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