rdbms: Avoid selectDB() call in LoadMonitor new connections

[mediawiki.git] / includes / externalstore / ExternalStoreDB.php

<?php
/**
 * External storage in SQL database.
 *
 * 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
 */

use Wikimedia\Rdbms\LoadBalancer;
use Wikimedia\Rdbms\IDatabase;
use Wikimedia\Rdbms\DBConnRef;
use Wikimedia\Rdbms\MaintainableDBConnRef;

/**
 * DB accessable external objects.
 *
 * In this system, each store "location" maps to a database "cluster".
 * The clusters must be defined in the normal LBFactory configuration.
 *
 * @ingroup ExternalStorage
 */
class ExternalStoreDB extends ExternalStoreMedium {
        /**
         * The provided URL is in the form of DB://cluster/id
         * or DB://cluster/id/itemid for concatened storage.
         *
         * @param string $url
         * @return string|bool False if missing
         * @see ExternalStoreMedium::fetchFromURL()
         */
        public function fetchFromURL( $url ) {
                list( $cluster, $id, $itemID ) = $this->parseURL( $url );
                $ret = $this->fetchBlob( $cluster, $id, $itemID );

                if ( $itemID !== false && $ret !== false ) {
                        return $ret->getItem( $itemID );
                }

                return $ret;
        }

        /**
         * Fetch data from given external store URLs.
         * The provided URLs are in the form of DB://cluster/id
         * or DB://cluster/id/itemid for concatened storage.
         *
         * @param array $urls An array of external store URLs
         * @return array A map from url to stored content. Failed results
         *     are not represented.
         */
        public function batchFetchFromURLs( array $urls ) {
                $batched = $inverseUrlMap = [];
                foreach ( $urls as $url ) {
                        list( $cluster, $id, $itemID ) = $this->parseURL( $url );
                        $batched[$cluster][$id][] = $itemID;
                        // false $itemID gets cast to int, but should be ok
                        // since we do === from the $itemID in $batched
                        $inverseUrlMap[$cluster][$id][$itemID] = $url;
                }
                $ret = [];
                foreach ( $batched as $cluster => $batchByCluster ) {
                        $res = $this->batchFetchBlobs( $cluster, $batchByCluster );
                        /** @var HistoryBlob $blob */
                        foreach ( $res as $id => $blob ) {
                                foreach ( $batchByCluster[$id] as $itemID ) {
                                        $url = $inverseUrlMap[$cluster][$id][$itemID];
                                        if ( $itemID === false ) {
                                                $ret[$url] = $blob;
                                        } else {
                                                $ret[$url] = $blob->getItem( $itemID );
                                        }
                                }
                        }
                }

                return $ret;
        }

        public function store( $location, $data ) {
                $dbw = $this->getMaster( $location );
                $id = $dbw->nextSequenceValue( 'blob_blob_id_seq' );
                $dbw->insert( $this->getTable( $dbw ),
                        [ 'blob_id' => $id, 'blob_text' => $data ],
                        __METHOD__ );
                $id = $dbw->insertId();
                if ( !$id ) {
                        throw new MWException( __METHOD__ . ': no insert ID' );
                }

                return "DB://$location/$id";
        }

        /**
         * Get a LoadBalancer for the specified cluster
         *
         * @param string $cluster Cluster name
         * @return LoadBalancer
         */
        private function getLoadBalancer( $cluster ) {
                return wfGetLBFactory()->getExternalLB( $cluster );
        }

        /**
         * Get a replica DB connection for the specified cluster
         *
         * @param string $cluster Cluster name
         * @return DBConnRef
         */
        public function getSlave( $cluster ) {
                global $wgDefaultExternalStore;

                $wiki = isset( $this->params['wiki'] ) ? $this->params['wiki'] : false;
                $lb = $this->getLoadBalancer( $cluster );

                if ( !in_array( "DB://" . $cluster, (array)$wgDefaultExternalStore ) ) {
                        wfDebug( "read only external store\n" );
                        $lb->allowLagged( true );
                } else {
                        wfDebug( "writable external store\n" );
                }

                $db = $lb->getConnectionRef( DB_REPLICA, [], $wiki );
                $db->clearFlag( DBO_TRX ); // sanity

                return $db;
        }

        /**
         * Get a master database connection for the specified cluster
         *
         * @param string $cluster Cluster name
         * @return MaintainableDBConnRef
         */
        public function getMaster( $cluster ) {
                $wiki = isset( $this->params['wiki'] ) ? $this->params['wiki'] : false;
                $lb = $this->getLoadBalancer( $cluster );

