includes/filebackend/lockmanager/QuorumLockManager.php

   1 <?php
   2 /**
   3  * Version of LockManager that uses a quorum from peer servers for locks.
   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 LockManager
  22  */
  23
  24 /**
  25  * Version of LockManager that uses a quorum from peer servers for locks.
  26  * The resource space can also be sharded into separate peer groups.
  27  *
  28  * @ingroup LockManager
  29  * @since 1.20
  30  */
  31 abstract class QuorumLockManager extends LockManager {
  32         /** @var Array Map of bucket indexes to peer server lists */
  33         protected $srvsByBucket = array(); // (bucket index => (lsrv1, lsrv2, ...))
  34
  35         /**
  36          * @see LockManager::doLock()
  37          * @param $paths array
  38          * @param $type int
  39          * @return Status
  40          */
  41         final protected function doLock( array $paths, $type ) {
  42                 $status = Status::newGood();
  43
  44                 $pathsToLock = array(); // (bucket => paths)
  45                 // Get locks that need to be acquired (buckets => locks)...
  46                 foreach ( $paths as $path ) {
  47                         if ( isset( $this->locksHeld[$path][$type] ) ) {
  48                                 ++$this->locksHeld[$path][$type];
  49                         } else {
  50                                 $bucket = $this->getBucketFromPath( $path );
  51                                 $pathsToLock[$bucket][] = $path;
  52                         }
  53                 }
  54
  55                 $lockedPaths = array(); // files locked in this attempt
  56                 // Attempt to acquire these locks...
  57                 foreach ( $pathsToLock as $bucket => $paths ) {
  58                         // Try to acquire the locks for this bucket
  59                         $status->merge( $this->doLockingRequestBucket( $bucket, $paths, $type ) );
  60                         if ( !$status->isOK() ) {
  61                                 $status->merge( $this->doUnlock( $lockedPaths, $type ) );
  62                                 return $status;
  63                         }
  64                         // Record these locks as active
  65                         foreach ( $paths as $path ) {
  66                                 $this->locksHeld[$path][$type] = 1; // locked
  67                         }
  68                         // Keep track of what locks were made in this attempt
  69                         $lockedPaths = array_merge( $lockedPaths, $paths );
  70                 }
  71
  72                 return $status;
  73         }
  74
  75         /**
  76          * @see LockManager::doUnlock()
  77          * @param $paths array
  78          * @param $type int
  79          * @return Status
  80          */
  81         final protected function doUnlock( array $paths, $type ) {
  82                 $status = Status::newGood();
  83
  84                 $pathsToUnlock = array();
  85                 foreach ( $paths as $path ) {
  86                         if ( !isset( $this->locksHeld[$path][$type] ) ) {
  87                                 $status->warning( 'lockmanager-notlocked', $path );
  88                         } else {
  89                                 --$this->locksHeld[$path][$type];
  90                                 // Reference count the locks held and release locks when zero
  91                                 if ( $this->locksHeld[$path][$type] <= 0 ) {
  92                                         unset( $this->locksHeld[$path][$type] );
  93                                         $bucket = $this->getBucketFromPath( $path );
  94                                         $pathsToUnlock[$bucket][] = $path;
  95                                 }
  96                                 if ( !count( $this->locksHeld[$path] ) ) {
  97                                         unset( $this->locksHeld[$path] ); // no SH or EX locks left for key
  98                                 }
  99                         }
 100                 }
 101
 102                 // Remove these specific locks if possible, or at least release
 103                 // all locks once this process is currently not holding any locks.
 104                 foreach ( $pathsToUnlock as $bucket => $paths ) {
 105                         $status->merge( $this->doUnlockingRequestBucket( $bucket, $paths, $type ) );
 106                 }
 107                 if ( !count( $this->locksHeld ) ) {
 108                         $status->merge( $this->releaseAllLocks() );
 109                 }
 110
 111                 return $status;
 112         }
 113
 114         /**
 115          * Attempt to acquire locks with the peers for a bucket.
 116          * This is all or nothing; if any key is locked then this totally fails.
