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          *  - chronologyCallback: Callback to run before the first connection attempt [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          * Use CONN_TRX_AUTOCOMMIT to only look for connections opened with that flag
 171          *
 172          * @param int $i Server index or DB_MASTER/DB_REPLICA
 173          * @param int $flags Bitfield of CONN_* class constants
 174          * @return Database|bool False if no such connection is open
 175          */
 176         public function getAnyOpenConnection( $i, $flags = 0 );
 177
 178         /**
 179          * Get a connection handle by server index
 180          *
 181          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 182          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 183          * can be used to check such flags beforehand.
 184          *
 185          * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must also
 186          * call ILoadBalancer::reuseConnection() on the handle when finished using it.
 187          * In all other cases, this is not necessary, though not harmful either.
 188          *
 189          * @param int $i Server index (overrides $groups) or DB_MASTER/DB_REPLICA
 190          * @param array|string|bool $groups Query group(s), or false for the generic reader
 191          * @param string|bool $domain Domain ID, or false for the current domain
 192          * @param int $flags Bitfield of CONN_* class constants
 193          *
 194          * @note This method throws DBAccessError if ILoadBalancer::disable() was called
 195          *
 196          * @throws DBError
 197          * @return Database
 198          */
 199         public function getConnection( $i, $groups = [], $domain = false, $flags = 0 );
 200
 201         /**
 202          * Mark a foreign connection as being available for reuse under a different DB domain
 203          *
 204          * This mechanism is reference-counted, and must be called the same number of times
 205          * as getConnection() to work.
 206          *
 207          * @param IDatabase $conn
 208          * @throws InvalidArgumentException
 209          */
 210         public function reuseConnection( IDatabase $conn );
 211
 212         /**
 213          * Get a database connection handle reference
 214          *
 215          * The handle's methods simply wrap those of a Database handle
 216          *
 217          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 218          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 219          * can be used to check such flags beforehand.
 220          *
 221          * @see ILoadBalancer::getConnection() for parameter information
 222          *
 223          * @param int $i Server index or DB_MASTER/DB_REPLICA
 224          * @param array|string|bool $groups Query group(s), or false for the generic reader
 225          * @param string|bool $domain Domain ID, or false for the current domain
 226          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 227          * @return DBConnRef
 228          */
 229         public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 230
 231         /**
 232          * Get a database connection handle reference without connecting yet
 233          *
 234          * The handle's methods simply wrap those of a Database handle
 235          *
 236          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 237          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 238          * can be used to check such flags beforehand.
 239          *
 240          * @see ILoadBalancer::getConnection() for parameter information
 241          *
 242          * @param int $i Server index or DB_MASTER/DB_REPLICA
 243          * @param array|string|bool $groups Query group(s), or false for the generic reader
 244          * @param string|bool $domain Domain ID, or false for the current domain
 245          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 246          * @return DBConnRef
 247          */
 248         public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 249
 250         /**
 251          * Get a maintenance database connection handle reference for migrations and schema changes
 252          *
 253          * The handle's methods simply wrap those of a Database handle
 254          *
 255          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 256          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 257          * can be used to check such flags beforehand.
 258          *
 259          * @see ILoadBalancer::getConnection() for parameter information
 260          *
 261          * @param int $db Server index or DB_MASTER/DB_REPLICA
 262          * @param array|string|bool $groups Query group(s), or false for the generic reader
 263          * @param string|bool $domain Domain ID, or false for the current domain
 264          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 265          * @return MaintainableDBConnRef
 266          */
 267         public function getMaintenanceConnectionRef( $db, $groups = [], $domain = false, $flags = 0 );
 268
 269         /**
 270          * Open a connection to the server given by the specified index
 271          *
 272          * The index must be an actual index into the array. If a connection to the server is
 273          * already open and not considered an "in use" foreign connection, this simply returns it.
