Move ILoadBalancer to Rdbms namespace

[mediawiki.git] / includes / libs / rdbms / ChronologyProtector.php

<?php
/**
 * Generator of database load balancing objects.
 *
 * 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 Database
 */

namespace Wikimedia\Rdbms;

use Psr\Log\LoggerAwareInterface;
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;
use Wikimedia\WaitConditionLoop;
use BagOStuff;
use DBMasterPos;

/**
 * Class for ensuring a consistent ordering of events as seen by the user, despite replication.
 * Kind of like Hawking's [[Chronology Protection Agency]].
 */
class ChronologyProtector implements LoggerAwareInterface {
        /** @var BagOStuff */
        protected $store;
        /** @var LoggerInterface */
        protected $logger;

        /** @var string Storage key name */
        protected $key;
        /** @var string Hash of client parameters */
        protected $clientId;
        /** @var float|null Minimum UNIX timestamp of 1+ expected startup positions */
        protected $waitForPosTime;
        /** @var int Max seconds to wait on positions to appear */
        protected $waitForPosTimeout = self::POS_WAIT_TIMEOUT;
        /** @var bool Whether to no-op all method calls */
        protected $enabled = true;
        /** @var bool Whether to check and wait on positions */
        protected $wait = true;

        /** @var bool Whether the client data was loaded */
        protected $initialized = false;
        /** @var DBMasterPos[] Map of (DB master name => position) */
        protected $startupPositions = [];
        /** @var DBMasterPos[] Map of (DB master name => position) */
        protected $shutdownPositions = [];
        /** @var float[] Map of (DB master name => 1) */
        protected $shutdownTouchDBs = [];

        /** @var integer Seconds to store positions */
        const POSITION_TTL = 60;
        /** @var integer Max time to wait for positions to appear */
        const POS_WAIT_TIMEOUT = 5;

        /**
         * @param BagOStuff $store
         * @param array $client Map of (ip: <IP>, agent: <user-agent>)
         * @param float $posTime UNIX timestamp
         * @since 1.27
         */
        public function __construct( BagOStuff $store, array $client, $posTime = null ) {
                $this->store = $store;
                $this->clientId = md5( $client['ip'] . "\n" . $client['agent'] );
                $this->key = $store->makeGlobalKey( __CLASS__, $this->clientId );
                $this->waitForPosTime = $posTime;
                $this->logger = new NullLogger();
        }

        public function setLogger( LoggerInterface $logger ) {
                $this->logger = $logger;
        }

        /**
         * @param bool $enabled Whether to no-op all method calls
         * @since 1.27
         */
        public function setEnabled( $enabled ) {
                $this->enabled = $enabled;
        }

        /**
         * @param bool $enabled Whether to check and wait on positions
         * @since 1.27
         */
        public function setWaitEnabled( $enabled ) {
                $this->wait = $enabled;
        }

        /**
         * Initialise a ILoadBalancer to give it appropriate chronology protection.
         *
         * If the stash has a previous master position recorded, this will try to
         * make sure that the next query to a replica DB of that master will see changes up
         * to that position by delaying execution. The delay may timeout and allow stale
         * data if no non-lagged replica DBs are available.
         *
         * @param ILoadBalancer $lb
         * @return void
         */
        public function initLB( ILoadBalancer $lb ) {
                if ( !$this->enabled || $lb->getServerCount() <= 1 ) {
                        return; // non-replicated setup or disabled
                }

                $this->initPositions();

                $masterName = $lb->getServerName( $lb->getWriterIndex() );
                if ( !empty( $this->startupPositions[$masterName] ) ) {
                        $pos = $this->startupPositions[$masterName];
                        $this->logger->info( __METHOD__ . ": LB for '$masterName' set to pos $pos\n" );
                        $lb->waitFor( $pos );
                }
        }

        /**
         * Notify the ChronologyProtector that the ILoadBalancer is about to shut
         * down. Saves replication positions.
         *
         * @param ILoadBalancer $lb
         * @return void
         */
        public function shutdownLB( ILoadBalancer $lb ) {
                if ( !$this->enabled ) {
                        return; // not enabled
                } elseif ( !$lb->hasOrMadeRecentMasterChanges( INF ) ) {
                        // Only save the position if writes have been done on the connection
                        return;
                }

                $masterName = $lb->getServerName( $lb->getWriterIndex() );
                if ( $lb->getServerCount() > 1 ) {
                        $pos = $lb->getMasterPos();
                        $this->logger->info( __METHOD__ . ": LB for '$masterName' has pos $pos\n" );
                        $this->shutdownPositions[$masterName] = $pos;
                } else {
                        $this->logger->info( __METHOD__ . ": DB '$masterName' touched\n" );
                }
                $this->shutdownTouchDBs[$masterName] = 1;
        }

        /**
         * Notify the ChronologyProtector that the LBFactory is done calling shutdownLB() for now.
         * May commit chronology data to persistent storage.
         *
         * @param callable|null $workCallback Work to do instead of waiting on syncing positions
         * @param string $mode One of (sync, async); whether to wait on remote datacenters
         * @return DBMasterPos[] Empty on success; returns the (db name => position) map on failure
         */
        public function shutdown( callable $workCallback = null, $mode = 'sync' ) {
                if ( !$this->enabled ) {
                        return [];
                }

