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[mediawiki.git] / includes / filebackend / lockmanager / QuorumLockManager.php

<?php
/**
 * Version of LockManager that uses a quorum from peer servers for locks.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup LockManager
 */

/**
 * Version of LockManager that uses a quorum from peer servers for locks.
 * The resource space can also be sharded into separate peer groups.
 *
 * @ingroup LockManager
 * @since 1.20
 */
abstract class QuorumLockManager extends LockManager {
        /** @var array Map of bucket indexes to peer server lists */
        protected $srvsByBucket = array(); // (bucket index => (lsrv1, lsrv2, ...))

        /** @var array Map of degraded buckets */
        protected $degradedBuckets = array(); // (buckey index => UNIX timestamp)

        final protected function doLock( array $paths, $type ) {
                return $this->doLockByType( array( $type => $paths ) );
        }

        final protected function doUnlock( array $paths, $type ) {
                return $this->doUnlockByType( array( $type => $paths ) );
        }

        protected function doLockByType( array $pathsByType ) {
                $status = Status::newGood();

                $pathsToLock = array(); // (bucket => type => paths)
                // Get locks that need to be acquired (buckets => locks)...
                foreach ( $pathsByType as $type => $paths ) {
                        foreach ( $paths as $path ) {
                                if ( isset( $this->locksHeld[$path][$type] ) ) {
                                        ++$this->locksHeld[$path][$type];
                                } else {
                                        $bucket = $this->getBucketFromPath( $path );
                                        $pathsToLock[$bucket][$type][] = $path;
                                }
                        }
                }

                $lockedPaths = array(); // files locked in this attempt (type => paths)
                // Attempt to acquire these locks...
                foreach ( $pathsToLock as $bucket => $pathsToLockByType ) {
                        // Try to acquire the locks for this bucket
                        $status->merge( $this->doLockingRequestBucket( $bucket, $pathsToLockByType ) );
                        if ( !$status->isOK() ) {
                                $status->merge( $this->doUnlockByType( $lockedPaths ) );

                                return $status;
                        }
                        // Record these locks as active
                        foreach ( $pathsToLockByType as $type => $paths ) {
                                foreach ( $paths as $path ) {
                                        $this->locksHeld[$path][$type] = 1; // locked
                                        // Keep track of what locks were made in this attempt
                                        $lockedPaths[$type][] = $path;
                                }
                        }
                }

                return $status;
        }

        protected function doUnlockByType( array $pathsByType ) {
                $status = Status::newGood();

                $pathsToUnlock = array(); // (bucket => type => paths)
                foreach ( $pathsByType as $type => $paths ) {
                        foreach ( $paths as $path ) {
                                if ( !isset( $this->locksHeld[$path][$type] ) ) {
                                        $status->warning( 'lockmanager-notlocked', $path );
                                } else {
                                        --$this->locksHeld[$path][$type];
                                        // Reference count the locks held and release locks when zero
                                        if ( $this->locksHeld[$path][$type] <= 0 ) {
                                                unset( $this->locksHeld[$path][$type] );
                                                $bucket = $this->getBucketFromPath( $path );
                                                $pathsToUnlock[$bucket][$type][] = $path;
                                        }
                                        if ( !count( $this->locksHeld[$path] ) ) {
                                                unset( $this->locksHeld[$path] ); // no SH or EX locks left for key
                                        }
                                }
                        }
                }

                // Remove these specific locks if possible, or at least release
                // all locks once this process is currently not holding any locks.
                foreach ( $pathsToUnlock as $bucket => $pathsToUnlockByType ) {
                        $status->merge( $this->doUnlockingRequestBucket( $bucket, $pathsToUnlockByType ) );
                }
                if ( !count( $this->locksHeld ) ) {
                        $status->merge( $this->releaseAllLocks() );
                        $this->degradedBuckets = array(); // safe to retry the normal quorum
                }

                return $status;
        }

        /**
         * Attempt to acquire locks with the peers for a bucket.
         * This is all or nothing; if any key is locked then this totally fails.
         *
         * @param int $bucket
         * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
         * @return Status
         */
        final protected function doLockingRequestBucket( $bucket, array $pathsByType ) {
                $status = Status::newGood();

