includes/db/ChronologyProtector.php

   1 <?php
   2 /**
   3  * Generator of database load balancing objects.
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License along
  16  * with this program; if not, write to the Free Software Foundation, Inc.,
  17  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  18  * http://www.gnu.org/copyleft/gpl.html
  19  *
  20  * @file
  21  * @ingroup Database
  22  */
  23
  24 /**
  25  * Class for ensuring a consistent ordering of events as seen by the user, despite replication.
  26  * Kind of like Hawking's [[Chronology Protection Agency]].
  27  */
  28 class ChronologyProtector {
  29         /** @var array (DB master name => position) */
  30         protected $startupPositions = array();
  31
  32         /** @var array (DB master name => position) */
  33         protected $shutdownPositions = array();
  34
  35         /** @var bool Whether the session data was loaded */
  36         protected $initialized = false;
  37
  38         /**
  39          * Initialise a LoadBalancer to give it appropriate chronology protection.
  40          *
  41          * If the session has a previous master position recorded, this will try to
  42          * make sure that the next query to a slave of that master will see changes up
  43          * to that position by delaying execution. The delay may timeout and allow stale
  44          * data if no non-lagged slaves are available.
  45          *
  46          * @param LoadBalancer $lb
  47          * @return void
  48          */
  49         public function initLB( LoadBalancer $lb ) {
  50                 if ( $lb->getServerCount() <= 1 ) {
  51                         return; // non-replicated setup
  52                 }
  53                 if ( !$this->initialized ) {
  54                         $this->initialized = true;
  55                         if ( isset( $_SESSION[__CLASS__] ) && is_array( $_SESSION[__CLASS__] ) ) {
  56                                 $this->startupPositions = $_SESSION[__CLASS__];
  57                         }
  58                 }
  59                 $masterName = $lb->getServerName( 0 );
  60                 if ( !empty( $this->startupPositions[$masterName] ) ) {
  61                         $info = $lb->parentInfo();
  62                         $pos = $this->startupPositions[$masterName];
  63                         wfDebug( __METHOD__ . ": LB " . $info['id'] . " waiting for master pos $pos\n" );
  64                         $lb->waitFor( $pos );
  65                 }
  66         }
  67
  68         /**
  69          * Notify the ChronologyProtector that the LoadBalancer is about to shut
  70          * down. Saves replication positions.
  71          *
  72          * @param LoadBalancer $lb
  73          * @return void
  74          */
  75         public function shutdownLB( LoadBalancer $lb ) {
  76                 if ( session_id() == '' || $lb->getServerCount() <= 1 ) {
  77                         return; // don't start a session; don't bother with non-replicated setups
  78                 }
  79                 $masterName = $lb->getServerName( 0 );
  80                 if ( isset( $this->shutdownPositions[$masterName] ) ) {
  81                         return; // already done
  82                 }
  83                 // Only save the position if writes have been done on the connection
  84                 $db = $lb->getAnyOpenConnection( 0 );
  85                 $info = $lb->parentInfo();
  86                 if ( !$db || !$db->doneWrites() ) {
  87                         wfDebug( __METHOD__ . ": LB {$info['id']}, no writes done\n" );
  88
  89                         return;
  90                 }
  91                 $pos = $db->getMasterPos();
  92                 wfDebug( __METHOD__ . ": LB {$info['id']} has master pos $pos\n" );
  93                 $this->shutdownPositions[$masterName] = $pos;
  94         }
  95
  96         /**
  97          * Notify the ChronologyProtector that the LBFactory is done calling shutdownLB() for now.
  98          * May commit chronology data to persistent storage.
  99          *
 100          * @return void
 101          */
 102         public function shutdown() {
 103                 if ( session_id() != '' && count( $this->shutdownPositions ) ) {
 104                         wfDebug( __METHOD__ . ": saving master pos for " .
 105                                 count( $this->shutdownPositions ) . " master(s)\n" );
 106                         $_SESSION[__CLASS__] = $this->shutdownPositions;
 107                 }
 108         }
 109 }