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          *  - deprecationLogger: Callback to log a deprecation warning. [optional]
 115          * @throws InvalidArgumentException
 116          */
 117         public function __construct( array $params );
 118
 119         /**
 120          * Get the index of the reader connection, which may be a replica DB
 121          *
 122          * This takes into account load ratios and lag times. It should
 123          * always return a consistent index during a given invocation.
 124          *
 125          * Side effect: opens connections to databases
 126          * @param string|bool $group Query group, or false for the generic reader
 127          * @param string|bool $domain Domain ID, or false for the current domain
 128          * @throws DBError
 129          * @return bool|int|string
 130          */
 131         public function getReaderIndex( $group = false, $domain = false );
 132
 133         /**
 134          * Set the master wait position
 135          *
 136          * If a DB_REPLICA connection has been opened already, then wait immediately.
 137          * Otherwise sets a variable telling it to wait if such a connection is opened.
 138          *
 139          * This only applies to connections to the generic replica DB for this request.
 140          * If a timeout happens when waiting, then getLaggedReplicaMode()/laggedReplicaUsed()
 141          * will return true.
 142          *
 143          * @param DBMasterPos|bool $pos Master position or false
 144          */
 145         public function waitFor( $pos );
 146
 147         /**
 148          * Set the master wait position and wait for a "generic" replica DB to catch up to it
 149          *
 150          * This can be used a faster proxy for waitForAll()
 151          *
 152          * @param DBMasterPos|bool $pos Master position or false
 153          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 154          * @return bool Success (able to connect and no timeouts reached)
 155          */
 156         public function waitForOne( $pos, $timeout = null );
 157
 158         /**
 159          * Set the master wait position and wait for ALL replica DBs to catch up to it
 160          *
 161          * @param DBMasterPos|bool $pos Master position or false
 162          * @param int $timeout Max seconds to wait; default is mWaitTimeout
 163          * @return bool Success (able to connect and no timeouts reached)
 164          */
 165         public function waitForAll( $pos, $timeout = null );
 166
 167         /**
 168          * Get any open connection to a given server index, local or foreign
 169          *
 170          * @param int $i Server index or DB_MASTER/DB_REPLICA
 171          * @return Database|bool False if no such connection is open
 172          */
 173         public function getAnyOpenConnection( $i );
 174
 175         /**
 176          * Get a connection handle by server index
 177          *
 178          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 179          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 180          * can be used to check such flags beforehand.
 181          *
 182          * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must also
 183          * call ILoadBalancer::reuseConnection() on the handle when finished using it.
 184          * In all other cases, this is not necessary, though not harmful either.
 185          *
 186          * @param int $i Server index or DB_MASTER/DB_REPLICA
 187          * @param array|string|bool $groups Query group(s), or false for the generic reader
 188          * @param string|bool $domain Domain ID, or false for the current domain
 189          * @param int $flags Bitfield of CONN_* class constants
 190          *
 191          * @note This method throws DBAccessError if ILoadBalancer::disable() was called
 192          *
 193          * @throws DBError
 194          * @return Database
 195          */
 196         public function getConnection( $i, $groups = [], $domain = false, $flags = 0 );
 197
 198         /**
 199          * Mark a foreign connection as being available for reuse under a different DB domain
 200          *
 201          * This mechanism is reference-counted, and must be called the same number of times
 202          * as getConnection() to work.
 203          *
 204          * @param IDatabase $conn
 205          * @throws InvalidArgumentException
 206          */
 207         public function reuseConnection( IDatabase $conn );
 208
 209         /**
 210          * Get a database connection handle reference
 211          *
 212          * The handle's methods simply wrap those of a Database handle
 213          *
 214          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 215          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 216          * can be used to check such flags beforehand.
 217          *
 218          * @see ILoadBalancer::getConnection() for parameter information
 219          *
 220          * @param int $i Server index or DB_MASTER/DB_REPLICA
 221          * @param array|string|bool $groups Query group(s), or false for the generic reader
 222          * @param string|bool $domain Domain ID, or false for the current domain
 223          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 224          * @return DBConnRef
 225          */
 226         public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 227
 228         /**
 229          * Get a database connection handle reference without connecting yet
 230          *
 231          * The handle's methods simply wrap those of a Database handle
 232          *
 233          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 234          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 235          * can be used to check such flags beforehand.
