includes/objectcache/SqlBagOStuff.php

   1 <?php
   2 /**
   3  * Object caching using a SQL database.
   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 Cache
  22  */
  23
  24 use \MediaWiki\MediaWikiServices;
  25
  26 /**
  27  * Class to store objects in the database
  28  *
  29  * @ingroup Cache
  30  */
  31 class SqlBagOStuff extends BagOStuff {
  32         /** @var array[] (server index => server config) */
  33         protected $serverInfos;
  34         /** @var string[] (server index => tag/host name) */
  35         protected $serverTags;
  36         /** @var int */
  37         protected $numServers;
  38         /** @var int */
  39         protected $lastExpireAll = 0;
  40         /** @var int */
  41         protected $purgePeriod = 100;
  42         /** @var int */
  43         protected $shards = 1;
  44         /** @var string */
  45         protected $tableName = 'objectcache';
  46         /** @var bool */
  47         protected $slaveOnly = false;
  48         /** @var int */
  49         protected $syncTimeout = 3;
  50
  51         /** @var array */
  52         protected $conns;
  53         /** @var array UNIX timestamps */
  54         protected $connFailureTimes = [];
  55         /** @var array Exceptions */
  56         protected $connFailureErrors = [];
  57
  58         /**
  59          * Constructor. Parameters are:
  60          *   - server:      A server info structure in the format required by each
  61          *                  element in $wgDBServers.
  62          *
  63          *   - servers:     An array of server info structures describing a set of database servers
  64          *                  to distribute keys to. If this is specified, the "server" option will be
  65          *                  ignored. If string keys are used, then they will be used for consistent
  66          *                  hashing *instead* of the host name (from the server config). This is useful
  67          *                  when a cluster is replicated to another site (with different host names)
  68          *                  but each server has a corresponding replica in the other cluster.
  69          *
  70          *   - purgePeriod: The average number of object cache requests in between
  71          *                  garbage collection operations, where expired entries
  72          *                  are removed from the database. Or in other words, the
  73          *                  reciprocal of the probability of purging on any given
  74          *                  request. If this is set to zero, purging will never be
  75          *                  done.
  76          *
  77          *   - tableName:   The table name to use, default is "objectcache".
  78          *
  79          *   - shards:      The number of tables to use for data storage on each server.
  80          *                  If this is more than 1, table names will be formed in the style
  81          *                  objectcacheNNN where NNN is the shard index, between 0 and
  82          *                  shards-1. The number of digits will be the minimum number
  83          *                  required to hold the largest shard index. Data will be
  84          *                  distributed across all tables by key hash. This is for
  85          *                  MySQL bugs 61735 and 61736.
  86          *   - slaveOnly:   Whether to only use slave DBs and avoid triggering
  87          *                  garbage collection logic of expired items. This only
  88          *                  makes sense if the primary DB is used and only if get()
  89          *                  calls will be used. This is used by ReplicatedBagOStuff.
  90          *   - syncTimeout: Max seconds to wait for slaves to catch up for WRITE_SYNC.
