includes/libs/rdbms/loadbalancer/ILoadBalancer.php

   1 <?php
   2 /**
   3  * Database load balancing interface
   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 Database
  22  */
  23 namespace Wikimedia\Rdbms;
  24
  25 use Exception;
  26 use InvalidArgumentException;
  27
  28 /**
  29  * Database cluster connection, tracking, load balancing, and transaction manager interface
  30  *
  31  * A "cluster" is considered to be one master database and zero or more replica databases.
  32  * Typically, the replica DBs replicate from the master asynchronously. The first node in the
  33  * "servers" configuration array is always considered the "master". However, this class can still
  34  * be used when all or some of the "replica" DBs are multi-master peers of the master or even
  35  * when all the DBs are non-replicating clones of each other holding read-only data. Thus, the
  36  * role of "master" is in some cases merely nominal.
  37  *
  38  * By default, each DB server uses DBO_DEFAULT for its 'flags' setting, unless explicitly set
  39  * otherwise in configuration. DBO_DEFAULT behavior depends on whether 'cliMode' is set:
  40  *   - In CLI mode, the flag has no effect with regards to LoadBalancer.
  41  *   - In non-CLI mode, the flag causes implicit transactions to be used; the first query on
  42  *     a database starts a transaction on that database. The transactions are meant to remain
  43  *     pending until either commitMasterChanges() or rollbackMasterChanges() is called. The
  44  *     application must have some point where it calls commitMasterChanges() near the end of
  45  *     the PHP request.
  46  * Every iteration of beginMasterChanges()/commitMasterChanges() is called a "transaction round".
  47  * Rounds are useful on the master DB connections because they make single-DB (and by and large
  48  * multi-DB) updates in web requests all-or-nothing. Also, transactions on replica DBs are useful
  49  * when REPEATABLE-READ or SERIALIZABLE isolation is used because all foriegn keys and constraints
  50  * hold across separate queries in the DB transaction since the data appears within a consistent
  51  * point-in-time snapshot.
  52  *
  53  * The typical caller will use LoadBalancer::getConnection( DB_* ) to yield a live database
  54  * connection handle. The choice of which DB server to use is based on pre-defined loads for
  55  * weighted random selection, adjustments thereof by LoadMonitor, and the amount of replication
  56  * lag on each DB server. Lag checks might cause problems in certain setups, so they should be
  57  * tuned in the server configuration maps as follows:
  58  *   - Master + N Replica(s): set 'max lag' to an appropriate threshold for avoiding any database
  59  *      lagged by this much or more. If all DBs are this lagged, then the load balancer considers
  60  *      the cluster to be read-only.
  61  *   - Galera Cluster: Seconds_Behind_Master will be 0, so there probably is nothing to tune.
  62  *      Note that lag is still possible depending on how wsrep-sync-wait is set server-side.
  63  *   - Read-only archive clones: set 'is static' in the server configuration maps. This will
  64  *      treat all such DBs as having 0 lag.
  65  *   - SQL load balancing proxy: any proxy should handle lag checks on its own, so the 'max lag'
  66  *      parameter should probably be set to INF in the server configuration maps. This will make
  67  *      the load balancer ignore whatever it detects as the lag of the logical replica is (which
  68  *      would probably just randomly bounce around).
  69  *
  70  * If using a SQL proxy service, it would probably be best to have two proxy hosts for the
  71  * load balancer to talk to. One would be the 'host' of the master server entry and another for
  72  * the (logical) replica server entry. The proxy could map the load balancer's "replica" DB to
  73  * any number of physical replica DBs.
  74  *
  75  * @since 1.28
  76  * @ingroup Database
  77  */
  78 interface ILoadBalancer {
  79         /** @var int Request a replica DB connection */
  80         const DB_REPLICA = -1;
  81         /** @var int Request a master DB connection */
  82         const DB_MASTER = -2;
  83
  84         /** @var string Domain specifier when no specific database needs to be selected */
  85         const DOMAIN_ANY = '';
  86
  87         /** @var int DB handle should have DBO_TRX disabled and the caller will leave it as such */
  88         const CONN_TRX_AUTOCOMMIT = 1;
  89         /** @var int Alias for CONN_TRX_AUTOCOMMIT for b/c; deprecated since 1.31 */
  90         const CONN_TRX_AUTO = 1;
  91
  92         /**
  93          * Construct a manager of IDatabase connection objects
  94          *
  95          * @param array $params Parameter map with keys:
  96          *  - servers : Required. Array of server info structures.
  97          *  - localDomain: A DatabaseDomain or domain ID string.