 236          *
 237          * @see ILoadBalancer::getConnection() for parameter information
 238          *
 239          * @param int $i Server index or DB_MASTER/DB_REPLICA
 240          * @param array|string|bool $groups Query group(s), or false for the generic reader
 241          * @param string|bool $domain Domain ID, or false for the current domain
 242          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 243          * @return DBConnRef
 244          */
 245         public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 246
 247         /**
 248          * Get a maintenance database connection handle reference for migrations and schema changes
 249          *
 250          * The handle's methods simply wrap those of a Database handle
 251          *
 252          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 253          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 254          * can be used to check such flags beforehand.
 255          *
 256          * @see ILoadBalancer::getConnection() for parameter information
 257          *
 258          * @param int $db Server index or DB_MASTER/DB_REPLICA
 259          * @param array|string|bool $groups Query group(s), or false for the generic reader
 260          * @param string|bool $domain Domain ID, or false for the current domain
 261          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 262          * @return MaintainableDBConnRef
 263          */
 264         public function getMaintenanceConnectionRef( $db, $groups = [], $domain = false, $flags = 0 );
 265
 266         /**
 267          * Open a connection to the server given by the specified index
 268          *
 269          * The index must be an actual index into the array. If a connection to the server is
 270          * already open and not considered an "in use" foreign connection, this simply returns it.
 271          *
 272          * Avoid using CONN_TRX_AUTOCOMMIT for databases with ATTR_DB_LEVEL_LOCKING (e.g. sqlite) in
 273          * order to avoid deadlocks. ILoadBalancer::getServerAttributes() can be used to check
 274          * such flags beforehand.
 275          *
 276          * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must also
 277          * call ILoadBalancer::reuseConnection() on the handle when finished using it.
 278          * In all other cases, this is not necessary, though not harmful either.
 279          *
 280          * @note This method throws DBAccessError if ILoadBalancer::disable() was called
 281          *
 282          * @param int $i Server index (does not support DB_MASTER/DB_REPLICA)
 283          * @param string|bool $domain Domain ID, or false for the current domain
 284          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 285          * @return Database|bool Returns false on errors
 286          * @throws DBAccessError
 287          */
 288         public function openConnection( $i, $domain = false, $flags = 0 );
 289
 290         /**
 291          * @return int
 292          */
 293         public function getWriterIndex();
 294
 295         /**
 296          * Returns true if the specified index is a valid server index
 297          *
 298          * @param int $i
 299          * @return bool
 300          */
 301         public function haveIndex( $i );
 302
 303         /**
 304          * Returns true if the specified index is valid and has non-zero load
 305          *
 306          * @param int $i
 307          * @return bool
 308          */
 309         public function isNonZeroLoad( $i );
 310
 311         /**
 312          * Get the number of defined servers (not the number of open connections)
 313          *
 314          * @return int
 315          */
 316         public function getServerCount();
 317
 318         /**
 319          * Get the host name or IP address of the server with the specified index
 320          *
 321          * @param int $i
 322          * @return string Readable name if available or IP/host otherwise
 323          */
 324         public function getServerName( $i );
 325
 326         /**
 327          * Get DB type of the server with the specified index
 328          *
 329          * @param int $i
 330          * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes
 331          * @since 1.30
 332          */
 333         public function getServerType( $i );
 334
 335         /**
 336          * @param int $i Server index
 337          * @return array (Database::ATTRIBUTE_* constant => value) for all such constants
 338          * @since 1.31
 339          */
 340         public function getServerAttributes( $i );
 341
 342         /**
 343          * Get the current master position for chronology control purposes
 344          * @return DBMasterPos|bool Returns false if not applicable
 345          */
 346         public function getMasterPos();
 347
 348         /**
 349          * Disable this load balancer. All connections are closed, and any attempt to
 350          * open a new connection will result in a DBAccessError.
 351          */
 352         public function disable();
 353
 354         /**
 355          * Close all open connections
 356          */
 357         public function closeAll();
 358
 359         /**
 360          * Close a connection
 361          *
 362          * Using this function makes sure the LoadBalancer knows the connection is closed.
 363          * If you use $conn->close() directly, the load balancer won't update its state.