  91          *
  92          * @param array $params
  93          */
  94         public function __construct( $params ) {
  95                 parent::__construct( $params );
  96
  97                 $this->attrMap[self::ATTR_EMULATION] = self::QOS_EMULATION_SQL;
  98
  99                 if ( isset( $params['servers'] ) ) {
 100                         $this->serverInfos = [];
 101                         $this->serverTags = [];
 102                         $this->numServers = count( $params['servers'] );
 103                         $index = 0;
 104                         foreach ( $params['servers'] as $tag => $info ) {
 105                                 $this->serverInfos[$index] = $info;
 106                                 if ( is_string( $tag ) ) {
 107                                         $this->serverTags[$index] = $tag;
 108                                 } else {
 109                                         $this->serverTags[$index] = isset( $info['host'] ) ? $info['host'] : "#$index";
 110                                 }
 111                                 ++$index;
 112                         }
 113                 } elseif ( isset( $params['server'] ) ) {
 114                         $this->serverInfos = [ $params['server'] ];
 115                         $this->numServers = count( $this->serverInfos );
 116                 } else {
 117                         $this->serverInfos = false;
 118                         $this->numServers = 1;
 119                 }
 120                 if ( isset( $params['purgePeriod'] ) ) {
 121                         $this->purgePeriod = intval( $params['purgePeriod'] );
 122                 }
 123                 if ( isset( $params['tableName'] ) ) {
 124                         $this->tableName = $params['tableName'];
 125                 }
 126                 if ( isset( $params['shards'] ) ) {
 127                         $this->shards = intval( $params['shards'] );
 128                 }
 129                 if ( isset( $params['syncTimeout'] ) ) {
 130                         $this->syncTimeout = $params['syncTimeout'];
 131                 }
 132                 $this->slaveOnly = !empty( $params['slaveOnly'] );
 133         }
 134
 135         /**
 136          * Get a connection to the specified database
 137          *
 138          * @param int $serverIndex
 139          * @return IDatabase
 140          * @throws MWException
 141          */
 142         protected function getDB( $serverIndex ) {
 143                 if ( !isset( $this->conns[$serverIndex] ) ) {
 144                         if ( $serverIndex >= $this->numServers ) {
 145                                 throw new MWException( __METHOD__ . ": Invalid server index \"$serverIndex\"" );
 146                         }
 147
 148                         # Don't keep timing out trying to connect for each call if the DB is down
 149                         if ( isset( $this->connFailureErrors[$serverIndex] )
 150                                 && ( time() - $this->connFailureTimes[$serverIndex] ) < 60
 151                         ) {
 152                                 throw $this->connFailureErrors[$serverIndex];
 153                         }
 154
 155                         # If server connection info was given, use that
 156                         if ( $this->serverInfos ) {
 157                                 $info = $this->serverInfos[$serverIndex];
 158                                 $type = isset( $info['type'] ) ? $info['type'] : 'mysql';
 159                                 $host = isset( $info['host'] ) ? $info['host'] : '[unknown]';
 160                                 $this->logger->debug( __CLASS__ . ": connecting to $host" );
 161                                 // Use a blank trx profiler to ignore expections as this is a cache
 162                                 $info['trxProfiler'] = new TransactionProfiler();
 163                                 $db = DatabaseBase::factory( $type, $info );
 164                                 $db->clearFlag( DBO_TRX );
 165                         } else {
 166                                 // We must keep a separate connection to MySQL in order to avoid deadlocks
 167                                 // However, SQLite has an opposite behavior. And PostgreSQL needs to know
 168                                 // if we are in transaction or not (@TODO: find some work-around).
 169                                 $index = $this->slaveOnly ? DB_SLAVE : DB_MASTER;
 170                                 if ( wfGetDB( $index )->getType() == 'mysql' ) {
 171                                         $lb = wfGetLBFactory()->newMainLB();
 172                                         $db = $lb->getConnection( $index );
 173                                         $db->clearFlag( DBO_TRX ); // auto-commit mode
 174                                 } else {
 175                                         $db = wfGetDB( $index );
 176                                 }
 177                         }
 178                         $this->logger->debug( sprintf( "Connection %s will be used for SqlBagOStuff", $db ) );
 179                         $this->conns[$serverIndex] = $db;
 180                 }
 181
 182                 return $this->conns[$serverIndex];
 183         }
 184
 185         /**
 186          * Get the server index and table name for a given key
 187          * @param string $key
 188          * @return array Server index and table name
 189          */
 190         protected function getTableByKey( $key ) {
 191                 if ( $this->shards > 1 ) {
 192                         $hash = hexdec( substr( md5( $key ), 0, 8 ) ) & 0x7fffffff;
 193                         $tableIndex = $hash % $this->shards;
 194                 } else {
 195                         $tableIndex = 0;
 196                 }
 197                 if ( $this->numServers > 1 ) {
 198                         $sortedServers = $this->serverTags;
 199                         ArrayUtils::consistentHashSort( $sortedServers, $key );
 200                         reset( $sortedServers );
 201                         $serverIndex = key( $sortedServers );
 202                 } else {
 203                         $serverIndex = 0;
 204                 }
 205                 return [ $serverIndex, $this->getTableNameByShard( $tableIndex ) ];
 206         }
 207
 208         /**
 209          * Get the table name for a given shard index
 210          * @param int $index
 211          * @return string
 212          */
 213         protected function getTableNameByShard( $index ) {
 214                 if ( $this->shards > 1 ) {
 215                         $decimals = strlen( $this->shards - 1 );
 216                         return $this->tableName .
