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                         } elseif ( isset( $this->locksHeld[$path][self::LOCK_EX] ) ) {
  50                                 $this->locksHeld[$path][$type] = 1;
  51                         } else {
  52                                 $bucket = $this->getBucketFromPath( $path );
  53                                 $pathsToLock[$bucket][] = $path;
  54                         }
  55                 }
  56
  57                 $lockedPaths = array(); // files locked in this attempt
  58                 // Attempt to acquire these locks...
  59                 foreach ( $pathsToLock as $bucket => $paths ) {
  60                         // Try to acquire the locks for this bucket
  61                         $status->merge( $this->doLockingRequestBucket( $bucket, $paths, $type ) );
  62                         if ( !$status->isOK() ) {
  63                                 $status->merge( $this->doUnlock( $lockedPaths, $type ) );
  64                                 return $status;
  65                         }
  66                         // Record these locks as active
  67                         foreach ( $paths as $path ) {
  68                                 $this->locksHeld[$path][$type] = 1; // locked
  69                         }
  70                         // Keep track of what locks were made in this attempt
  71                         $lockedPaths = array_merge( $lockedPaths, $paths );
  72                 }
  73
  74                 return $status;
  75         }
  76
  77         /**
  78          * @see LockManager::doUnlock()
  79          * @param $paths array
  80          * @param $type int
  81          * @return Status
  82          */
  83         final protected function doUnlock( array $paths, $type ) {
  84                 $status = Status::newGood();
  85
  86                 $pathsToUnlock = array();
  87                 foreach ( $paths as $path ) {
  88                         if ( !isset( $this->locksHeld[$path][$type] ) ) {
  89                                 $status->warning( 'lockmanager-notlocked', $path );
  90                         } else {
  91                                 --$this->locksHeld[$path][$type];
  92                                 // Reference count the locks held and release locks when zero
  93                                 if ( $this->locksHeld[$path][$type] <= 0 ) {
  94                                         unset( $this->locksHeld[$path][$type] );
  95                                         $bucket = $this->getBucketFromPath( $path );
  96                                         $pathsToUnlock[$bucket][] = $path;
  97                                 }
  98                                 if ( !count( $this->locksHeld[$path] ) ) {
  99                                         unset( $this->locksHeld[$path] ); // no SH or EX locks left for key
 100                                 }
 101                         }
 102                 }
 103
 104                 // Remove these specific locks if possible, or at least release
 105                 // all locks once this process is currently not holding any locks.
 106                 foreach ( $pathsToUnlock as $bucket => $paths ) {
 107                         $status->merge( $this->doUnlockingRequestBucket( $bucket, $paths, $type ) );
 108                 }
 109                 if ( !count( $this->locksHeld ) ) {
 110                         $status->merge( $this->releaseAllLocks() );
 111                 }
 112
 113                 return $status;
 114         }
 115
 116         /**
 117          * Attempt to acquire locks with the peers for a bucket.
 118          * This is all or nothing; if any key is locked then this totally fails.
 119          *
 120          * @param $bucket integer
 121          * @param $paths Array List of resource keys to lock
 122          * @param $type integer LockManager::LOCK_EX or LockManager::LOCK_SH
 123          * @return Status
 124          */
 125         final protected function doLockingRequestBucket( $bucket, array $paths, $type ) {
 126                 $status = Status::newGood();
 127
 128                 $yesVotes = 0; // locks made on trustable servers
 129                 $votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
 130                 $quorum = floor( $votesLeft/2 + 1 ); // simple majority
 131                 // Get votes for each peer, in order, until we have enough...
 132                 foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
 133                         if ( !$this->isServerUp( $lockSrv ) ) {
 134                                 --$votesLeft;
 135                                 $status->warning( 'lockmanager-fail-svr-acquire', $lockSrv );
 136                                 continue; // server down?
 137                         }
 138                         // Attempt to acquire the lock on this peer
 139                         $status->merge( $this->getLocksOnServer( $lockSrv, $paths, $type ) );
 140                         if ( !$status->isOK() ) {
 141                                 return $status; // vetoed; resource locked
 142                         }
 143                         ++$yesVotes; // success for this peer
 144                         if ( $yesVotes >= $quorum ) {
 145                                 return $status; // lock obtained
 146                         }
 147                         --$votesLeft;
 148                         $votesNeeded = $quorum - $yesVotes;
 149                         if ( $votesNeeded > $votesLeft ) {
 150                                 break; // short-circuit
 151                         }
 152                 }
 153                 // At this point, we must not have met the quorum
 154                 $status->setResult( false );
 155
 156                 return $status;
 157         }
 158
 159         /**
 160          * Attempt to release locks with the peers for a bucket
 161          *
 162          * @param $bucket integer
 163          * @param $paths Array List of resource keys to lock
 164          * @param $type integer LockManager::LOCK_EX or LockManager::LOCK_SH
 165          * @return Status
 166          */
 167         final protected function doUnlockingRequestBucket( $bucket, array $paths, $type ) {
 168                 $status = Status::newGood();
 169
 170                 foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
 171                         if ( !$this->isServerUp( $lockSrv ) ) {
 172                                 $status->fatal( 'lockmanager-fail-svr-release', $lockSrv );
 173                         // Attempt to release the lock on this peer
 174                         } else {
 175                                 $status->merge( $this->freeLocksOnServer( $lockSrv, $paths, $type ) );
 176                         }
 177                 }
 178
 179                 return $status;
 180         }
 181
 182         /**
 183          * Get the bucket for resource path.
 184          * This should avoid throwing any exceptions.
 185          *
 186          * @param $path string
 187          * @return integer
 188          */
 189         protected function getBucketFromPath( $path ) {
 190                 $prefix = substr( sha1( $path ), 0, 2 ); // first 2 hex chars (8 bits)
 191                 return (int)base_convert( $prefix, 16, 10 ) % count( $this->srvsByBucket );
 192         }
 193
 194         /**
 195          * Check if a lock server is up
 196          *
 197          * @param $lockSrv string
 198          * @return bool
 199          */
 200         abstract protected function isServerUp( $lockSrv );
 201
 202         /**
 203          * Get a connection to a lock server and acquire locks on $paths
 204          *
 205          * @param $lockSrv string
 206          * @param $paths array
 207          * @param $type integer
 208          * @return Status
 209          */
 210         abstract protected function getLocksOnServer( $lockSrv, array $paths, $type );
 211
 212         /**
 213          * Get a connection to a lock server and release locks on $paths.
 214          *
 215          * Subclasses must effectively implement this or releaseAllLocks().
 216          *
 217          * @param $lockSrv string
 218          * @param $paths array
 219          * @param $type integer
 220          * @return Status
 221          */
 222         abstract protected function freeLocksOnServer( $lockSrv, array $paths, $type );
 223
 224         /**
 225          * Release all locks that this session is holding.
 226          *
 227          * Subclasses must effectively implement this or freeLocksOnServer().
 228          *
 229          * @return Status
 230          */
 231         abstract protected function releaseAllLocks();
 232 }