 364          *
 365          * @param IDatabase $conn
 366          */
 367         public function closeConnection( IDatabase $conn );
 368
 369         /**
 370          * Commit transactions on all open connections
 371          * @param string $fname Caller name
 372          * @throws DBExpectedError
 373          */
 374         public function commitAll( $fname = __METHOD__ );
 375
 376         /**
 377          * Perform all pre-commit callbacks that remain part of the atomic transactions
 378          * and disable any post-commit callbacks until runMasterPostTrxCallbacks()
 379          *
 380          * Use this only for mutli-database commits
 381          */
 382         public function finalizeMasterChanges();
 383
 384         /**
 385          * Perform all pre-commit checks for things like replication safety
 386          *
 387          * Use this only for mutli-database commits
 388          *
 389          * @param array $options Includes:
 390          *   - maxWriteDuration : max write query duration time in seconds
 391          * @throws DBTransactionError
 392          */
 393         public function approveMasterChanges( array $options );
 394
 395         /**
 396          * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
 397          *
 398          * The DBO_TRX setting will be reverted to the default in each of these methods:
 399          *   - commitMasterChanges()
 400          *   - rollbackMasterChanges()
 401          *   - commitAll()
 402          * This allows for custom transaction rounds from any outer transaction scope.
 403          *
 404          * @param string $fname
 405          * @throws DBExpectedError
 406          */
 407         public function beginMasterChanges( $fname = __METHOD__ );
 408
 409         /**
 410          * Issue COMMIT on all master connections where writes where done
 411          * @param string $fname Caller name
 412          * @throws DBExpectedError
 413          */
 414         public function commitMasterChanges( $fname = __METHOD__ );
 415
 416         /**
 417          * Issue all pending post-COMMIT/ROLLBACK callbacks
 418          *
 419          * Use this only for mutli-database commits
 420          *
 421          * @param int $type IDatabase::TRIGGER_* constant
 422          * @return Exception|null The first exception or null if there were none
 423          */
 424         public function runMasterPostTrxCallbacks( $type );
 425
 426         /**
 427          * Issue ROLLBACK only on master, only if queries were done on connection
 428          * @param string $fname Caller name
 429          * @throws DBExpectedError
 430          */
 431         public function rollbackMasterChanges( $fname = __METHOD__ );
 432
 433         /**
 434          * Suppress all pending post-COMMIT/ROLLBACK callbacks
 435          *
 436          * Use this only for mutli-database commits
 437          *
 438          * @return Exception|null The first exception or null if there were none
 439          */
 440         public function suppressTransactionEndCallbacks();
 441
 442         /**
 443          * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshot
 444          *
 445          * @param string $fname Caller name
 446          */
 447         public function flushReplicaSnapshots( $fname = __METHOD__ );
 448
 449         /**
 450          * @return bool Whether a master connection is already open
 451          */
 452         public function hasMasterConnection();
 453
 454         /**
 455          * Determine if there are pending changes in a transaction by this thread
 456          * @return bool
 457          */
 458         public function hasMasterChanges();
 459
 460         /**
 461          * Get the timestamp of the latest write query done by this thread
 462          * @return float|bool UNIX timestamp or false
 463          */
 464         public function lastMasterChangeTimestamp();
 465
 466         /**
 467          * Check if this load balancer object had any recent or still
 468          * pending writes issued against it by this PHP thread
 469          *
 470          * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
 471          * @return bool
 472          */
 473         public function hasOrMadeRecentMasterChanges( $age = null );
 474
 475         /**
 476          * Get the list of callers that have pending master changes
 477          *
 478          * @return string[] List of method names
 479          */
 480         public function pendingMasterChangeCallers();
 481
 482         /**
 483          * @note This method will trigger a DB connection if not yet done
 484          * @param string|bool $domain Domain ID, or false for the current domain
 485          * @return bool Whether the database for generic connections this request is highly "lagged"
 486          */
 487         public function getLaggedReplicaMode( $domain = false );
 488
 489         /**
 490          * Checks whether the database for generic connections this request was both:
 491          *   - a) Already choosen due to a prior connection attempt
 492          *   - b) Considered highly "lagged"
 493          *
 494          * @note This method will never cause a new DB connection
 495          * @return bool
 496          */
 497         public function laggedReplicaUsed();
 498
 499         /**
 500          * @note This method may trigger a DB connection if not yet done
 501          * @param string|bool $domain Domain ID, or false for the current domain
 502          * @param IDatabase|null $conn DB master connection; used to avoid loops [optional]
 503          * @return string|bool Reason the master is read-only or false if it is not
 504          */
 505         public function getReadOnlyReason( $domain = false, IDatabase $conn = null );
 506
 507         /**
 508          * Disables/enables lag checks
 509          * @param null|bool $mode
 510          * @return bool
 511          */
 512         public function allowLagged( $mode = null );
 513
 514         /**
 515          * @return bool
 516          */
 517         public function pingAll();
 518
 519         /**
 520          * Call a function with each open connection object
 521          * @param callable $callback
 522          * @param array $params
 523          */
 524         public function forEachOpenConnection( $callback, array $params = [] );
 525
 526         /**
 527          * Call a function with each open connection object to a master
 528          * @param callable $callback
 529          * @param array $params
 530          */
 531         public function forEachOpenMasterConnection( $callback, array $params = [] );
 532
 533         /**
 534          * Call a function with each open replica DB connection object
 535          * @param callable $callback
 536          * @param array $params
 537          */
 538         public function forEachOpenReplicaConnection( $callback, array $params = [] );
 539
 540         /**
 541          * Get the hostname and lag time of the most-lagged replica DB
 542          *
 543          * This is useful for maintenance scripts that need to throttle their updates.
