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                 $pathsToLock = []; // (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                                         $pathsToLock[$bucket][$type][] = $path;
  50                                 }
  51                         }
  52                 }
  53
  54                 $lockedPaths = []; // files locked in this attempt (type => paths)
  55                 // Attempt to acquire these locks...
  56                 foreach ( $pathsToLock as $bucket => $pathsToLockByType ) {
  57                         // Try to acquire the locks for this bucket
  58                         $status->merge( $this->doLockingRequestBucket( $bucket, $pathsToLockByType ) );
  59                         if ( !$status->isOK() ) {
  60                                 $status->merge( $this->doUnlockByType( $lockedPaths ) );
  61
  62                                 return $status;
  63                         }
  64                         // Record these locks as active
  65                         foreach ( $pathsToLockByType as $type => $paths ) {
  66                                 foreach ( $paths as $path ) {
  67                                         $this->locksHeld[$path][$type] = 1; // locked
  68                                         // Keep track of what locks were made in this attempt
  69                                         $lockedPaths[$type][] = $path;
  70                                 }
  71                         }
  72                 }
  73
  74                 return $status;
  75         }
  76
  77         protected function doUnlockByType( array $pathsByType ) {
  78                 $status = StatusValue::newGood();
  79
  80                 $pathsToUnlock = []; // (bucket => type => paths)
  81                 foreach ( $pathsByType as $type => $paths ) {
  82                         foreach ( $paths as $path ) {
  83                                 if ( !isset( $this->locksHeld[$path][$type] ) ) {
  84                                         $status->warning( 'lockmanager-notlocked', $path );
  85                                 } else {
  86                                         --$this->locksHeld[$path][$type];
  87                                         // Reference count the locks held and release locks when zero
  88                                         if ( $this->locksHeld[$path][$type] <= 0 ) {
  89                                                 unset( $this->locksHeld[$path][$type] );
  90                                                 $bucket = $this->getBucketFromPath( $path );
  91                                                 $pathsToUnlock[$bucket][$type][] = $path;
  92                                         }
  93                                         if ( $this->locksHeld[$path] === [] ) {
  94                                                 unset( $this->locksHeld[$path] ); // no SH or EX locks left for key
  95                                         }
  96                                 }
  97                         }
  98                 }
  99
 100                 // Remove these specific locks if possible, or at least release
 101                 // all locks once this process is currently not holding any locks.
 102                 foreach ( $pathsToUnlock as $bucket => $pathsToUnlockByType ) {
 103                         $status->merge( $this->doUnlockingRequestBucket( $bucket, $pathsToUnlockByType ) );
 104                 }
 105                 if ( $this->locksHeld === [] ) {
 106                         $status->merge( $this->releaseAllLocks() );
 107                         $this->degradedBuckets = []; // safe to retry the normal quorum
 108                 }
 109
 110                 return $status;
 111         }
 112
 113         /**
 114          * Attempt to acquire locks with the peers for a bucket.
 115          * This is all or nothing; if any key is locked then this totally fails.
 116          *
 117          * @param int $bucket
 118          * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
 119          * @return StatusValue
 120          */
 121         final protected function doLockingRequestBucket( $bucket, array $pathsByType ) {
 122                 return $this->collectPledgeQuorum(
 123                         $bucket,
 124                         function ( $lockSrv ) use ( $pathsByType ) {
 125                                 return $this->getLocksOnServer( $lockSrv, $pathsByType );
 126                         }
 127                 );
 128         }
 129
 130         /**
 131          * Attempt to release locks with the peers for a bucket
 132          *
 133          * @param int $bucket
 134          * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
 135          * @return StatusValue
 136          */
 137         final protected function doUnlockingRequestBucket( $bucket, array $pathsByType ) {
 138                 return $this->releasePledges(
 139                         $bucket,
 140                         function ( $lockSrv ) use ( $pathsByType ) {
 141                                 return $this->freeLocksOnServer( $lockSrv, $pathsByType );
 142                         }
 143                 );
 144         }
 145
 146         /**
 147          * Attempt to acquire pledges with the peers for a bucket.
 148          * This is all or nothing; if any key is already pledged then this totally fails.
 149          *
 150          * @param int $bucket
 151          * @param callable $callback Pledge method taking a server name and yeilding a StatusValue
 152          * @return StatusValue
 153          */
 154         final protected function collectPledgeQuorum( $bucket, callable $callback ) {
 155                 $status = StatusValue::newGood();
 156
 157                 $yesVotes = 0; // locks made on trustable servers
 158                 $votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
 159                 $quorum = floor( $votesLeft / 2 + 1 ); // simple majority
 160                 // Get votes for each peer, in order, until we have enough...