  98          *  - loadMonitor : Name of a class used to fetch server lag and load.
  99          *  - readOnlyReason : Reason the master DB is read-only if so [optional]
 100          *  - waitTimeout : Maximum time to wait for replicas for consistency [optional]
 101          *  - maxLag: Avoid replica DB servers with more lag than this [optional]
 102          *  - srvCache : BagOStuff object for server cache [optional]
 103          *  - wanCache : WANObjectCache object [optional]
 104          *  - chronologyProtector: ChronologyProtector object [optional]
 105          *  - hostname : The name of the current server [optional]
 106          *  - cliMode: Whether the execution context is a CLI script. [optional]
 107          *  - profiler : Class name or instance with profileIn()/profileOut() methods. [optional]
 108          *  - trxProfiler: TransactionProfiler instance. [optional]
 109          *  - replLogger: PSR-3 logger instance. [optional]
 110          *  - connLogger: PSR-3 logger instance. [optional]
 111          *  - queryLogger: PSR-3 logger instance. [optional]
 112          *  - perfLogger: PSR-3 logger instance. [optional]
 113          *  - errorLogger : Callback that takes an Exception and logs it. [optional]
 114          * @throws InvalidArgumentException
 115          */
 116         public function __construct( array $params );
 117
 118         /**
 119          * Get the index of the reader connection, which may be a replica DB
 120          *
 121          * This takes into account load ratios and lag times. It should
 122          * always return a consistent index during a given invocation.
 123          *
 124          * Side effect: opens connections to databases
 125          * @param string|bool $group Query group, or false for the generic reader
 126          * @param string|bool $domain Domain ID, or false for the current domain
 127          * @throws DBError
 128          * @return bool|int|string
 129          */
 130         public function getReaderIndex( $group = false, $domain = false );
 131
 132         /**
 133          * Set the master wait position
 134          *
 135          * If a DB_REPLICA connection has been opened already, then wait immediately.
 136          * Otherwise sets a variable telling it to wait if such a connection is opened.
 137          *
 138          * This only applies to connections to the generic replica DB for this request.
 139          * If a timeout happens when waiting, then getLaggedReplicaMode()/laggedReplicaUsed()
 140          * will return true.
 141          *
 142          * @param DBMasterPos|bool $pos Master position or false
 143          */
 144         public function waitFor( $pos );
 145
 146         /**
 147          * Set the master wait position and wait for a "generic" replica DB to catch up to it
 148          *
 149          * This can be used a faster proxy for waitForAll()
 150          *
 151          * @param DBMasterPos|bool $pos Master position or false
 152          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 153          * @return bool Success (able to connect and no timeouts reached)
 154          */
 155         public function waitForOne( $pos, $timeout = null );
 156
 157         /**
 158          * Set the master wait position and wait for ALL replica DBs to catch up to it
 159          *
 160          * @param DBMasterPos|bool $pos Master position or false
 161          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 162          * @return bool Success (able to connect and no timeouts reached)
 163          */
 164         public function waitForAll( $pos, $timeout = null );
 165
 166         /**
 167          * Get any open connection to a given server index, local or foreign
 168          *
 169          * @param int $i Server index or DB_MASTER/DB_REPLICA
 170          * @return Database|bool False if no such connection is open
 171          */
 172         public function getAnyOpenConnection( $i );
 173
 174         /**
 175          * Get a connection handle by server index
 176          *
 177          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 178          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 179          * can be used to check such flags beforehand.
 180          *
 181          * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must also
 182          * call ILoadBalancer::reuseConnection() on the handle when finished using it.
 183          * In all other cases, this is not necessary, though not harmful either.
 184          *
 185          * @param int $i Server index or DB_MASTER/DB_REPLICA
 186          * @param array|string|bool $groups Query group(s), or false for the generic reader
 187          * @param string|bool $domain Domain ID, or false for the current domain
 188          * @param int $flags Bitfield of CONN_* class constants
 189          *
 190          * @note This method throws DBAccessError if ILoadBalancer::disable() was called
 191          *
 192          * @throws DBError
 193          * @return Database
 194          */
 195         public function getConnection( $i, $groups = [], $domain = false, $flags = 0 );
 196
 197         /**
 198          * Mark a foreign connection as being available for reuse under a different DB domain
 199          *
 200          * This mechanism is reference-counted, and must be called the same number of times
 201          * as getConnection() to work.
 202          *
 203          * @param IDatabase $conn
 204          * @throws InvalidArgumentException
 205          */
 206         public function reuseConnection( IDatabase $conn );
 207
 208         /**
 209          * Get a database connection handle reference
 210          *
 211          * The handle's methods simply wrap those of a Database handle
 212          *
 213          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 214          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 215          * can be used to check such flags beforehand.