 544          * May attempt to open connections to replica DBs on the default DB. If there is
 545          * no lag, the maximum lag will be reported as -1.
 546          *
 547          * @param bool|string $domain Domain ID, or false for the default database
 548          * @return array ( host, max lag, index of max lagged host )
 549          */
 550         public function getMaxLag( $domain = false );
 551
 552         /**
 553          * Get an estimate of replication lag (in seconds) for each server
 554          *
 555          * Results are cached for a short time in memcached/process cache
 556          *
 557          * Values may be "false" if replication is too broken to estimate
 558          *
 559          * @param string|bool $domain
 560          * @return int[] Map of (server index => float|int|bool)
 561          */
 562         public function getLagTimes( $domain = false );
 563
 564         /**
 565          * Get the lag in seconds for a given connection, or zero if this load
 566          * balancer does not have replication enabled.
 567          *
 568          * This should be used in preference to Database::getLag() in cases where
 569          * replication may not be in use, since there is no way to determine if
 570          * replication is in use at the connection level without running
 571          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
 572          * function instead of Database::getLag() avoids a fatal error in this
 573          * case on many installations.
 574          *
 575          * @param IDatabase $conn
 576          * @return int|bool Returns false on error
 577          */
 578         public function safeGetLag( IDatabase $conn );
 579
 580         /**
 581          * Wait for a replica DB to reach a specified master position
 582          *
 583          * This will connect to the master to get an accurate position if $pos is not given
 584          *
 585          * @param IDatabase $conn Replica DB
 586          * @param DBMasterPos|bool $pos Master position; default: current position
 587          * @param int $timeout Timeout in seconds [optional]
 588          * @return bool Success
 589          */
 590         public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = 10 );
 591
 592         /**
 593          * Set a callback via IDatabase::setTransactionListener() on
 594          * all current and future master connections of this load balancer
 595          *
 596          * @param string $name Callback name
 597          * @param callable|null $callback
 598          */
 599         public function setTransactionListener( $name, callable $callback = null );
 600
 601         /**
 602          * Set a new table prefix for the existing local domain ID for testing
 603          *
 604          * @param string $prefix
 605          */
 606         public function setDomainPrefix( $prefix );
 607
 608         /**
 609          * Make certain table names use their own database, schema, and table prefix
 610          * when passed into SQL queries pre-escaped and without a qualified database name
 611          *
 612          * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
 613          * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
 614          *
 615          * Calling this twice will completely clear any old table aliases. Also, note that
 616          * callers are responsible for making sure the schemas and databases actually exist.
 617          *
 618          * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
 619          */
 620         public function setTableAliases( array $aliases );
 621
 622         /**
 623          * Convert certain index names to alternative names before querying the DB
 624          *
 625          * Note that this applies to indexes regardless of the table they belong to.
 626          *
 627          * This can be employed when an index was renamed X => Y in code, but the new Y-named
 628          * indexes were not yet built on all DBs. After all the Y-named ones are added by the DBA,
 629          * the aliases can be removed, and then the old X-named indexes dropped.
 630          *
 631          * @param string[] $aliases
 632          * @return mixed
 633          * @since 1.31
 634          */
 635         public function setIndexAliases( array $aliases );
 636 }