 161                 foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
 162                         if ( !$this->isServerUp( $lockSrv ) ) {
 163                                 --$votesLeft;
 164                                 $status->warning( 'lockmanager-fail-svr-acquire', $lockSrv );
 165                                 $this->degradedBuckets[$bucket] = time();
 166                                 continue; // server down?
 167                         }
 168                         // Attempt to acquire the lock on this peer
 169                         $status->merge( $callback( $lockSrv ) );
 170                         if ( !$status->isOK() ) {
 171                                 return $status; // vetoed; resource locked
 172                         }
 173                         ++$yesVotes; // success for this peer
 174                         if ( $yesVotes >= $quorum ) {
 175                                 return $status; // lock obtained
 176                         }
 177                         --$votesLeft;
 178                         $votesNeeded = $quorum - $yesVotes;
 179                         if ( $votesNeeded > $votesLeft ) {
 180                                 break; // short-circuit
 181                         }
 182                 }
 183                 // At this point, we must not have met the quorum
 184                 $status->setResult( false );
 185
 186                 return $status;
 187         }
 188
 189         /**
 190          * Attempt to release pledges with the peers for a bucket
 191          *
 192          * @param int $bucket
 193          * @param callable $callback Pledge method taking a server name and yeilding a StatusValue
 194          * @return StatusValue
 195          */
 196         final protected function releasePledges( $bucket, callable $callback ) {
 197                 $status = StatusValue::newGood();
 198
 199                 $yesVotes = 0; // locks freed on trustable servers
 200                 $votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
 201                 $quorum = floor( $votesLeft / 2 + 1 ); // simple majority
 202                 $isDegraded = isset( $this->degradedBuckets[$bucket] ); // not the normal quorum?
 203                 foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
 204                         if ( !$this->isServerUp( $lockSrv ) ) {
 205                                 $status->warning( 'lockmanager-fail-svr-release', $lockSrv );
 206                         } else {
 207                                 // Attempt to release the lock on this peer
 208                                 $status->merge( $callback( $lockSrv ) );
 209                                 ++$yesVotes; // success for this peer
 210                                 // Normally the first peers form the quorum, and the others are ignored.
 211                                 // Ignore them in this case, but not when an alternative quorum was used.
 212                                 if ( $yesVotes >= $quorum && !$isDegraded ) {
 213                                         break; // lock released
 214                                 }
 215                         }
 216                 }
 217                 // Set a bad StatusValue if the quorum was not met.
 218                 // Assumes the same "up" servers as during the acquire step.
 219                 $status->setResult( $yesVotes >= $quorum );
 220
 221                 return $status;
 222         }
 223
 224         /**
 225          * Get the bucket for resource path.
 226          * This should avoid throwing any exceptions.
 227          *
 228          * @param string $path
 229          * @return int
 230          */
 231         protected function getBucketFromPath( $path ) {
 232                 $prefix = substr( sha1( $path ), 0, 2 ); // first 2 hex chars (8 bits)
 233                 return (int)base_convert( $prefix, 16, 10 ) % count( $this->srvsByBucket );
 234         }
 235
 236         /**
 237          * Check if a lock server is up.
 238          * This should process cache results to reduce RTT.
 239          *
 240          * @param string $lockSrv
 241          * @return bool
 242          */
 243         abstract protected function isServerUp( $lockSrv );
 244
 245         /**
 246          * Get a connection to a lock server and acquire locks
 247          *
 248          * @param string $lockSrv
 249          * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
 250          * @return StatusValue
 251          */
 252         abstract protected function getLocksOnServer( $lockSrv, array $pathsByType );
 253
 254         /**
 255          * Get a connection to a lock server and release locks on $paths.
 256          *
 257          * Subclasses must effectively implement this or releaseAllLocks().
 258          *
 259          * @param string $lockSrv
 260          * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
 261          * @return StatusValue
 262          */
 263         abstract protected function freeLocksOnServer( $lockSrv, array $pathsByType );
 264
 265         /**
 266          * Release all locks that this session is holding.
 267          *
 268          * Subclasses must effectively implement this or freeLocksOnServer().
 269          *
 270          * @return StatusValue
 271          */
 272         abstract protected function releaseAllLocks();
 273
 274         final protected function doLock( array $paths, $type ) {
 275                 throw new LogicException( __METHOD__ . ': proxy class does not need this method.' );
 276         }
 277
 278         final protected function doUnlock( array $paths, $type ) {
 279                 throw new LogicException( __METHOD__ . ': proxy class does not need this method.' );
 280         }
 281 }