includes/BacklinkCache.php

   1 <?php
   2 /**
   3  * Class for fetching backlink lists, approximate backlink counts and
   4  * partitions.
   5  *
   6  * This program is free software; you can redistribute it and/or modify
   7  * it under the terms of the GNU General Public License as published by
   8  * the Free Software Foundation; either version 2 of the License, or
   9  * (at your option) any later version.
  10  *
  11  * This program is distributed in the hope that it will be useful,
  12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  14  * GNU General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU General Public License along
  17  * with this program; if not, write to the Free Software Foundation, Inc.,
  18  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  19  * http://www.gnu.org/copyleft/gpl.html
  20  *
  21  * @file
  22  * @author Tim Starling
  23  * @copyright © 2009, Tim Starling, Domas Mituzas
  24  * @copyright © 2010, Max Sem
  25  * @copyright © 2011, Antoine Musso
  26  */
  27
  28 /**
  29  * Class for fetching backlink lists, approximate backlink counts and
  30  * partitions. This is a shared cache.
  31  *
  32  * Instances of this class should typically be fetched with the method
  33  * $title->getBacklinkCache().
  34  *
  35  * Ideally you should only get your backlinks from here when you think
  36  * there is some advantage in caching them. Otherwise it's just a waste
  37  * of memory.
  38  *
  39  * Introduced by r47317
  40  *
  41  * @internal documentation reviewed on 18 Mar 2011 by hashar
  42  */
  43 class BacklinkCache {
  44         /** @var ProcessCacheLRU */
  45         protected static $cache;
  46
  47         /**
  48          * Multi dimensions array representing batches. Keys are:
  49          *  > (string) links table name
  50          *    > 'numRows' : Number of rows for this link table
  51          *    > 'batches' : array( $start, $end )
  52          *
  53          * @see BacklinkCache::partitionResult()
  54          *
  55          * Cleared with BacklinkCache::clear()
  56          */
  57         protected $partitionCache = array();
  58
  59         /**
  60          * Contains the whole links from a database result.
  61          * This is raw data that will be partitioned in $partitionCache
  62          *
  63          * Initialized with BacklinkCache::getLinks()
  64          * Cleared with BacklinkCache::clear()
  65          */
  66         protected $fullResultCache = array();
  67
  68         /**
  69          * Local copy of a database object.
  70          *
  71          * Accessor: BacklinkCache::getDB()
  72          * Mutator : BacklinkCache::setDB()
  73          * Cleared with BacklinkCache::clear()
  74          */
  75         protected $db;
  76
  77         /**
  78          * Local copy of a Title object
  79          */
  80         protected $title;
  81
  82         const CACHE_EXPIRY = 3600;
  83
  84         /**
  85          * Create a new BacklinkCache
  86          *
  87          * @param Title $title : Title object to create a backlink cache for
  88          */
  89         public function __construct( Title $title ) {
  90                 $this->title = $title;
  91         }
  92
  93         /**
  94          * Create a new BacklinkCache or reuse any existing one.
  95          * Currently, only one cache instance can exist; callers that
  96          * need multiple backlink cache objects should keep them in scope.
  97          *
  98          * @param Title $title : Title object to get a backlink cache for
  99          * @return BacklinkCache
 100          */
 101         public static function get( Title $title ) {
 102                 if ( !self::$cache ) { // init cache
 103                         self::$cache = new ProcessCacheLRU( 1 );
 104                 }
 105                 $dbKey = $title->getPrefixedDBkey();
 106                 if ( !self::$cache->has( $dbKey, 'obj' ) ) {
 107                         self::$cache->set( $dbKey, 'obj', new self( $title ) );
 108                 }
 109                 return self::$cache->get( $dbKey, 'obj' );
 110         }
 111
 112         /**
 113          * Serialization handler, diasallows to serialize the database to prevent
 114          * failures after this class is deserialized from cache with dead DB
 115          * connection.
 116          *
 117          * @return array
 118          */
 119         function __sleep() {
 120                 return array( 'partitionCache', 'fullResultCache', 'title' );
 121         }
 122
 123         /**
 124          * Clear locally stored data and database object.
 125          */
 126         public function clear() {
 127                 $this->partitionCache = array();
 128                 $this->fullResultCache = array();
 129                 unset( $this->db );
 130         }
 131
 132         /**
 133          * Set the Database object to use
 134          *
 135          * @param $db DatabaseBase
 136          */
 137         public function setDB( $db ) {
 138                 $this->db = $db;
 139         }
 140
 141         /**
 142          * Get the slave connection to the database
 143          * When non existing, will initialize the connection.
 144          * @return DatabaseBase object
 145          */
 146         protected function getDB() {
 147                 if ( !isset( $this->db ) ) {
 148                         $this->db = wfGetDB( DB_SLAVE );
 149                 }
 150
 151                 return $this->db;
 152         }
 153
 154         /**
 155          * Get the backlinks for a given table. Cached in process memory only.
