php'; }`
Then add in an abstraction layer to extend wpdb
class m_wpdb extends wpdb {
Once the class is extended and options defined, a method to connect to the database server/docker and select the correct database
` public function construct( $dbuser, $dbpassword, $dbname, $dbhost ) { register_shutdown_function( array( $this, 'destruct' ) );
if ( WP_DEBUG && WP_DEBUG_DISPLAY ) {
$this->show_errors();
}
$this->init_charset();
$this->dbuser = $dbuser;
$this->dbpassword = $dbpassword;
$this->dbname = $dbname;
$this->dbhost = $dbhost;
// Try to connect to the database
$global = $this->_get_global_read();
$this->dbhglobal = @mysql_connect( $global['host'], $global['user'], $global['password'], true );
$this->dbh = @mysql_connect( $global['host'], $global['user'], $global['password'], true );
if ( !$this->dbhglobal ) {
$this->_bail_db_connection_error();
}
$this->set_charset( $this->dbhglobal );
$this->ready = true;
$this->select( $global['name'], $this->dbhglobal );
}`Then returning the information
` /* * Returns global database information * @access private * @global array $db_servers The array of databases. * @return boolean|array The global database information on success, otherwise FALSE. */ private function _get_global_read() { global $db_servers;
if ( is_array( $db_servers['global'] ) ) {
if ( count( $db_servers['global'] ) > 1 ) {
$dc = defined( 'DATACENTER' ) ? DATACENTER : false;
foreach ( $db_servers['global'] as $global ) {
if ( $global['dc'] == $dc && $global['read'] > 0 ) {
return $global;
}
}
// If still here we can't find a local readable global database so return first readable one
foreach ( $db_servers['global'] as $global ) {
if ( $global['read'] > 0 ) {
return $global;
}
}
// Nope, none of those either so exit.
return false;
} else {
return $db_servers['global'][0];
}
}
return false;
}`If this is the direction you want to go, let me know and I'll send you the rest of the multi-db code
Data center awareness
WordPress + HyperDB supports running in multiple data centers. The HyperDB config includes comments on how to configure it for data center awareness:
Though MySQL is not the only service that needs data center awareness:
mcroutersupport for replicated pools claims to solve that, but I've never used it personally)Given the current implementation, it might be necessary to ignore performance issues with Memached and NFS transactions over the WAN. However, a better implementation would:
mcrouterand replicated pools or ditching Memcached for Couchbase, which provides a Memcached-compatible interface with cross-data center replicationRequirements for full active-active data center support
Story: The application will be deployed in data centers in two different regions connected by a WAN. Browsers may reach either data center with approximately equal frequency. Operators will specify one data center for the primary database instance, and the application will route requests internally to the correct primary instance in the correct DC.
Questions to answer:
Requirements for a standby data center
Story: We need a minimal foot print of the application running in a remote data center so that we can quickly recover if the the primary data center fails. The replica data center is not handling any end-user requests under normal use, and there is no provision for automatic fail-over. This approach seeks to reduce challenges by eliminating activity in the replica data center that would cause frustration due to slow performance of requests over the WAN or inconsistency due to writes in separate DCs (Memcached and NFS).
incomplete issue