includes/libs/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 = []; // (bucket index => UNIX timestamp)
  37
  38         final protected function doLockByType( array $pathsByType ) {
  39                 $status = StatusValue::newGood();
  40
  41                 $pathsByTypeByBucket = []; // (bucket => type => paths)
  42                 // Get locks that need to be acquired (buckets => locks)...
  43                 foreach ( $pathsByType as $type => $paths ) {
  44                         foreach ( $paths as $path ) {
  45                                 if ( isset( $this->locksHeld[$path][$type] ) ) {
  46                                         ++$this->locksHeld[$path][$type];
  47                                 } else {
  48                                         $bucket = $this->getBucketFromPath( $path );
  49                                         $pathsByTypeByBucket[$bucket][$type][] = $path;
  50                                 }
  51                         }
  52                 }
  53
  54                 // Acquire locks in each bucket in bucket order to reduce contention. Any blocking
  55                 // mutexes during the acquisition step will not involve circular waiting on buckets.
  56                 ksort( $pathsByTypeByBucket );
  57
  58                 $lockedPaths = []; // files locked in this attempt (type => paths)
  59                 // Attempt to acquire these locks...
  60                 foreach ( $pathsByTypeByBucket as $bucket => $bucketPathsByType ) {
  61                         // Try to acquire the locks for this bucket
  62                         $status->merge( $this->doLockingRequestBucket( $bucket, $bucketPathsByType ) );
  63                         if ( !$status->isOK() ) {
  64                                 $status->merge( $this->doUnlockByType( $lockedPaths ) );
  65
  66                                 return $status;
  67                         }
  68                         // Record these locks as active
  69                         foreach ( $bucketPathsByType as $type => $paths ) {
  70                                 foreach ( $paths as $path ) {
  71                                         $this->locksHeld[$path][$type] = 1; // locked
  72                                         // Keep track of what locks were made in this attempt
  73                                         $lockedPaths[$type][] = $path;
  74                                 }
  75                         }
  76                 }
  77
  78                 return $status;
  79         }
  80
  81         protected function doUnlockByType( array $pathsByType ) {
  82                 $status = StatusValue::newGood();
  83
  84                 $pathsByTypeByBucket = []; // (bucket => type => paths)
  85                 foreach ( $pathsByType as $type => $paths ) {
  86                         foreach ( $paths as $path ) {
  87                                 if ( !isset( $this->locksHeld[$path][$type] ) ) {
  88                                         $status->warning( 'lockmanager-notlocked', $path );
  89                                 } else {
  90                                         --$this->locksHeld[$path][$type];
  91                                         // Reference count the locks held and release locks when zero
  92                                         if ( $this->locksHeld[$path][$type] <= 0 ) {
  93                                                 unset( $this->locksHeld[$path][$type] );
  94                                                 $bucket = $this->getBucketFromPath( $path );
  95                                                 $pathsByTypeByBucket[$bucket][$type][] = $path;
  96                                         }
  97                                         if ( $this->locksHeld[$path] === [] ) {
  98                                                 unset( $this->locksHeld[$path] ); // no SH or EX locks left for key
  99                                         }
 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 ( $pathsByTypeByBucket as $bucket => $bucketPathsByType ) {
 107                         $status->merge( $this->doUnlockingRequestBucket( $bucket, $bucketPathsByType ) );
 108                 }
 109                 if ( $this->locksHeld === [] ) {
 110                         $status->merge( $this->releaseAllLocks() );
 111                         $this->degradedBuckets = []; // safe to retry the normal quorum
 112                 }
 113
 114                 return $status;
 115         }
 116
 117         /**
 118          * Attempt to acquire locks with the peers for a bucket.
 119          * This is all or nothing; if any key is locked then this totally fails.
 120          *
 121          * @param int $bucket
 122          * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
 123          * @return StatusValue
 124          */
 125         final protected function doLockingRequestBucket( $bucket, array $pathsByType ) {
 126                 return $this->collectPledgeQuorum(
 127                         $bucket,
 128                         function ( $lockSrv ) use ( $pathsByType ) {
 129                                 return $this->getLocksOnServer( $lockSrv, $pathsByType );
 130                         }
 131                 );
 132         }
 133
 134         /**
 135          * Attempt to release locks with the peers for a bucket
 136          *
 137          * @param int $bucket
 138          * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
 139          * @return StatusValue
 140          */
 141         final protected function doUnlockingRequestBucket( $bucket, array $pathsByType ) {
 142                 return $this->releasePledges(
 143                         $bucket,
 144                         function ( $lockSrv ) use ( $pathsByType ) {
 145                                 return $this->freeLocksOnServer( $lockSrv, $pathsByType );
 146                         }
 147                 );
 148         }
 149
 150         /**
 151          * Attempt to acquire pledges with the peers for a bucket.
 152          * This is all or nothing; if any key is already pledged then this totally fails.
