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 = []; // (bucket index => (lsrv1, lsrv2, ...))
  34
  35         /** @var array Map of degraded buckets */
  36         protected $degradedBuckets = []; // (buckey index => UNIX timestamp)
  37
  38         final protected function doLock( array $paths, $type ) {
  39                 return $this->doLockByType( [ $type => $paths ] );
  40         }
  41
  42         final protected function doUnlock( array $paths, $type ) {
  43                 return $this->doUnlockByType( [ $type => $paths ] );
  44         }
  45
  46         protected function doLockByType( array $pathsByType ) {
  47                 $status = Status::newGood();
  48
  49                 $pathsToLock = []; // (bucket => type => paths)
  50                 // Get locks that need to be acquired (buckets => locks)...
  51                 foreach ( $pathsByType as $type => $paths ) {
  52                         foreach ( $paths as $path ) {
  53                                 if ( isset( $this->locksHeld[$path][$type] ) ) {
  54                                         ++$this->locksHeld[$path][$type];
  55                                 } else {
  56                                         $bucket = $this->getBucketFromPath( $path );
  57                                         $pathsToLock[$bucket][$type][] = $path;
  58                                 }
  59                         }
  60                 }
  61
  62                 $lockedPaths = []; // files locked in this attempt (type => paths)
  63                 // Attempt to acquire these locks...
  64                 foreach ( $pathsToLock as $bucket => $pathsToLockByType ) {
  65                         // Try to acquire the locks for this bucket
  66                         $status->merge( $this->doLockingRequestBucket( $bucket, $pathsToLockByType ) );
  67                         if ( !$status->isOK() ) {
  68                                 $status->merge( $this->doUnlockByType( $lockedPaths ) );
  69
  70                                 return $status;
  71                         }
  72                         // Record these locks as active
  73                         foreach ( $pathsToLockByType as $type => $paths ) {
  74                                 foreach ( $paths as $path ) {
  75                                         $this->locksHeld[$path][$type] = 1; // locked
  76                                         // Keep track of what locks were made in this attempt
  77                                         $lockedPaths[$type][] = $path;
  78                                 }
  79                         }
  80                 }
  81
  82                 return $status;
  83         }
  84
  85         protected function doUnlockByType( array $pathsByType ) {
  86                 $status = Status::newGood();
  87
  88                 $pathsToUnlock = []; // (bucket => type => paths)
  89                 foreach ( $pathsByType as $type => $paths ) {
  90                         foreach ( $paths as $path ) {
  91                                 if ( !isset( $this->locksHeld[$path][$type] ) ) {
  92                                         $status->warning( 'lockmanager-notlocked', $path );
  93                                 } else {
  94                                         --$this->locksHeld[$path][$type];
  95                                         // Reference count the locks held and release locks when zero
  96                                         if ( $this->locksHeld[$path][$type] <= 0 ) {
  97                                                 unset( $this->locksHeld[$path][$type] );
  98                                                 $bucket = $this->getBucketFromPath( $path );
  99                                                 $pathsToUnlock[$bucket][$type][] = $path;
 100                                         }
 101                                         if ( !count( $this->locksHeld[$path] ) ) {
 102                                                 unset( $this->locksHeld[$path] ); // no SH or EX locks left for key
 103                                         }
 104                                 }
 105                         }
 106                 }
 107
 108                 // Remove these specific locks if possible, or at least release
 109                 // all locks once this process is currently not holding any locks.
 110                 foreach ( $pathsToUnlock as $bucket => $pathsToUnlockByType ) {
 111                         $status->merge( $this->doUnlockingRequestBucket( $bucket, $pathsToUnlockByType ) );
 112                 }
 113                 if ( !count( $this->locksHeld ) ) {
 114                         $status->merge( $this->releaseAllLocks() );
 115                         $this->degradedBuckets = []; // safe to retry the normal quorum
 116                 }
 117
 118                 return $status;
 119         }
 120
 121         /**
 122          * Attempt to acquire locks with the peers for a bucket.
 123          * This is all or nothing; if any key is locked then this totally fails.
 124          *
 125          * @param int $bucket
 126          * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
 127          * @return Status
 128          */
 129         final protected function doLockingRequestBucket( $bucket, array $pathsByType ) {
 130                 $status = Status::newGood();
 131
 132                 $yesVotes = 0; // locks made on trustable servers
 133                 $votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
 134                 $quorum = floor( $votesLeft / 2 + 1 ); // simple majority
 135                 // Get votes for each peer, in order, until we have enough...