 274          *
 275          * Avoid using CONN_TRX_AUTOCOMMIT for databases with ATTR_DB_LEVEL_LOCKING (e.g. sqlite) in
 276          * order to avoid deadlocks. ILoadBalancer::getServerAttributes() can be used to check
 277          * such flags beforehand.
 278          *
 279          * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must also
 280          * call ILoadBalancer::reuseConnection() on the handle when finished using it.
 281          * In all other cases, this is not necessary, though not harmful either.
 282          *
 283          * @note This method throws DBAccessError if ILoadBalancer::disable() was called
 284          *
 285          * @param int $i Server index (does not support DB_MASTER/DB_REPLICA)
 286          * @param string|bool $domain Domain ID, or false for the current domain
 287          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 288          * @return Database|bool Returns false on errors
 289          * @throws DBAccessError
 290          */
 291         public function openConnection( $i, $domain = false, $flags = 0 );
 292
 293         /**
 294          * @return int
 295          */
 296         public function getWriterIndex();
 297
 298         /**
 299          * Returns true if the specified index is a valid server index
 300          *
 301          * @param int $i
 302          * @return bool
 303          */
 304         public function haveIndex( $i );
 305
 306         /**
 307          * Returns true if the specified index is valid and has non-zero load
 308          *
 309          * @param int $i
 310          * @return bool
 311          */
 312         public function isNonZeroLoad( $i );
 313
 314         /**
 315          * Get the number of defined servers (not the number of open connections)
 316          *
 317          * @return int
 318          */
 319         public function getServerCount();
 320
 321         /**
 322          * Get the host name or IP address of the server with the specified index
 323          *
 324          * @param int $i
 325          * @return string Readable name if available or IP/host otherwise
 326          */
 327         public function getServerName( $i );
 328
 329         /**
 330          * Get DB type of the server with the specified index
 331          *
 332          * @param int $i
 333          * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes
 334          * @since 1.30
 335          */
 336         public function getServerType( $i );
 337
 338         /**
 339          * Return the server info structure for a given index, or false if the index is invalid.
 340          * @param int $i
 341          * @return array|bool
 342          * @since 1.31
 343          */
 344         public function getServerInfo( $i );
 345
 346         /**
 347          * @param int $i Server index
 348          * @return array (Database::ATTRIBUTE_* constant => value) for all such constants
 349          * @since 1.31
 350          */
 351         public function getServerAttributes( $i );
 352
 353         /**
 354          * Get the current master position for chronology control purposes
 355          * @return DBMasterPos|bool Returns false if not applicable
 356          */
 357         public function getMasterPos();
 358
 359         /**
 360          * Disable this load balancer. All connections are closed, and any attempt to
 361          * open a new connection will result in a DBAccessError.
 362          */
 363         public function disable();
 364
 365         /**
 366          * Close all open connections
 367          */
 368         public function closeAll();
 369
 370         /**
 371          * Close a connection
 372          *
 373          * Using this function makes sure the LoadBalancer knows the connection is closed.
 374          * If you use $conn->close() directly, the load balancer won't update its state.
 375          *
 376          * @param IDatabase $conn
 377          */
 378         public function closeConnection( IDatabase $conn );
 379
 380         /**
 381          * Commit transactions on all open connections
 382          * @param string $fname Caller name
 383          * @throws DBExpectedError
 384          */
 385         public function commitAll( $fname = __METHOD__ );
 386
 387         /**
 388          * Perform all pre-commit callbacks that remain part of the atomic transactions
 389          * and disable any post-commit callbacks until runMasterPostTrxCallbacks()
 390          *
 391          * Use this only for mutli-database commits
 392          */
 393         public function finalizeMasterChanges();
 394
 395         /**
 396          * Perform all pre-commit checks for things like replication safety
 397          *
 398          * Use this only for mutli-database commits
 399          *
 400          * @param array $options Includes:
 401          *   - maxWriteDuration : max write query duration time in seconds
 402          * @throws DBTransactionError
 403          */
 404         public function approveMasterChanges( array $options );
 405
 406         /**
 407          * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
 408          *
 409          * The DBO_TRX setting will be reverted to the default in each of these methods:
 410          *   - commitMasterChanges()
 411          *   - rollbackMasterChanges()
 412          *   - commitAll()
 413          * This allows for custom transaction rounds from any outer transaction scope.