 153          *
 154          * @param int $bucket
 155          * @param callable $callback Pledge method taking a server name and yielding a StatusValue
 156          * @return StatusValue
 157          */
 158         final protected function collectPledgeQuorum( $bucket, callable $callback ) {
 159                 $status = StatusValue::newGood();
 160
 161                 $yesVotes = 0; // locks made on trustable servers
 162                 $votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
 163                 $quorum = floor( $votesLeft / 2 + 1 ); // simple majority
 164                 // Get votes for each peer, in order, until we have enough...
 165                 foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
 166                         if ( !$this->isServerUp( $lockSrv ) ) {
 167                                 --$votesLeft;
 168                                 $status->warning( 'lockmanager-fail-svr-acquire', $lockSrv );
 169                                 $this->degradedBuckets[$bucket] = time();
 170                                 continue; // server down?
 171                         }
 172                         // Attempt to acquire the lock on this peer
 173                         $status->merge( $callback( $lockSrv ) );
 174                         if ( !$status->isOK() ) {
 175                                 return $status; // vetoed; resource locked
 176                         }
 177                         ++$yesVotes; // success for this peer
 178                         if ( $yesVotes >= $quorum ) {
 179                                 return $status; // lock obtained
 180                         }
 181                         --$votesLeft;
 182                         $votesNeeded = $quorum - $yesVotes;
 183                         if ( $votesNeeded > $votesLeft ) {
 184                                 break; // short-circuit
 185                         }
 186                 }
 187                 // At this point, we must not have met the quorum
 188                 $status->setResult( false );
 189
 190                 return $status;
 191         }
 192
 193         /**
 194          * Attempt to release pledges with the peers for a bucket
 195          *
 196          * @param int $bucket
 197          * @param callable $callback Pledge method taking a server name and yielding a StatusValue
 198          * @return StatusValue
 199          */
 200         final protected function releasePledges( $bucket, callable $callback ) {
 201                 $status = StatusValue::newGood();
 202
 203                 $yesVotes = 0; // locks freed on trustable servers
 204                 $votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
 205                 $quorum = floor( $votesLeft / 2 + 1 ); // simple majority
 206                 $isDegraded = isset( $this->degradedBuckets[$bucket] ); // not the normal quorum?
 207                 foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
 208                         if ( !$this->isServerUp( $lockSrv ) ) {
 209                                 $status->warning( 'lockmanager-fail-svr-release', $lockSrv );
 210                         } else {
 211                                 // Attempt to release the lock on this peer
 212                                 $status->merge( $callback( $lockSrv ) );
 213                                 ++$yesVotes; // success for this peer
 214                                 // Normally the first peers form the quorum, and the others are ignored.
 215                                 // Ignore them in this case, but not when an alternative quorum was used.
 216                                 if ( $yesVotes >= $quorum && !$isDegraded ) {
 217                                         break; // lock released
 218                                 }
 219                         }
 220                 }
 221                 // Set a bad StatusValue if the quorum was not met.
 222                 // Assumes the same "up" servers as during the acquire step.
 223                 $status->setResult( $yesVotes >= $quorum );
 224
 225                 return $status;
 226         }
 227
 228         /**
 229          * Get the bucket for resource path.
 230          * This should avoid throwing any exceptions.
 231          *
 232          * @param string $path
 233          * @return int
 234          */
 235         protected function getBucketFromPath( $path ) {
 236                 $prefix = substr( sha1( $path ), 0, 2 ); // first 2 hex chars (8 bits)
 237                 return (int)base_convert( $prefix, 16, 10 ) % count( $this->srvsByBucket );
 238         }
 239
 240         /**
 241          * Check if a lock server is up.
 242          * This should process cache results to reduce RTT.
 243          *
 244          * @param string $lockSrv
 245          * @return bool
 246          */
 247         abstract protected function isServerUp( $lockSrv );
 248
 249         /**
 250          * Get a connection to a lock server and acquire locks
 251          *
 252          * @param string $lockSrv
 253          * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
 254          * @return StatusValue
 255          */
 256         abstract protected function getLocksOnServer( $lockSrv, array $pathsByType );
 257
 258         /**
 259          * Get a connection to a lock server and release locks on $paths.
 260          *
 261          * Subclasses must effectively implement this or releaseAllLocks().
 262          *
 263          * @param string $lockSrv
 264          * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
 265          * @return StatusValue
 266          */
 267         abstract protected function freeLocksOnServer( $lockSrv, array $pathsByType );
 268
 269         /**
 270          * Release all locks that this session is holding.
 271          *
 272          * Subclasses must effectively implement this or freeLocksOnServer().
 273          *
 274          * @return StatusValue
 275          */
 276         abstract protected function releaseAllLocks();
 277
 278         final protected function doLock( array $paths, $type ) {
 279                 throw new LogicException( __METHOD__ . ': proxy class does not need this method.' );
 280         }
 281
 282         final protected function doUnlock( array $paths, $type ) {
 283                 throw new LogicException( __METHOD__ . ': proxy class does not need this method.' );
 284         }
 285 }