                $store = $this->store;
                // Some callers might want to know if a user recently touched a DB.
                // These writes do not need to block on all datacenters receiving them.
                foreach ( $this->shutdownTouchDBs as $dbName => $unused ) {
                        $store->set(
                                $this->getTouchedKey( $this->store, $dbName ),
                                microtime( true ),
                                $store::TTL_DAY
                        );
                }

                if ( !count( $this->shutdownPositions ) ) {
                        return []; // nothing to save
                }

                $this->logger->info( __METHOD__ . ": saving master pos for " .
                        implode( ', ', array_keys( $this->shutdownPositions ) ) . "\n"
                );

                // CP-protected writes should overwhemingly go to the master datacenter, so get DC-local
                // lock to merge the values. Use a DC-local get() and a synchronous all-DC set(). This
                // makes it possible for the BagOStuff class to write in parallel to all DCs with one RTT.
                if ( $store->lock( $this->key, 3 ) ) {
                        if ( $workCallback ) {
                                // Let the store run the work before blocking on a replication sync barrier. By the
                                // time it's done with the work, the barrier should be fast if replication caught up.
                                $store->addBusyCallback( $workCallback );
                        }
                        $ok = $store->set(
                                $this->key,
                                self::mergePositions( $store->get( $this->key ), $this->shutdownPositions ),
                                self::POSITION_TTL,
                                ( $mode === 'sync' ) ? $store::WRITE_SYNC : 0
                        );
                        $store->unlock( $this->key );
                } else {
                        $ok = false;
                }

                if ( !$ok ) {
                        $bouncedPositions = $this->shutdownPositions;
                        // Raced out too many times or stash is down
                        $this->logger->warning( __METHOD__ . ": failed to save master pos for " .
                                implode( ', ', array_keys( $this->shutdownPositions ) ) . "\n"
                        );
                } elseif ( $mode === 'sync' &&
                        $store->getQoS( $store::ATTR_SYNCWRITES ) < $store::QOS_SYNCWRITES_BE
                ) {
                        // Positions may not be in all datacenters, force LBFactory to play it safe
                        $this->logger->info( __METHOD__ . ": store may not support synchronous writes." );
                        $bouncedPositions = $this->shutdownPositions;
                } else {
                        $bouncedPositions = [];
                }

                return $bouncedPositions;
        }

        /**
         * @param string $dbName DB master name (e.g. "db1052")
         * @return float|bool UNIX timestamp when client last touched the DB; false if not on record
         * @since 1.28
         */
        public function getTouched( $dbName ) {
                return $this->store->get( $this->getTouchedKey( $this->store, $dbName ) );
        }

        /**
         * @param BagOStuff $store
         * @param string $dbName
         * @return string
         */
        private function getTouchedKey( BagOStuff $store, $dbName ) {
                return $store->makeGlobalKey( __CLASS__, 'mtime', $this->clientId, $dbName );
        }

        /**
         * Load in previous master positions for the client
         */
        protected function initPositions() {
                if ( $this->initialized ) {
                        return;
                }

                $this->initialized = true;
                if ( $this->wait ) {
                        // If there is an expectation to see master positions with a certain min
                        // timestamp, then block until they appear, or until a timeout is reached.
                        if ( $this->waitForPosTime > 0.0 ) {
                                $data = null;
                                $loop = new WaitConditionLoop(
                                        function () use ( &$data ) {
                                                $data = $this->store->get( $this->key );

                                                return ( self::minPosTime( $data ) >= $this->waitForPosTime )
                                                        ? WaitConditionLoop::CONDITION_REACHED
                                                        : WaitConditionLoop::CONDITION_CONTINUE;
                                        },
                                        $this->waitForPosTimeout
                                );
                                $result = $loop->invoke();
                                $waitedMs = $loop->getLastWaitTime() * 1e3;

                                if ( $result == $loop::CONDITION_REACHED ) {
                                        $msg = "expected and found pos time {$this->waitForPosTime} ({$waitedMs}ms)";
                                        $this->logger->debug( $msg );
                                } else {
                                        $msg = "expected but missed pos time {$this->waitForPosTime} ({$waitedMs}ms)";
                                        $this->logger->info( $msg );
                                }
                        } else {
                                $data = $this->store->get( $this->key );
                        }

                        $this->startupPositions = $data ? $data['positions'] : [];
                        $this->logger->info( __METHOD__ . ": key is {$this->key} (read)\n" );
                } else {
                        $this->startupPositions = [];
                        $this->logger->info( __METHOD__ . ": key is {$this->key} (unread)\n" );
                }
        }

        /**
         * @param array|bool $data
         * @return float|null
         */
        private static function minPosTime( $data ) {
                if ( !isset( $data['positions'] ) ) {
                        return null;
                }

                $min = null;
                foreach ( $data['positions'] as $pos ) {
                        /** @var DBMasterPos $pos */
                        $min = $min ? min( $pos->asOfTime(), $min ) : $pos->asOfTime();
                }

                return $min;
        }

        /**
         * @param array|bool $curValue
         * @param DBMasterPos[] $shutdownPositions
         * @return array
         */
        private static function mergePositions( $curValue, array $shutdownPositions ) {
                /** @var $curPositions DBMasterPos[] */
                if ( $curValue === false ) {
                        $curPositions = $shutdownPositions;
                } else {
                        $curPositions = $curValue['positions'];
                        // Use the newest positions for each DB master
                        foreach ( $shutdownPositions as $db => $pos ) {
                                if ( !isset( $curPositions[$db] )
                                        || $pos->asOfTime() > $curPositions[$db]->asOfTime()
                                ) {
                                        $curPositions[$db] = $pos;
                                }
                        }
                }

                return [ 'positions' => $curPositions ];
        }
}