 156          * @param $table String
 157          * @param $startId Integer or false
 158          * @param $endId Integer or false
 159          * @return TitleArrayFromResult
 160          */
 161         public function getLinks( $table, $startId = false, $endId = false ) {
 162                 wfProfileIn( __METHOD__ );
 163
 164                 $fromField = $this->getPrefix( $table ) . '_from';
 165
 166                 if ( $startId || $endId ) {
 167                         // Partial range, not cached
 168                         wfDebug( __METHOD__ . ": from DB (uncacheable range)\n" );
 169                         $conds = $this->getConditions( $table );
 170
 171                         // Use the from field in the condition rather than the joined page_id,
 172                         // because databases are stupid and don't necessarily propagate indexes.
 173                         if ( $startId ) {
 174                                 $conds[] = "$fromField >= " . intval( $startId );
 175                         }
 176
 177                         if ( $endId ) {
 178                                 $conds[] = "$fromField <= " . intval( $endId );
 179                         }
 180
 181                         $res = $this->getDB()->select(
 182                                 array( $table, 'page' ),
 183                                 array( 'page_namespace', 'page_title', 'page_id' ),
 184                                 $conds,
 185                                 __METHOD__,
 186                                 array(
 187                                         'STRAIGHT_JOIN',
 188                                         'ORDER BY' => $fromField
 189                                 ) );
 190                         $ta = TitleArray::newFromResult( $res );
 191
 192                         wfProfileOut( __METHOD__ );
 193                         return $ta;
 194                 }
 195
 196                 // @todo FIXME: Make this a function?
 197                 if ( !isset( $this->fullResultCache[$table] ) ) {
 198                         wfDebug( __METHOD__ . ": from DB\n" );
 199                         $res = $this->getDB()->select(
 200                                 array( $table, 'page' ),
 201                                 array( 'page_namespace', 'page_title', 'page_id' ),
 202                                 $this->getConditions( $table ),
 203                                 __METHOD__,
 204                                 array(
 205                                         'STRAIGHT_JOIN',
 206                                         'ORDER BY' => $fromField,
 207                                 ) );
 208                         $this->fullResultCache[$table] = $res;
 209                 }
 210
 211                 $ta = TitleArray::newFromResult( $this->fullResultCache[$table] );
 212
 213                 wfProfileOut( __METHOD__ );
 214                 return $ta;
 215         }
 216
 217         /**
 218          * Get the field name prefix for a given table
 219          * @param $table String
 220          * @return null|string
 221          */
 222         protected function getPrefix( $table ) {
 223                 static $prefixes = array(
 224                         'pagelinks'     => 'pl',
 225                         'imagelinks'    => 'il',
 226                         'categorylinks' => 'cl',
 227                         'templatelinks' => 'tl',
 228                         'redirect'      => 'rd',
 229                 );
 230
 231                 if ( isset( $prefixes[$table] ) ) {
 232                         return $prefixes[$table];
 233                 } else {
 234                         $prefix = null;
 235                         wfRunHooks( 'BacklinkCacheGetPrefix', array( $table, &$prefix ) );
 236                         if( $prefix ) {
 237                                 return $prefix;
 238                         } else {
 239                                 throw new MWException( "Invalid table \"$table\" in " . __CLASS__ );
 240                         }
 241                 }
 242         }
 243
 244         /**
 245          * Get the SQL condition array for selecting backlinks, with a join
 246          * on the page table.
 247          * @param $table String
 248          * @return array|null
 249          */
 250         protected function getConditions( $table ) {
 251                 $prefix = $this->getPrefix( $table );
 252
 253                 // @todo FIXME: imagelinks and categorylinks do not rely on getNamespace,
 254                 // they could be moved up for nicer case statements
 255                 switch ( $table ) {
 256                         case 'pagelinks':
 257                         case 'templatelinks':
 258                                 $conds = array(
 259                                         "{$prefix}_namespace" => $this->title->getNamespace(),
 260                                         "{$prefix}_title"     => $this->title->getDBkey(),
 261                                         "page_id={$prefix}_from"
 262                                 );
 263                                 break;
 264                         case 'redirect':
 265                                 $conds = array(
 266                                         "{$prefix}_namespace" => $this->title->getNamespace(),
 267                                         "{$prefix}_title"     => $this->title->getDBkey(),
 268                                         $this->getDb()->makeList( array(
 269                                                 "{$prefix}_interwiki = ''",
 270                                                 "{$prefix}_interwiki is null",
 271                                         ), LIST_OR ),
 272                                         "page_id={$prefix}_from"
 273                                 );
 274                                 break;
 275                         case 'imagelinks':
 276                                 $conds = array(
 277                                         'il_to' => $this->title->getDBkey(),
 278                                         'page_id=il_from'
 279                                 );
 280                                 break;
 281                         case 'categorylinks':
 282                                 $conds = array(
 283                                         'cl_to' => $this->title->getDBkey(),
 284                                         'page_id=cl_from',
 285                                 );
 286                                 break;
 287                         default:
 288                                 $conds = null;
 289                                 wfRunHooks( 'BacklinkCacheGetConditions', array( $table, $this->title, &$conds ) );
 290                                 if( !$conds ) {
 291                                         throw new MWException( "Invalid table \"$table\" in " . __CLASS__ );
 292                                 }
 293                 }
 294
 295                 return $conds;
 296         }
 297
 298         /**
 299          * Get the approximate number of backlinks
 300          * @param $table String
 301          * @return integer
 302          */
 303         public function getNumLinks( $table ) {
 304                 if ( isset( $this->fullResultCache[$table] ) ) {
 305                         return $this->fullResultCache[$table]->numRows();
 306                 }
 307
 308                 if ( isset( $this->partitionCache[$table] ) ) {
 309                         $entry = reset( $this->partitionCache[$table] );
 310                         return $entry['numRows'];
 311                 }
 312
 313                 $titleArray = $this->getLinks( $table );
 314
 315                 return $titleArray->count();
 316         }
 317
 318         /**
 319          * Partition the backlinks into batches.
