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<?php
/**
* Generator of database load balancing objects.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
* http://www.gnu.org/copyleft/gpl.html
*
* @file
* @ingroup Database
*/
/**
* Class for ensuring a consistent ordering of events as seen by the user, despite replication.
* Kind of like Hawking's [[Chronology Protection Agency]].
*/
class ChronologyProtector {
/** @var Array (DB master name => position) */
protected $startupPositions = array();
/** @var Array (DB master name => position) */
protected $shutdownPositions = array();
protected $initialized = false; // bool; whether the session data was loaded
/**
* Initialise a LoadBalancer to give it appropriate chronology protection.
*
* If the session has a previous master position recorded, this will try to
* make sure that the next query to a slave of that master will see changes up
* to that position by delaying execution. The delay may timeout and allow stale
* data if no non-lagged slaves are available.
*
* @param $lb LoadBalancer
* @return void
*/
public function initLB( LoadBalancer $lb ) {
if ( $lb->getServerCount() <= 1 ) {
return; // non-replicated setup
}
if ( !$this->initialized ) {
$this->initialized = true;
if ( isset( $_SESSION[__CLASS__] ) && is_array( $_SESSION[__CLASS__] ) ) {
$this->startupPositions = $_SESSION[__CLASS__];
}
}
$masterName = $lb->getServerName( 0 );
if ( !empty( $this->startupPositions[$masterName] ) ) {
$info = $lb->parentInfo();
$pos = $this->startupPositions[$masterName];
wfDebug( __METHOD__ . ": LB " . $info['id'] . " waiting for master pos $pos\n" );
$lb->waitFor( $pos );
}
}
/**
* Notify the ChronologyProtector that the LoadBalancer is about to shut
* down. Saves replication positions.
*
* @param $lb LoadBalancer
* @return void
*/
public function shutdownLB( LoadBalancer $lb ) {
if ( session_id() == '' || $lb->getServerCount() <= 1 ) {
return; // don't start a session; don't bother with non-replicated setups
}
$masterName = $lb->getServerName( 0 );
if ( isset( $this->shutdownPositions[$masterName] ) ) {
return; // already done
}
// Only save the position if writes have been done on the connection
$db = $lb->getAnyOpenConnection( 0 );
$info = $lb->parentInfo();
if ( !$db || !$db->doneWrites() ) {
wfDebug( __METHOD__ . ": LB {$info['id']}, no writes done\n" );
return;
}
$pos = $db->getMasterPos();
wfDebug( __METHOD__ . ": LB {$info['id']} has master pos $pos\n" );
$this->shutdownPositions[$masterName] = $pos;
}
/**
* Notify the ChronologyProtector that the LBFactory is done calling shutdownLB() for now.
* May commit chronology data to persistent storage.
*
* @return void
*/
public function shutdown() {
if ( session_id() != '' && count( $this->shutdownPositions ) ) {
wfDebug( __METHOD__ . ": saving master pos for " .
count( $this->shutdownPositions ) . " master(s)\n" );
$_SESSION[__CLASS__] = $this->shutdownPositions;
}
}
}
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