 117          *
 118          * @param $bucket integer
 119          * @param array $paths List of resource keys to lock
 120          * @param $type integer LockManager::LOCK_EX or LockManager::LOCK_SH
 121          * @return Status
 122          */
 123         final protected function doLockingRequestBucket( $bucket, array $paths, $type ) {
 124                 $status = Status::newGood();
 125
 126                 $yesVotes = 0; // locks made on trustable servers
 127                 $votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
 128                 $quorum = floor( $votesLeft / 2 + 1 ); // simple majority
 129                 // Get votes for each peer, in order, until we have enough...
 130                 foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
 131                         if ( !$this->isServerUp( $lockSrv ) ) {
 132                                 --$votesLeft;
 133                                 $status->warning( 'lockmanager-fail-svr-acquire', $lockSrv );
 134                                 continue; // server down?
 135                         }
 136                         // Attempt to acquire the lock on this peer
 137                         $status->merge( $this->getLocksOnServer( $lockSrv, $paths, $type ) );
 138                         if ( !$status->isOK() ) {
 139                                 return $status; // vetoed; resource locked
 140                         }
 141                         ++$yesVotes; // success for this peer
 142                         if ( $yesVotes >= $quorum ) {
 143                                 return $status; // lock obtained
 144                         }
 145                         --$votesLeft;
 146                         $votesNeeded = $quorum - $yesVotes;
 147                         if ( $votesNeeded > $votesLeft ) {
 148                                 break; // short-circuit
 149                         }
 150                 }
 151                 // At this point, we must not have met the quorum
 152                 $status->setResult( false );
 153
 154                 return $status;
 155         }
 156
 157         /**
 158          * Attempt to release locks with the peers for a bucket
 159          *
 160          * @param $bucket integer
 161          * @param array $paths List of resource keys to lock
 162          * @param $type integer LockManager::LOCK_EX or LockManager::LOCK_SH
 163          * @return Status
 164          */
 165         final protected function doUnlockingRequestBucket( $bucket, array $paths, $type ) {
 166                 $status = Status::newGood();
 167
 168                 foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
 169                         if ( !$this->isServerUp( $lockSrv ) ) {
 170                                 $status->fatal( 'lockmanager-fail-svr-release', $lockSrv );
 171                         // Attempt to release the lock on this peer
 172                         } else {
 173                                 $status->merge( $this->freeLocksOnServer( $lockSrv, $paths, $type ) );
 174                         }
 175                 }
 176
 177                 return $status;
 178         }
 179
 180         /**
 181          * Get the bucket for resource path.
 182          * This should avoid throwing any exceptions.
 183          *
 184          * @param $path string
 185          * @return integer
 186          */
 187         protected function getBucketFromPath( $path ) {
 188                 $prefix = substr( sha1( $path ), 0, 2 ); // first 2 hex chars (8 bits)
 189                 return (int)base_convert( $prefix, 16, 10 ) % count( $this->srvsByBucket );
 190         }
 191
 192         /**
 193          * Check if a lock server is up
 194          *
 195          * @param $lockSrv string
 196          * @return bool
 197          */
 198         abstract protected function isServerUp( $lockSrv );
 199
 200         /**
 201          * Get a connection to a lock server and acquire locks on $paths
 202          *
 203          * @param $lockSrv string
 204          * @param $paths array
 205          * @param $type integer
 206          * @return Status
 207          */
 208         abstract protected function getLocksOnServer( $lockSrv, array $paths, $type );
 209
 210         /**
 211          * Get a connection to a lock server and release locks on $paths.
 212          *
 213          * Subclasses must effectively implement this or releaseAllLocks().
 214          *
 215          * @param $lockSrv string
 216          * @param $paths array
 217          * @param $type integer
 218          * @return Status
 219          */
 220         abstract protected function freeLocksOnServer( $lockSrv, array $paths, $type );
 221
 222         /**
 223          * Release all locks that this session is holding.
 224          *
 225          * Subclasses must effectively implement this or freeLocksOnServer().
 226          *
 227          * @return Status
 228          */
 229         abstract protected function releaseAllLocks();
 230 }