 320          * Returns an array giving the start and end of each range. The first
 321          * batch has a start of false, and the last batch has an end of false.
 322          *
 323          * @param $table String: the links table name
 324          * @param $batchSize Integer
 325          * @return Array
 326          */
 327         public function partition( $table, $batchSize ) {
 328
 329                 // 1) try partition cache ...
 330
 331                 if ( isset( $this->partitionCache[$table][$batchSize] ) ) {
 332                         wfDebug( __METHOD__ . ": got from partition cache\n" );
 333                         return $this->partitionCache[$table][$batchSize]['batches'];
 334                 }
 335
 336                 $this->partitionCache[$table][$batchSize] = false;
 337                 $cacheEntry =& $this->partitionCache[$table][$batchSize];
 338
 339                 // 2) ... then try full result cache ...
 340
 341                 if ( isset( $this->fullResultCache[$table] ) ) {
 342                         $cacheEntry = $this->partitionResult( $this->fullResultCache[$table], $batchSize );
 343                         wfDebug( __METHOD__ . ": got from full result cache\n" );
 344
 345                         return $cacheEntry['batches'];
 346                 }
 347
 348                 // 3) ... fallback to memcached ...
 349
 350                 global $wgMemc;
 351
 352                 $memcKey = wfMemcKey(
 353                         'backlinks',
 354                         md5( $this->title->getPrefixedDBkey() ),
 355                         $table,
 356                         $batchSize
 357                 );
 358
 359                 $memcValue = $wgMemc->get( $memcKey );
 360
 361                 if ( is_array( $memcValue ) ) {
 362                         $cacheEntry = $memcValue;
 363                         wfDebug( __METHOD__ . ": got from memcached $memcKey\n" );
 364
 365                         return $cacheEntry['batches'];
 366                 }
 367
 368
 369                 // 4) ... finally fetch from the slow database :(
 370
 371                 $this->getLinks( $table );
 372                 $cacheEntry = $this->partitionResult( $this->fullResultCache[$table], $batchSize );
 373                 // Save to memcached
 374                 $wgMemc->set( $memcKey, $cacheEntry, self::CACHE_EXPIRY );
 375
 376                 wfDebug( __METHOD__ . ": got from database\n" );
 377                 return $cacheEntry['batches'];
 378         }
 379
 380         /**
 381          * Partition a DB result with backlinks in it into batches
 382          * @param $res ResultWrapper database result
 383          * @param $batchSize integer
 384          * @return array @see
 385          */
 386         protected function partitionResult( $res, $batchSize ) {
 387                 $batches = array();
 388                 $numRows = $res->numRows();
 389                 $numBatches = ceil( $numRows / $batchSize );
 390
 391                 for ( $i = 0; $i < $numBatches; $i++ ) {
 392                         if ( $i == 0  ) {
 393                                 $start = false;
 394                         } else {
 395                                 $rowNum = intval( $numRows * $i / $numBatches );
 396                                 $res->seek( $rowNum );
 397                                 $row = $res->fetchObject();
 398                                 $start = $row->page_id;
 399                         }
 400
 401                         if ( $i == $numBatches - 1 ) {
 402                                 $end = false;
 403                         } else {
 404                                 $rowNum = intval( $numRows * ( $i + 1 ) / $numBatches );
 405                                 $res->seek( $rowNum );
 406                                 $row = $res->fetchObject();
 407                                 $end = $row->page_id - 1;
 408                         }
 409
 410                         # Sanity check order
 411                         if ( $start && $end && $start > $end ) {
 412                                 throw new MWException( __METHOD__ . ': Internal error: query result out of order' );
 413                         }
 414
 415                         $batches[] = array( $start, $end );
 416                 }
 417
 418                 return array( 'numRows' => $numRows, 'batches' => $batches );
 419         }
 420 }