 217                                 sprintf( "%0{$decimals}d", $index );
 218                 } else {
 219                         return $this->tableName;
 220                 }
 221         }
 222
 223         protected function doGet( $key, $flags = 0 ) {
 224                 $casToken = null;
 225
 226                 return $this->getWithToken( $key, $casToken, $flags );
 227         }
 228
 229         protected function getWithToken( $key, &$casToken, $flags = 0 ) {
 230                 $values = $this->getMulti( [ $key ] );
 231                 if ( array_key_exists( $key, $values ) ) {
 232                         $casToken = $values[$key];
 233                         return $values[$key];
 234                 }
 235                 return false;
 236         }
 237
 238         public function getMulti( array $keys, $flags = 0 ) {
 239                 $values = []; // array of (key => value)
 240
 241                 $keysByTable = [];
 242                 foreach ( $keys as $key ) {
 243                         list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
 244                         $keysByTable[$serverIndex][$tableName][] = $key;
 245                 }
 246
 247                 $this->garbageCollect(); // expire old entries if any
 248
 249                 $dataRows = [];
 250                 foreach ( $keysByTable as $serverIndex => $serverKeys ) {
 251                         try {
 252                                 $db = $this->getDB( $serverIndex );
 253                                 foreach ( $serverKeys as $tableName => $tableKeys ) {
 254                                         $res = $db->select( $tableName,
 255                                                 [ 'keyname', 'value', 'exptime' ],
 256                                                 [ 'keyname' => $tableKeys ],
 257                                                 __METHOD__,
 258                                                 // Approximate write-on-the-fly BagOStuff API via blocking.
 259                                                 // This approximation fails if a ROLLBACK happens (which is rare).
 260                                                 // We do not want to flush the TRX as that can break callers.
 261                                                 $db->trxLevel() ? [ 'LOCK IN SHARE MODE' ] : []
 262                                         );
 263                                         if ( $res === false ) {
 264                                                 continue;
 265                                         }
 266                                         foreach ( $res as $row ) {
 267                                                 $row->serverIndex = $serverIndex;
 268                                                 $row->tableName = $tableName;
 269                                                 $dataRows[$row->keyname] = $row;
 270                                         }
 271                                 }
 272                         } catch ( DBError $e ) {
 273                                 $this->handleReadError( $e, $serverIndex );
 274                         }
 275                 }
 276
 277                 foreach ( $keys as $key ) {
 278                         if ( isset( $dataRows[$key] ) ) { // HIT?
