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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;
+ }
+ }
+}