 136                 foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
 137                         if ( !$this->isServerUp( $lockSrv ) ) {
 138                                 --$votesLeft;
 139                                 $status->warning( 'lockmanager-fail-svr-acquire', $lockSrv );
 140                                 $this->degradedBuckets[$bucket] = time();
 141                                 continue; // server down?
 142                         }
 143                         // Attempt to acquire the lock on this peer
 144                         $status->merge( $this->getLocksOnServer( $lockSrv, $pathsByType ) );
 145                         if ( !$status->isOK() ) {
 146                                 return $status; // vetoed; resource locked
 147                         }
 148                         ++$yesVotes; // success for this peer
 149                         if ( $yesVotes >= $quorum ) {
 150                                 return $status; // lock obtained
 151                         }
 152                         --$votesLeft;
 153                         $votesNeeded = $quorum - $yesVotes;
 154                         if ( $votesNeeded > $votesLeft ) {
 155                                 break; // short-circuit
 156                         }
 157                 }
 158                 // At this point, we must not have met the quorum
 159                 $status->setResult( false );
 160
 161                 return $status;
 162         }
 163
 164         /**
 165          * Attempt to release locks with the peers for a bucket
 166          *
 167          * @param int $bucket
 168          * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
 169          * @return Status
 170          */
 171         final protected function doUnlockingRequestBucket( $bucket, array $pathsByType ) {
 172                 $status = Status::newGood();
 173
 174                 $yesVotes = 0; // locks freed on trustable servers
 175                 $votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
 176                 $quorum = floor( $votesLeft / 2 + 1 ); // simple majority
 177                 $isDegraded = isset( $this->degradedBuckets[$bucket] ); // not the normal quorum?
 178                 foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
 179                         if ( !$this->isServerUp( $lockSrv ) ) {
 180                                 $status->warning( 'lockmanager-fail-svr-release', $lockSrv );
 181                         } else {
 182                                 // Attempt to release the lock on this peer
 183                                 $status->merge( $this->freeLocksOnServer( $lockSrv, $pathsByType ) );
 184                                 ++$yesVotes; // success for this peer
 185                                 // Normally the first peers form the quorum, and the others are ignored.
 186                                 // Ignore them in this case, but not when an alternative quorum was used.
 187                                 if ( $yesVotes >= $quorum && !$isDegraded ) {
 188                                         break; // lock released
 189                                 }
 190                         }
 191                 }
 192                 // Set a bad status if the quorum was not met.
 193                 // Assumes the same "up" servers as during the acquire step.
 194                 $status->setResult( $yesVotes >= $quorum );
 195
 196                 return $status;
 197         }
 198
 199         /**
 200          * Get the bucket for resource path.
 201          * This should avoid throwing any exceptions.
 202          *
 203          * @param string $path
 204          * @return int
 205          */
 206         protected function getBucketFromPath( $path ) {
 207                 $prefix = substr( sha1( $path ), 0, 2 ); // first 2 hex chars (8 bits)
 208                 return (int)base_convert( $prefix, 16, 10 ) % count( $this->srvsByBucket );
 209         }
 210
 211         /**
 212          * Check if a lock server is up.
 213          * This should process cache results to reduce RTT.
 214          *
 215          * @param string $lockSrv
 216          * @return bool
 217          */
 218         abstract protected function isServerUp( $lockSrv );
 219
 220         /**
 221          * Get a connection to a lock server and acquire locks
 222          *
 223          * @param string $lockSrv
 224          * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
 225          * @return Status
 226          */
 227         abstract protected function getLocksOnServer( $lockSrv, array $pathsByType );
 228
 229         /**
 230          * Get a connection to a lock server and release locks on $paths.
 231          *
 232          * Subclasses must effectively implement this or releaseAllLocks().
 233          *
 234          * @param string $lockSrv
 235          * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
 236          * @return Status
 237          */
 238         abstract protected function freeLocksOnServer( $lockSrv, array $pathsByType );
 239
 240         /**
 241          * Release all locks that this session is holding.
 242          *
 243          * Subclasses must effectively implement this or freeLocksOnServer().
 244          *
 245          * @return Status
 246          */
 247         abstract protected function releaseAllLocks();
 248 }