 414          *
 415          * @param string $fname
 416          * @throws DBExpectedError
 417          */
 418         public function beginMasterChanges( $fname = __METHOD__ );
 419
 420         /**
 421          * Issue COMMIT on all master connections where writes where done
 422          * @param string $fname Caller name
 423          * @throws DBExpectedError
 424          */
 425         public function commitMasterChanges( $fname = __METHOD__ );
 426
 427         /**
 428          * Issue all pending post-COMMIT/ROLLBACK callbacks
 429          *
 430          * Use this only for mutli-database commits
 431          *
 432          * @param int $type IDatabase::TRIGGER_* constant
 433          * @return Exception|null The first exception or null if there were none
 434          */
 435         public function runMasterPostTrxCallbacks( $type );
 436
 437         /**
 438          * Issue ROLLBACK only on master, only if queries were done on connection
 439          * @param string $fname Caller name
 440          * @throws DBExpectedError
 441          */
 442         public function rollbackMasterChanges( $fname = __METHOD__ );
 443
 444         /**
 445          * Suppress all pending post-COMMIT/ROLLBACK callbacks
 446          *
 447          * Use this only for mutli-database commits
 448          *
 449          * @return Exception|null The first exception or null if there were none
 450          */
 451         public function suppressTransactionEndCallbacks();
 452
 453         /**
 454          * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshot
 455          *
 456          * @param string $fname Caller name
 457          */
 458         public function flushReplicaSnapshots( $fname = __METHOD__ );
 459
 460         /**
 461          * @return bool Whether a master connection is already open
 462          */
 463         public function hasMasterConnection();
 464
 465         /**
 466          * Determine if there are pending changes in a transaction by this thread
 467          * @return bool
 468          */
 469         public function hasMasterChanges();
 470
 471         /**
 472          * Get the timestamp of the latest write query done by this thread
 473          * @return float|bool UNIX timestamp or false
 474          */
 475         public function lastMasterChangeTimestamp();
 476
 477         /**
 478          * Check if this load balancer object had any recent or still
 479          * pending writes issued against it by this PHP thread
 480          *
 481          * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
 482          * @return bool
 483          */
 484         public function hasOrMadeRecentMasterChanges( $age = null );
 485
 486         /**
 487          * Get the list of callers that have pending master changes
 488          *
 489          * @return string[] List of method names
 490          */
 491         public function pendingMasterChangeCallers();
 492
 493         /**
 494          * @note This method will trigger a DB connection if not yet done
 495          * @param string|bool $domain Domain ID, or false for the current domain
 496          * @return bool Whether the database for generic connections this request is highly "lagged"
 497          */
 498         public function getLaggedReplicaMode( $domain = false );
 499
 500         /**
 501          * Checks whether the database for generic connections this request was both:
 502          *   - a) Already choosen due to a prior connection attempt
 503          *   - b) Considered highly "lagged"
 504          *
 505          * @note This method will never cause a new DB connection
 506          * @return bool
 507          */
 508         public function laggedReplicaUsed();
 509
 510         /**
 511          * @note This method may trigger a DB connection if not yet done
 512          * @param string|bool $domain Domain ID, or false for the current domain
 513          * @param IDatabase|null $conn DB master connection; used to avoid loops [optional]
 514          * @return string|bool Reason the master is read-only or false if it is not
 515          */
 516         public function getReadOnlyReason( $domain = false, IDatabase $conn = null );
 517
 518         /**
 519          * Disables/enables lag checks
 520          * @param null|bool $mode
 521          * @return bool
 522          */
 523         public function allowLagged( $mode = null );
 524
 525         /**
 526          * @return bool
 527          */
 528         public function pingAll();
 529
 530         /**
 531          * Call a function with each open connection object
 532          * @param callable $callback
 533          * @param array $params
 534          */
 535         public function forEachOpenConnection( $callback, array $params = [] );
 536
 537         /**
 538          * Call a function with each open connection object to a master
 539          * @param callable $callback
 540          * @param array $params
 541          */
 542         public function forEachOpenMasterConnection( $callback, array $params = [] );
 543
 544         /**
 545          * Call a function with each open replica DB connection object
 546          * @param callable $callback
 547          * @param array $params
 548          */
 549         public function forEachOpenReplicaConnection( $callback, array $params = [] );
 550
 551         /**
 552          * Get the hostname and lag time of the most-lagged replica DB
 553          *
 554          * This is useful for maintenance scripts that need to throttle their updates.