 279                                 $row = $dataRows[$key];
 280                                 $this->debug( "get: retrieved data; expiry time is " . $row->exptime );
 281                                 $db = null;
 282                                 try {
 283                                         $db = $this->getDB( $row->serverIndex );
 284                                         if ( $this->isExpired( $db, $row->exptime ) ) { // MISS
 285                                                 $this->debug( "get: key has expired" );
 286                                         } else { // HIT
 287                                                 $values[$key] = $this->unserialize( $db->decodeBlob( $row->value ) );
 288                                         }
 289                                 } catch ( DBQueryError $e ) {
 290                                         $this->handleWriteError( $e, $db, $row->serverIndex );
 291                                 }
 292                         } else { // MISS
 293                                 $this->debug( 'get: no matching rows' );
 294                         }
 295                 }
 296
 297                 return $values;
 298         }
 299
 300         public function setMulti( array $data, $expiry = 0 ) {
 301                 $keysByTable = [];
 302                 foreach ( $data as $key => $value ) {
 303                         list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
 304                         $keysByTable[$serverIndex][$tableName][] = $key;
 305                 }
 306
 307                 $this->garbageCollect(); // expire old entries if any
 308
 309                 $result = true;
 310                 $exptime = (int)$expiry;
 311                 foreach ( $keysByTable as $serverIndex => $serverKeys ) {
 312                         $db = null;
 313                         try {
 314                                 $db = $this->getDB( $serverIndex );
 315                         } catch ( DBError $e ) {
 316                                 $this->handleWriteError( $e, $db, $serverIndex );
 317                                 $result = false;
 318                                 continue;
 319                         }
 320
 321                         if ( $exptime < 0 ) {
 322                                 $exptime = 0;
 323                         }
 324
 325                         if ( $exptime == 0 ) {
 326                                 $encExpiry = $this->getMaxDateTime( $db );
 327                         } else {
 328                                 $exptime = $this->convertExpiry( $exptime );
 329                                 $encExpiry = $db->timestamp( $exptime );
 330                         }
 331                         foreach ( $serverKeys as $tableName => $tableKeys ) {
 332                                 $rows = [];
 333                                 foreach ( $tableKeys as $key ) {
 334                                         $rows[] = [
 335                                                 'keyname' => $key,
 336                                                 'value' => $db->encodeBlob( $this->serialize( $data[$key] ) ),
 337                                                 'exptime' => $encExpiry,
 338                                         ];
 339                                 }
 340
 341                                 try {
 342                                         $db->replace(
 343                                                 $tableName,
 344                                                 [ 'keyname' ],
 345                                                 $rows,
 346                                                 __METHOD__
 347                                         );
 348                                 } catch ( DBError $e ) {
 349                                         $this->handleWriteError( $e, $db, $serverIndex );
 350                                         $result = false;
 351                                 }
 352
 353                         }
 354
 355                 }
 356
 357                 return $result;
 358         }
 359
 360         public function set( $key, $value, $exptime = 0, $flags = 0 ) {
 361                 $ok = $this->setMulti( [ $key => $value ], $exptime );
 362                 if ( ( $flags & self::WRITE_SYNC ) == self::WRITE_SYNC ) {
 363                         $ok = $ok && $this->waitForSlaves();
 364                 }
 365
 366                 return $ok;
 367         }
 368
 369         protected function cas( $casToken, $key, $value, $exptime = 0 ) {
 370                 list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
 371                 $db = null;
 372                 try {
 373                         $db = $this->getDB( $serverIndex );
 374                         $exptime = intval( $exptime );
 375
 376                         if ( $exptime < 0 ) {
 377                                 $exptime = 0;
 378                         }
 379
 380                         if ( $exptime == 0 ) {
 381                                 $encExpiry = $this->getMaxDateTime( $db );
 382                         } else {
 383                                 $exptime = $this->convertExpiry( $exptime );
 384                                 $encExpiry = $db->timestamp( $exptime );
 385                         }
 386                         // (bug 24425) use a replace if the db supports it instead of
 387                         // delete/insert to avoid clashes with conflicting keynames
 388                         $db->update(
 389                                 $tableName,
 390                                 [
 391                                         'keyname' => $key,
 392                                         'value' => $db->encodeBlob( $this->serialize( $value ) ),
 393                                         'exptime' => $encExpiry
 394                                 ],
 395                                 [