                $db = $lb->getMaintenanceConnectionRef( DB_MASTER, [], $wiki );
                $db->clearFlag( DBO_TRX ); // sanity

                return $db;
        }

        /**
         * Get the 'blobs' table name for this database
         *
         * @param IDatabase $db
         * @return string Table name ('blobs' by default)
         */
        public function getTable( $db ) {
                $table = $db->getLBInfo( 'blobs table' );
                if ( is_null( $table ) ) {
                        $table = 'blobs';
                }

                return $table;
        }

        /**
         * Fetch a blob item out of the database; a cache of the last-loaded
         * blob will be kept so that multiple loads out of a multi-item blob
         * can avoid redundant database access and decompression.
         * @param string $cluster
         * @param string $id
         * @param string $itemID
         * @return HistoryBlob|bool Returns false if missing
         */
        private function fetchBlob( $cluster, $id, $itemID ) {
                /**
                 * One-step cache variable to hold base blobs; operations that
                 * pull multiple revisions may often pull multiple times from
                 * the same blob. By keeping the last-used one open, we avoid
                 * redundant unserialization and decompression overhead.
                 */
                static $externalBlobCache = [];

                $cacheID = ( $itemID === false ) ? "$cluster/$id" : "$cluster/$id/";
                if ( isset( $externalBlobCache[$cacheID] ) ) {
                        wfDebugLog( 'ExternalStoreDB-cache',
                                "ExternalStoreDB::fetchBlob cache hit on $cacheID" );

                        return $externalBlobCache[$cacheID];
                }

                wfDebugLog( 'ExternalStoreDB-cache',
                        "ExternalStoreDB::fetchBlob cache miss on $cacheID" );

                $dbr = $this->getSlave( $cluster );
                $ret = $dbr->selectField( $this->getTable( $dbr ),
                        'blob_text', [ 'blob_id' => $id ], __METHOD__ );
                if ( $ret === false ) {
                        wfDebugLog( 'ExternalStoreDB',
                                "ExternalStoreDB::fetchBlob master fallback on $cacheID" );
                        // Try the master
                        $dbw = $this->getMaster( $cluster );
                        $ret = $dbw->selectField( $this->getTable( $dbw ),
                                'blob_text', [ 'blob_id' => $id ], __METHOD__ );
                        if ( $ret === false ) {
                                wfDebugLog( 'ExternalStoreDB',
                                        "ExternalStoreDB::fetchBlob master failed to find $cacheID" );
                        }
                }
                if ( $itemID !== false && $ret !== false ) {
                        // Unserialise object; caller extracts item
                        $ret = unserialize( $ret );
                }

                $externalBlobCache = [ $cacheID => $ret ];

                return $ret;
        }

        /**
         * Fetch multiple blob items out of the database
         *
         * @param string $cluster A cluster name valid for use with LBFactory
         * @param array $ids A map from the blob_id's to look for to the requested itemIDs in the blobs
         * @return array A map from the blob_id's requested to their content.
         *   Unlocated ids are not represented
         */
        private function batchFetchBlobs( $cluster, array $ids ) {
                $dbr = $this->getSlave( $cluster );
                $res = $dbr->select( $this->getTable( $dbr ),
                        [ 'blob_id', 'blob_text' ], [ 'blob_id' => array_keys( $ids ) ], __METHOD__ );
                $ret = [];
                if ( $res !== false ) {
                        $this->mergeBatchResult( $ret, $ids, $res );
                }
                if ( $ids ) {
                        wfDebugLog( __CLASS__, __METHOD__ .
                                " master fallback on '$cluster' for: " .
                                implode( ',', array_keys( $ids ) ) );
                        // Try the master
                        $dbw = $this->getMaster( $cluster );
                        $res = $dbw->select( $this->getTable( $dbr ),
                                [ 'blob_id', 'blob_text' ],
                                [ 'blob_id' => array_keys( $ids ) ],
                                __METHOD__ );
                        if ( $res === false ) {
                                wfDebugLog( __CLASS__, __METHOD__ . " master failed on '$cluster'" );
                        } else {
                                $this->mergeBatchResult( $ret, $ids, $res );
                        }
                }
                if ( $ids ) {
                        wfDebugLog( __CLASS__, __METHOD__ .
                                " master on '$cluster' failed locating items: " .
                                implode( ',', array_keys( $ids ) ) );
                }

                return $ret;
        }

        /**
         * Helper function for self::batchFetchBlobs for merging master/replica DB results
         * @param array &$ret Current self::batchFetchBlobs return value
         * @param array &$ids Map from blob_id to requested itemIDs
         * @param mixed $res DB result from Database::select
         */
        private function mergeBatchResult( array &$ret, array &$ids, $res ) {
                foreach ( $res as $row ) {
                        $id = $row->blob_id;
                        $itemIDs = $ids[$id];
                        unset( $ids[$id] ); // to track if everything is found
                        if ( count( $itemIDs ) === 1 && reset( $itemIDs ) === false ) {
                                // single result stored per blob
                                $ret[$id] = $row->blob_text;
                        } else {
                                // multi result stored per blob
                                $ret[$id] = unserialize( $row->blob_text );
                        }
                }
        }

        /**
         * @param string $url
         * @return array
         */
        protected function parseURL( $url ) {
                $path = explode( '/', $url );

                return [
                        $path[2], // cluster
                        $path[3], // id
                        isset( $path[4] ) ? $path[4] : false // itemID
                ];
        }
}