 555          * May attempt to open connections to replica DBs on the default DB. If there is
 556          * no lag, the maximum lag will be reported as -1.
 557          *
 558          * @param bool|string $domain Domain ID, or false for the default database
 559          * @return array ( host, max lag, index of max lagged host )
 560          */
 561         public function getMaxLag( $domain = false );
 562
 563         /**
 564          * Get an estimate of replication lag (in seconds) for each server
 565          *
 566          * Results are cached for a short time in memcached/process cache
 567          *
 568          * Values may be "false" if replication is too broken to estimate
 569          *
 570          * @param string|bool $domain
 571          * @return int[] Map of (server index => float|int|bool)
 572          */
 573         public function getLagTimes( $domain = false );
 574
 575         /**
 576          * Get the lag in seconds for a given connection, or zero if this load
 577          * balancer does not have replication enabled.
 578          *
 579          * This should be used in preference to Database::getLag() in cases where
 580          * replication may not be in use, since there is no way to determine if
 581          * replication is in use at the connection level without running
 582          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
 583          * function instead of Database::getLag() avoids a fatal error in this
 584          * case on many installations.
 585          *
 586          * @param IDatabase $conn
 587          * @return int|bool Returns false on error
 588          */
 589         public function safeGetLag( IDatabase $conn );
 590
 591         /**
 592          * Wait for a replica DB to reach a specified master position
 593          *
 594          * This will connect to the master to get an accurate position if $pos is not given
 595          *
 596          * @param IDatabase $conn Replica DB
 597          * @param DBMasterPos|bool $pos Master position; default: current position
 598          * @param int $timeout Timeout in seconds [optional]
 599          * @return bool Success
 600          */
 601         public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = 10 );
 602
 603         /**
 604          * Set a callback via IDatabase::setTransactionListener() on
 605          * all current and future master connections of this load balancer
 606          *
 607          * @param string $name Callback name
 608          * @param callable|null $callback
 609          */
 610         public function setTransactionListener( $name, callable $callback = null );
 611
 612         /**
 613          * Set a new table prefix for the existing local domain ID for testing
 614          *
 615          * @param string $prefix
 616          */
 617         public function setDomainPrefix( $prefix );
 618
 619         /**
 620          * Make certain table names use their own database, schema, and table prefix
 621          * when passed into SQL queries pre-escaped and without a qualified database name
 622          *
 623          * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
 624          * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
 625          *
 626          * Calling this twice will completely clear any old table aliases. Also, note that
 627          * callers are responsible for making sure the schemas and databases actually exist.
 628          *
 629          * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
 630          */
 631         public function setTableAliases( array $aliases );
 632
 633         /**
 634          * Convert certain index names to alternative names before querying the DB
 635          *
 636          * Note that this applies to indexes regardless of the table they belong to.
 637          *
 638          * This can be employed when an index was renamed X => Y in code, but the new Y-named
 639          * indexes were not yet built on all DBs. After all the Y-named ones are added by the DBA,
 640          * the aliases can be removed, and then the old X-named indexes dropped.
 641          *
 642          * @param string[] $aliases
 643          * @return mixed
 644          * @since 1.31
 645          */
 646         public function setIndexAliases( array $aliases );
 647 }