 396                                         'keyname' => $key,
 397                                         'value' => $db->encodeBlob( $this->serialize( $casToken ) )
 398                                 ],
 399                                 __METHOD__
 400                         );
 401                 } catch ( DBQueryError $e ) {
 402                         $this->handleWriteError( $e, $db, $serverIndex );
 403
 404                         return false;
 405                 }
 406
 407                 return (bool)$db->affectedRows();
 408         }
 409
 410         public function delete( $key ) {
 411                 list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
 412                 $db = null;
 413                 try {
 414                         $db = $this->getDB( $serverIndex );
 415                         $db->delete(
 416                                 $tableName,
 417                                 [ 'keyname' => $key ],
 418                                 __METHOD__ );
 419                 } catch ( DBError $e ) {
 420                         $this->handleWriteError( $e, $db, $serverIndex );
 421                         return false;
 422                 }
 423
 424                 return true;
 425         }
 426
 427         public function incr( $key, $step = 1 ) {
 428                 list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
 429                 $db = null;
 430                 try {
 431                         $db = $this->getDB( $serverIndex );
 432                         $step = intval( $step );
 433                         $row = $db->selectRow(
 434                                 $tableName,
 435                                 [ 'value', 'exptime' ],
 436                                 [ 'keyname' => $key ],
 437                                 __METHOD__,
 438                                 [ 'FOR UPDATE' ] );
 439                         if ( $row === false ) {
 440                                 // Missing
 441
 442                                 return null;
 443                         }
 444                         $db->delete( $tableName, [ 'keyname' => $key ], __METHOD__ );
 445                         if ( $this->isExpired( $db, $row->exptime ) ) {
 446                                 // Expired, do not reinsert
 447
 448                                 return null;
 449                         }
 450
 451                         $oldValue = intval( $this->unserialize( $db->decodeBlob( $row->value ) ) );
 452                         $newValue = $oldValue + $step;
 453                         $db->insert( $tableName,
 454                                 [
 455                                         'keyname' => $key,
 456                                         'value' => $db->encodeBlob( $this->serialize( $newValue ) ),
 457                                         'exptime' => $row->exptime
 458                                 ], __METHOD__, 'IGNORE' );
 459
 460                         if ( $db->affectedRows() == 0 ) {
 461                                 // Race condition. See bug 28611
 462                                 $newValue = null;
 463                         }
 464                 } catch ( DBError $e ) {
 465                         $this->handleWriteError( $e, $db, $serverIndex );
 466                         return null;
 467                 }
 468
 469                 return $newValue;
 470         }
 471
 472         public function merge( $key, callable $callback, $exptime = 0, $attempts = 10, $flags = 0 ) {
 473                 $ok = $this->mergeViaCas( $key, $callback, $exptime, $attempts );
 474                 if ( ( $flags & self::WRITE_SYNC ) == self::WRITE_SYNC ) {
 475                         $ok = $ok && $this->waitForSlaves();
 476                 }
 477
 478                 return $ok;
 479         }
 480
 481         public function changeTTL( $key, $expiry = 0 ) {
 482                 list( $serverIndex, $tableName ) = $this->getTableByKey( $key );
 483                 $db = null;
 484                 try {
 485                         $db = $this->getDB( $serverIndex );
 486                         $db->update(
 487                                 $tableName,
 488                                 [ 'exptime' => $db->timestamp( $this->convertExpiry( $expiry ) ) ],
 489                                 [ 'keyname' => $key, 'exptime > ' . $db->addQuotes( $db->timestamp( time() ) ) ],
 490                                 __METHOD__
 491                         );
 492                         if ( $db->affectedRows() == 0 ) {
 493                                 return false;
 494                         }
 495                 } catch ( DBError $e ) {
 496                         $this->handleWriteError( $e, $db, $serverIndex );
 497                         return false;
 498                 }
 499
 500                 return true;
 501         }
 502
 503         /**
 504          * @param IDatabase $db
 505          * @param string $exptime
 506          * @return bool
 507          */
 508         protected function isExpired( $db, $exptime ) {
 509                 return $exptime != $this->getMaxDateTime( $db ) && wfTimestamp( TS_UNIX, $exptime ) < time();
 510         }
 511
 512         /**
 513          * @param IDatabase $db
 514          * @return string
 515          */
 516         protected function getMaxDateTime( $db ) {
 517                 if ( time() > 0x7fffffff ) {
 518                         return $db->timestamp( 1 << 62 );
 519                 } else {
 520                         return $db->timestamp( 0x7fffffff );
 521                 }
 522         }
 523
 524         protected function garbageCollect() {
 525                 if ( !$this->purgePeriod || $this->slaveOnly ) {
 526                         // Disabled
 527                         return;
 528                 }
 529                 // Only purge on one in every $this->purgePeriod requests.