                $yesVotes = 0; // locks made on trustable servers
                $votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
                $quorum = floor( $votesLeft / 2 + 1 ); // simple majority
                // Get votes for each peer, in order, until we have enough...
                foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
                        if ( !$this->isServerUp( $lockSrv ) ) {
                                --$votesLeft;
                                $status->warning( 'lockmanager-fail-svr-acquire', $lockSrv );
                                $this->degradedBuckets[$bucket] = time();
                                continue; // server down?
                        }
                        // Attempt to acquire the lock on this peer
                        $status->merge( $this->getLocksOnServer( $lockSrv, $pathsByType ) );
                        if ( !$status->isOK() ) {
                                return $status; // vetoed; resource locked
                        }
                        ++$yesVotes; // success for this peer
                        if ( $yesVotes >= $quorum ) {
                                return $status; // lock obtained
                        }
                        --$votesLeft;
                        $votesNeeded = $quorum - $yesVotes;
                        if ( $votesNeeded > $votesLeft ) {
                                break; // short-circuit
                        }
                }
                // At this point, we must not have met the quorum
                $status->setResult( false );

                return $status;
        }

        /**
         * Attempt to release locks with the peers for a bucket
         *
         * @param int $bucket
         * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
         * @return Status
         */
        final protected function doUnlockingRequestBucket( $bucket, array $pathsByType ) {
                $status = Status::newGood();

                $yesVotes = 0; // locks freed on trustable servers
                $votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
                $quorum = floor( $votesLeft / 2 + 1 ); // simple majority
                $isDegraded = isset( $this->degradedBuckets[$bucket] ); // not the normal quorum?
                foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
                        if ( !$this->isServerUp( $lockSrv ) ) {
                                $status->warning( 'lockmanager-fail-svr-release', $lockSrv );
                        } else {
                                // Attempt to release the lock on this peer
                                $status->merge( $this->freeLocksOnServer( $lockSrv, $pathsByType ) );
                                ++$yesVotes; // success for this peer
                                // Normally the first peers form the quorum, and the others are ignored.
                                // Ignore them in this case, but not when an alternative quorum was used.
                                if ( $yesVotes >= $quorum && !$isDegraded ) {
                                        break; // lock released
                                }
                        }
                }
                // Set a bad status if the quorum was not met.
                // Assumes the same "up" servers as during the acquire step.
                $status->setResult( $yesVotes >= $quorum );

                return $status;
        }

        /**
         * Get the bucket for resource path.
         * This should avoid throwing any exceptions.
         *
         * @param string $path
         * @return int
         */
        protected function getBucketFromPath( $path ) {
                $prefix = substr( sha1( $path ), 0, 2 ); // first 2 hex chars (8 bits)
                return (int)base_convert( $prefix, 16, 10 ) % count( $this->srvsByBucket );
        }

        /**
         * Check if a lock server is up.
         * This should process cache results to reduce RTT.
         *
         * @param string $lockSrv
         * @return bool
         */
        abstract protected function isServerUp( $lockSrv );

        /**
         * Get a connection to a lock server and acquire locks
         *
         * @param string $lockSrv
         * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
         * @return Status
         */
        abstract protected function getLocksOnServer( $lockSrv, array $pathsByType );

        /**
         * Get a connection to a lock server and release locks on $paths.
         *
         * Subclasses must effectively implement this or releaseAllLocks().
         *
         * @param string $lockSrv
         * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
         * @return Status
         */
        abstract protected function freeLocksOnServer( $lockSrv, array $pathsByType );

        /**
         * Release all locks that this session is holding.
         *
         * Subclasses must effectively implement this or freeLocksOnServer().
         *
         * @return Status
         */
        abstract protected function releaseAllLocks();
}