 216          *
 217          * @see ILoadBalancer::getConnection() for parameter information
 218          *
 219          * @param int $i Server index or DB_MASTER/DB_REPLICA
 220          * @param array|string|bool $groups Query group(s), or false for the generic reader
 221          * @param string|bool $domain Domain ID, or false for the current domain
 222          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 223          * @return DBConnRef
 224          */
 225         public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 226
 227         /**
 228          * Get a database connection handle reference without connecting yet
 229          *
 230          * The handle's methods simply wrap those of a Database handle
 231          *
 232          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 233          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 234          * can be used to check such flags beforehand.
 235          *
 236          * @see ILoadBalancer::getConnection() for parameter information
 237          *
 238          * @param int $i Server index or DB_MASTER/DB_REPLICA
 239          * @param array|string|bool $groups Query group(s), or false for the generic reader
 240          * @param string|bool $domain Domain ID, or false for the current domain
 241          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 242          * @return DBConnRef
 243          */
 244         public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 245
 246         /**
 247          * Get a maintenance database connection handle reference for migrations and schema changes
 248          *
 249          * The handle's methods simply wrap those of a Database handle
 250          *
 251          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 252          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 253          * can be used to check such flags beforehand.
 254          *
 255          * @see ILoadBalancer::getConnection() for parameter information
 256          *
 257          * @param int $db Server index or DB_MASTER/DB_REPLICA
 258          * @param array|string|bool $groups Query group(s), or false for the generic reader
 259          * @param string|bool $domain Domain ID, or false for the current domain
 260          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 261          * @return MaintainableDBConnRef
 262          */
 263         public function getMaintenanceConnectionRef( $db, $groups = [], $domain = false, $flags = 0 );
 264
 265         /**
 266          * Open a connection to the server given by the specified index
 267          *
 268          * The index must be an actual index into the array. If a connection to the server is
 269          * already open and not considered an "in use" foreign connection, this simply returns it.
 270          *
 271          * Avoid using CONN_TRX_AUTOCOMMIT for databases with ATTR_DB_LEVEL_LOCKING (e.g. sqlite) in
 272          * order to avoid deadlocks. ILoadBalancer::getServerAttributes() can be used to check
 273          * such flags beforehand.
 274          *
 275          * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must also
 276          * call ILoadBalancer::reuseConnection() on the handle when finished using it.
 277          * In all other cases, this is not necessary, though not harmful either.
 278          *
 279          * @note This method throws DBAccessError if ILoadBalancer::disable() was called
 280          *
 281          * @param int $i Server index (does not support DB_MASTER/DB_REPLICA)
 282          * @param string|bool $domain Domain ID, or false for the current domain
 283          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 284          * @return Database|bool Returns false on errors
 285          * @throws DBAccessError
 286          */
 287         public function openConnection( $i, $domain = false, $flags = 0 );
 288
 289         /**
 290          * @return int
 291          */
 292         public function getWriterIndex();
 293
 294         /**
 295          * Returns true if the specified index is a valid server index
 296          *
 297          * @param int $i
 298          * @return bool
 299          */
 300         public function haveIndex( $i );
 301
 302         /**
 303          * Returns true if the specified index is valid and has non-zero load
 304          *
 305          * @param int $i
 306          * @return bool
 307          */
 308         public function isNonZeroLoad( $i );
 309
 310         /**
 311          * Get the number of defined servers (not the number of open connections)
 312          *
 313          * @return int
 314          */
 315         public function getServerCount();
 316
 317         /**
 318          * Get the host name or IP address of the server with the specified index
 319          *
 320          * @param int $i
 321          * @return string Readable name if available or IP/host otherwise
 322          */
 323         public function getServerName( $i );
 324
 325         /**
 326          * Get DB type of the server with the specified index
 327          *
 328          * @param int $i
 329          * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes
 330          * @since 1.30
 331          */
 332         public function getServerType( $i );
 333
 334         /**
 335          * @param int $i Server index
 336          * @return array (Database::ATTRIBUTE_* constant => value) for all such constants
 337          * @since 1.31
 338          */
 339         public function getServerAttributes( $i );
 340
 341         /**
 342          * Get the current master position for chronology control purposes
 343          * @return DBMasterPos|bool Returns false if not applicable
 344          */
 345         public function getMasterPos();
 346
 347         /**
 348          * Disable this load balancer. All connections are closed, and any attempt to
 349          * open a new connection will result in a DBAccessError.