 530                 if ( $this->purgePeriod !== 1 && mt_rand( 0, $this->purgePeriod - 1 ) ) {
 531                         return;
 532                 }
 533                 $now = time();
 534                 // Avoid repeating the delete within a few seconds
 535                 if ( $now > ( $this->lastExpireAll + 1 ) ) {
 536                         $this->lastExpireAll = $now;
 537                         $this->expireAll();
 538                 }
 539         }
 540
 541         public function expireAll() {
 542                 $this->deleteObjectsExpiringBefore( wfTimestampNow() );
 543         }
 544
 545         /**
 546          * Delete objects from the database which expire before a certain date.
 547          * @param string $timestamp
 548          * @param bool|callable $progressCallback
 549          * @return bool
 550          */
 551         public function deleteObjectsExpiringBefore( $timestamp, $progressCallback = false ) {
 552                 for ( $serverIndex = 0; $serverIndex < $this->numServers; $serverIndex++ ) {
 553                         $db = null;
 554                         try {
 555                                 $db = $this->getDB( $serverIndex );
 556                                 $dbTimestamp = $db->timestamp( $timestamp );
 557                                 $totalSeconds = false;
 558                                 $baseConds = [ 'exptime < ' . $db->addQuotes( $dbTimestamp ) ];
 559                                 for ( $i = 0; $i < $this->shards; $i++ ) {
 560                                         $maxExpTime = false;
 561                                         while ( true ) {
 562                                                 $conds = $baseConds;
 563                                                 if ( $maxExpTime !== false ) {
 564                                                         $conds[] = 'exptime > ' . $db->addQuotes( $maxExpTime );
 565                                                 }
 566                                                 $rows = $db->select(
 567                                                         $this->getTableNameByShard( $i ),
 568                                                         [ 'keyname', 'exptime' ],
 569                                                         $conds,
 570                                                         __METHOD__,
 571                                                         [ 'LIMIT' => 100, 'ORDER BY' => 'exptime' ] );
 572                                                 if ( $rows === false || !$rows->numRows() ) {
 573                                                         break;
 574                                                 }
 575                                                 $keys = [];
 576                                                 $row = $rows->current();
 577                                                 $minExpTime = $row->exptime;
 578                                                 if ( $totalSeconds === false ) {
 579                                                         $totalSeconds = wfTimestamp( TS_UNIX, $timestamp )
 580                                                                 - wfTimestamp( TS_UNIX, $minExpTime );
 581                                                 }
 582                                                 foreach ( $rows as $row ) {
 583                                                         $keys[] = $row->keyname;
 584                                                         $maxExpTime = $row->exptime;
 585                                                 }
 586
 587                                                 $db->delete(
 588                                                         $this->getTableNameByShard( $i ),
 589                                                         [
 590                                                                 'exptime >= ' . $db->addQuotes( $minExpTime ),
 591                                                                 'exptime < ' . $db->addQuotes( $dbTimestamp ),
 592                                                                 'keyname' => $keys
 593                                                         ],
 594                                                         __METHOD__ );
 595
 596                                                 if ( $progressCallback ) {
 597                                                         if ( intval( $totalSeconds ) === 0 ) {
 598                                                                 $percent = 0;
 599                                                         } else {
 600                                                                 $remainingSeconds = wfTimestamp( TS_UNIX, $timestamp )
 601                                                                         - wfTimestamp( TS_UNIX, $maxExpTime );
 602                                                                 if ( $remainingSeconds > $totalSeconds ) {
 603                                                                         $totalSeconds = $remainingSeconds;
 604                                                                 }
 605                                                                 $processedSeconds = $totalSeconds - $remainingSeconds;
 606                                                                 $percent = ( $i + $processedSeconds / $totalSeconds )
 607                                                                         / $this->shards * 100;
 608                                                         }
 609                                                         $percent = ( $percent / $this->numServers )
 610                                                                 + ( $serverIndex / $this->numServers * 100 );
 611                                                         call_user_func( $progressCallback, $percent );
 612                                                 }
 613                                         }
 614                                 }
 615                         } catch ( DBError $e ) {
 616                                 $this->handleWriteError( $e, $db, $serverIndex );
 617                                 return false;
 618                         }
 619                 }
 620                 return true;
 621         }
 622
 623         /**
 624          * Delete content of shard tables in every server.
 625          * Return true if the operation is successful, false otherwise.
 626          * @return bool
 627          */
 628         public function deleteAll() {
 629                 for ( $serverIndex = 0; $serverIndex < $this->numServers; $serverIndex++ ) {
 630                         $db = null;
 631                         try {
 632                                 $db = $this->getDB( $serverIndex );
 633                                 for ( $i = 0; $i < $this->shards; $i++ ) {
 634                                         $db->delete( $this->getTableNameByShard( $i ), '*', __METHOD__ );
 635                                 }
 636                         } catch ( DBError $e ) {
 637                                 $this->handleWriteError( $e, $db, $serverIndex );
 638                                 return false;
 639                         }
 640                 }
 641                 return true;
 642         }
 643
 644         /**
 645          * Serialize an object and, if possible, compress the representation.
 646          * On typical message and page data, this can provide a 3X decrease
 647          * in storage requirements.
 648          *
 649          * @param mixed $data
 650          * @return string
 651          */
 652         protected function serialize( &$data ) {
 653                 $serial = serialize( $data );
 654
 655                 if ( function_exists( 'gzdeflate' ) ) {
 656                         return gzdeflate( $serial );
 657                 } else {
 658                         return $serial;
 659                 }
 660         }
 661
 662         /**
 663          * Unserialize and, if necessary, decompress an object.
 664          * @param string $serial
 665          * @return mixed
 666          */
 667         protected function unserialize( $serial ) {
 668                 if ( function_exists( 'gzinflate' ) ) {
 669                         MediaWiki\suppressWarnings();
 670                         $decomp = gzinflate( $serial );
 671                         MediaWiki\restoreWarnings();
 672
 673                         if ( false !== $decomp ) {
 674                                 $serial = $decomp;
 675                         }
 676                 }
 677
 678                 $ret = unserialize( $serial );
 679
 680                 return $ret;
 681         }
 682
 683         /**
 684          * Handle a DBError which occurred during a read operation.
 685          *
 686          * @param DBError $exception
 687          * @param int $serverIndex
 688          */
 689         protected function handleReadError( DBError $exception, $serverIndex ) {
 690                 if ( $exception instanceof DBConnectionError ) {
 691                         $this->markServerDown( $exception, $serverIndex );
 692                 }
 693                 $this->logger->error( "DBError: {$exception->getMessage()}" );
 694                 if ( $exception instanceof DBConnectionError ) {
 695                         $this->setLastError( BagOStuff::ERR_UNREACHABLE );
 696                         $this->logger->debug( __METHOD__ . ": ignoring connection error" );
 697                 } else {
 698                         $this->setLastError( BagOStuff::ERR_UNEXPECTED );
 699                         $this->logger->debug( __METHOD__ . ": ignoring query error" );
 700                 }
 701         }
 702
 703         /**
 704          * Handle a DBQueryError which occurred during a write operation.