 350          */
 351         public function disable();
 352
 353         /**
 354          * Close all open connections
 355          */
 356         public function closeAll();
 357
 358         /**
 359          * Close a connection
 360          *
 361          * Using this function makes sure the LoadBalancer knows the connection is closed.
 362          * If you use $conn->close() directly, the load balancer won't update its state.
 363          *
 364          * @param IDatabase $conn
 365          */
 366         public function closeConnection( IDatabase $conn );
 367
 368         /**
 369          * Commit transactions on all open connections
 370          * @param string $fname Caller name
 371          * @throws DBExpectedError
 372          */
 373         public function commitAll( $fname = __METHOD__ );
 374
 375         /**
 376          * Perform all pre-commit callbacks that remain part of the atomic transactions
 377          * and disable any post-commit callbacks until runMasterPostTrxCallbacks()
 378          *
 379          * Use this only for mutli-database commits
 380          */
 381         public function finalizeMasterChanges();
 382
 383         /**
 384          * Perform all pre-commit checks for things like replication safety
 385          *
 386          * Use this only for mutli-database commits
 387          *
 388          * @param array $options Includes:
 389          *   - maxWriteDuration : max write query duration time in seconds
 390          * @throws DBTransactionError
 391          */
 392         public function approveMasterChanges( array $options );
 393
 394         /**
 395          * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
 396          *
 397          * The DBO_TRX setting will be reverted to the default in each of these methods:
 398          *   - commitMasterChanges()
 399          *   - rollbackMasterChanges()
 400          *   - commitAll()
 401          * This allows for custom transaction rounds from any outer transaction scope.
 402          *
 403          * @param string $fname
 404          * @throws DBExpectedError
 405          */
 406         public function beginMasterChanges( $fname = __METHOD__ );
 407
 408         /**
 409          * Issue COMMIT on all master connections where writes where done
 410          * @param string $fname Caller name
 411          * @throws DBExpectedError
 412          */
 413         public function commitMasterChanges( $fname = __METHOD__ );
 414
 415         /**
 416          * Issue all pending post-COMMIT/ROLLBACK callbacks
 417          *
 418          * Use this only for mutli-database commits
 419          *
 420          * @param int $type IDatabase::TRIGGER_* constant
 421          * @return Exception|null The first exception or null if there were none
 422          */
 423         public function runMasterPostTrxCallbacks( $type );
 424
 425         /**
 426          * Issue ROLLBACK only on master, only if queries were done on connection
 427          * @param string $fname Caller name
 428          * @throws DBExpectedError
 429          */
 430         public function rollbackMasterChanges( $fname = __METHOD__ );
 431
 432         /**
 433          * Suppress all pending post-COMMIT/ROLLBACK callbacks
 434          *
 435          * Use this only for mutli-database commits
 436          *
 437          * @return Exception|null The first exception or null if there were none
 438          */
 439         public function suppressTransactionEndCallbacks();
 440
 441         /**
 442          * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshot
 443          *
 444          * @param string $fname Caller name
 445          */
 446         public function flushReplicaSnapshots( $fname = __METHOD__ );
 447
 448         /**
 449          * @return bool Whether a master connection is already open
 450          */
 451         public function hasMasterConnection();
 452
 453         /**
 454          * Determine if there are pending changes in a transaction by this thread
 455          * @return bool
 456          */
 457         public function hasMasterChanges();
 458
 459         /**
 460          * Get the timestamp of the latest write query done by this thread
 461          * @return float|bool UNIX timestamp or false
 462          */
 463         public function lastMasterChangeTimestamp();
 464
 465         /**
 466          * Check if this load balancer object had any recent or still
 467          * pending writes issued against it by this PHP thread
 468          *
 469          * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
 470          * @return bool
 471          */
 472         public function hasOrMadeRecentMasterChanges( $age = null );
 473
 474         /**
 475          * Get the list of callers that have pending master changes
 476          *
 477          * @return string[] List of method names
 478          */
 479         public function pendingMasterChangeCallers();
 480
 481         /**
 482          * @note This method will trigger a DB connection if not yet done
 483          * @param string|bool $domain Domain ID, or false for the current domain
 484          * @return bool Whether the database for generic connections this request is highly "lagged"
 485          */
 486         public function getLaggedReplicaMode( $domain = false );
 487
 488         /**
 489          * Checks whether the database for generic connections this request was both:
 490          *   - a) Already choosen due to a prior connection attempt
 491          *   - b) Considered highly "lagged"
 492          *
 493          * @note This method will never cause a new DB connection
 494          * @return bool
 495          */
 496         public function laggedReplicaUsed();
 497
 498         /**
 499          * @note This method may trigger a DB connection if not yet done
 500          * @param string|bool $domain Domain ID, or false for the current domain