 705          *
 706          * @param DBError $exception
 707          * @param IDatabase|null $db DB handle or null if connection failed
 708          * @param int $serverIndex
 709          * @throws Exception
 710          */
 711         protected function handleWriteError( DBError $exception, IDatabase $db = null, $serverIndex ) {
 712                 if ( !$db ) {
 713                         $this->markServerDown( $exception, $serverIndex );
 714                 } elseif ( $db->wasReadOnlyError() ) {
 715                         if ( $db->trxLevel() && $this->usesMainDB() ) {
 716                                 // Errors like deadlocks and connection drops already cause rollback.
 717                                 // For consistency, we have no choice but to throw an error and trigger
 718                                 // complete rollback if the main DB is also being used as the cache DB.
 719                                 throw $exception;
 720                         }
 721                 }
 722
 723                 $this->logger->error( "DBError: {$exception->getMessage()}" );
 724                 if ( $exception instanceof DBConnectionError ) {
 725                         $this->setLastError( BagOStuff::ERR_UNREACHABLE );
 726                         $this->logger->debug( __METHOD__ . ": ignoring connection error" );
 727                 } else {
 728                         $this->setLastError( BagOStuff::ERR_UNEXPECTED );
 729                         $this->logger->debug( __METHOD__ . ": ignoring query error" );
 730                 }
 731         }
 732
 733         /**
 734          * Mark a server down due to a DBConnectionError exception
 735          *
 736          * @param DBError $exception
 737          * @param int $serverIndex
 738          */
 739         protected function markServerDown( DBError $exception, $serverIndex ) {
 740                 unset( $this->conns[$serverIndex] ); // bug T103435
 741
 742                 if ( isset( $this->connFailureTimes[$serverIndex] ) ) {
 743                         if ( time() - $this->connFailureTimes[$serverIndex] >= 60 ) {
 744                                 unset( $this->connFailureTimes[$serverIndex] );
 745                                 unset( $this->connFailureErrors[$serverIndex] );
 746                         } else {
 747                                 $this->logger->debug( __METHOD__ . ": Server #$serverIndex already down" );
 748                                 return;
 749                         }
 750                 }
 751                 $now = time();
 752                 $this->logger->info( __METHOD__ . ": Server #$serverIndex down until " . ( $now + 60 ) );
 753                 $this->connFailureTimes[$serverIndex] = $now;
 754                 $this->connFailureErrors[$serverIndex] = $exception;
 755         }
 756
 757         /**
 758          * Create shard tables. For use from eval.php.
 759          */
 760         public function createTables() {
 761                 for ( $serverIndex = 0; $serverIndex < $this->numServers; $serverIndex++ ) {
 762                         $db = $this->getDB( $serverIndex );
 763                         if ( $db->getType() !== 'mysql' ) {
 764                                 throw new MWException( __METHOD__ . ' is not supported on this DB server' );
 765                         }
 766
 767                         for ( $i = 0; $i < $this->shards; $i++ ) {
 768                                 $db->query(
 769                                         'CREATE TABLE ' . $db->tableName( $this->getTableNameByShard( $i ) ) .
 770                                         ' LIKE ' . $db->tableName( 'objectcache' ),
 771                                         __METHOD__ );
 772                         }
 773                 }
 774         }
 775
 776         /**
 777          * @return bool Whether the main DB is used, e.g. wfGetDB( DB_MASTER )
 778          */
 779         protected function usesMainDB() {
 780                 return !$this->serverInfos;
 781         }
 782
 783         protected function waitForSlaves() {
 784                 if ( $this->usesMainDB() ) {
 785                         // Main LB is used; wait for any slaves to catch up
 786                         try {
 787                                 $lbFactory = MediaWikiServices::getInstance()->getDBLoadBalancerFactory();
 788                                 $lbFactory->waitForReplication( [ 'wiki' => wfWikiID() ] );
 789                                 return true;
 790                         } catch ( DBReplicationWaitError $e ) {
 791                                 return false;
 792                         }
 793                 } else {
 794                         // Custom DB server list; probably doesn't use replication
 795                         return true;
 796                 }
 797         }
 798 }