 501          * @param IDatabase|null $conn DB master connection; used to avoid loops [optional]
 502          * @return string|bool Reason the master is read-only or false if it is not
 503          */
 504         public function getReadOnlyReason( $domain = false, IDatabase $conn = null );
 505
 506         /**
 507          * Disables/enables lag checks
 508          * @param null|bool $mode
 509          * @return bool
 510          */
 511         public function allowLagged( $mode = null );
 512
 513         /**
 514          * @return bool
 515          */
 516         public function pingAll();
 517
 518         /**
 519          * Call a function with each open connection object
 520          * @param callable $callback
 521          * @param array $params
 522          */
 523         public function forEachOpenConnection( $callback, array $params = [] );
 524
 525         /**
 526          * Call a function with each open connection object to a master
 527          * @param callable $callback
 528          * @param array $params
 529          */
 530         public function forEachOpenMasterConnection( $callback, array $params = [] );
 531
 532         /**
 533          * Call a function with each open replica DB connection object
 534          * @param callable $callback
 535          * @param array $params
 536          */
 537         public function forEachOpenReplicaConnection( $callback, array $params = [] );
 538
 539         /**
 540          * Get the hostname and lag time of the most-lagged replica DB
 541          *
 542          * This is useful for maintenance scripts that need to throttle their updates.
 543          * May attempt to open connections to replica DBs on the default DB. If there is
 544          * no lag, the maximum lag will be reported as -1.
 545          *
 546          * @param bool|string $domain Domain ID, or false for the default database
 547          * @return array ( host, max lag, index of max lagged host )
 548          */
 549         public function getMaxLag( $domain = false );
 550
 551         /**
 552          * Get an estimate of replication lag (in seconds) for each server
 553          *
 554          * Results are cached for a short time in memcached/process cache
 555          *
 556          * Values may be "false" if replication is too broken to estimate
 557          *
 558          * @param string|bool $domain
 559          * @return int[] Map of (server index => float|int|bool)
 560          */
 561         public function getLagTimes( $domain = false );
 562
 563         /**
 564          * Get the lag in seconds for a given connection, or zero if this load
 565          * balancer does not have replication enabled.
 566          *
 567          * This should be used in preference to Database::getLag() in cases where
 568          * replication may not be in use, since there is no way to determine if
 569          * replication is in use at the connection level without running
 570          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
 571          * function instead of Database::getLag() avoids a fatal error in this
 572          * case on many installations.
 573          *
 574          * @param IDatabase $conn
 575          * @return int|bool Returns false on error
 576          */
 577         public function safeGetLag( IDatabase $conn );
 578
 579         /**
 580          * Wait for a replica DB to reach a specified master position
 581          *
 582          * This will connect to the master to get an accurate position if $pos is not given
 583          *
 584          * @param IDatabase $conn Replica DB
 585          * @param DBMasterPos|bool $pos Master position; default: current position
 586          * @param int $timeout Timeout in seconds [optional]
 587          * @return bool Success
 588          */
 589         public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = 10 );
 590
 591         /**
 592          * Set a callback via IDatabase::setTransactionListener() on
 593          * all current and future master connections of this load balancer
 594          *
 595          * @param string $name Callback name
 596          * @param callable|null $callback
 597          */
 598         public function setTransactionListener( $name, callable $callback = null );
 599
 600         /**
 601          * Set a new table prefix for the existing local domain ID for testing
 602          *
 603          * @param string $prefix
 604          */
 605         public function setDomainPrefix( $prefix );
 606
 607         /**
 608          * Make certain table names use their own database, schema, and table prefix
 609          * when passed into SQL queries pre-escaped and without a qualified database name
 610          *
 611          * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
 612          * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
 613          *
 614          * Calling this twice will completely clear any old table aliases. Also, note that
 615          * callers are responsible for making sure the schemas and databases actually exist.
 616          *
 617          * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
 618          */
 619         public function setTableAliases( array $aliases );
 620
 621         /**
 622          * Convert certain index names to alternative names before querying the DB
 623          *
 624          * Note that this applies to indexes regardless of the table they belong to.
 625          *
 626          * This can be employed when an index was renamed X => Y in code, but the new Y-named
 627          * indexes were not yet built on all DBs. After all the Y-named ones are added by the DBA,
 628          * the aliases can be removed, and then the old X-named indexes dropped.
 629          *
 630          * @param string[] $aliases
 631          * @return mixed
 632          * @since 1.31
 633          */
 634         public function setIndexAliases( array $aliases );
 635 }