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
  90         /** @var string Manager of ILoadBalancer instances is running post-commit callbacks */
  91         const STAGE_POSTCOMMIT_CALLBACKS = 'stage-postcommit-callbacks';
  92         /** @var string Manager of ILoadBalancer instances is running post-rollback callbacks */
  93         const STAGE_POSTROLLBACK_CALLBACKS = 'stage-postrollback-callbacks';
  94
  95         /**
  96          * Construct a manager of IDatabase connection objects
  97          *
  98          * @param array $params Parameter map with keys:
  99          *  - servers : Required. Array of server info structures.
 100          *  - localDomain: A DatabaseDomain or domain ID string.
 101          *  - loadMonitor : Name of a class used to fetch server lag and load.
 102          *  - readOnlyReason : Reason the master DB is read-only if so [optional]
 103          *  - waitTimeout : Maximum time to wait for replicas for consistency [optional]
 104          *  - maxLag: Avoid replica DB servers with more lag than this [optional]
 105          *  - srvCache : BagOStuff object for server cache [optional]
 106          *  - wanCache : WANObjectCache object [optional]
 107          *  - chronologyCallback: Callback to run before the first connection attempt [optional]
 108          *  - hostname : The name of the current server [optional]
 109          *  - cliMode: Whether the execution context is a CLI script. [optional]
 110          *  - profiler : Class name or instance with profileIn()/profileOut() methods. [optional]
 111          *  - trxProfiler: TransactionProfiler instance. [optional]
 112          *  - replLogger: PSR-3 logger instance. [optional]
 113          *  - connLogger: PSR-3 logger instance. [optional]
 114          *  - queryLogger: PSR-3 logger instance. [optional]
 115          *  - perfLogger: PSR-3 logger instance. [optional]
 116          *  - errorLogger : Callback that takes an Exception and logs it. [optional]
 117          *  - deprecationLogger: Callback to log a deprecation warning. [optional]
 118          *  - roundStage: STAGE_POSTCOMMIT_* class constant; for internal use [optional]
 119          * @throws InvalidArgumentException
 120          */
 121         public function __construct( array $params );
 122
 123         /**
 124          * Get the local (and default) database domain ID of connection handles
 125          *
 126          * @see DatabaseDomain
 127          * @return string Database domain ID; this specifies DB name, schema, and table prefix
 128          * @since 1.31
 129          */
 130         public function getLocalDomainID();
 131
 132         /**
 133          * @param DatabaseDomain|string|bool $domain Database domain
 134          * @return string Value of $domain if provided or the local domain otherwise
 135          * @since 1.32
 136          */
 137         public function resolveDomainID( $domain );
 138
 139         /**
 140          * Get the index of the reader connection, which may be a replica DB
 141          *
 142          * This takes into account load ratios and lag times. It should
 143          * always return a consistent index during a given invocation.
 144          *
 145          * Side effect: opens connections to databases
 146          * @param string|bool $group Query group, or false for the generic reader
 147          * @param string|bool $domain Domain ID, or false for the current domain
 148          * @throws DBError
 149          * @return bool|int|string
 150          */
 151         public function getReaderIndex( $group = false, $domain = false );
 152
 153         /**
 154          * Set the master wait position
 155          *
 156          * If a DB_REPLICA connection has been opened already, then wait immediately.
 157          * Otherwise sets a variable telling it to wait if such a connection is opened.
 158          *
 159          * This only applies to connections to the generic replica DB for this request.
 160          * If a timeout happens when waiting, then getLaggedReplicaMode()/laggedReplicaUsed()
 161          * will return true.
 162          *
 163          * @param DBMasterPos|bool $pos Master position or false
 164          */
 165         public function waitFor( $pos );
 166
 167         /**
 168          * Set the master wait position and wait for a "generic" replica DB to catch up to it
 169          *
 170          * This can be used a faster proxy for waitForAll()
 171          *
 172          * @param DBMasterPos|bool $pos Master position or false
 173          * @param int|null $timeout Max seconds to wait; default is mWaitTimeout
 174          * @return bool Success (able to connect and no timeouts reached)
 175          */
 176         public function waitForOne( $pos, $timeout = null );
 177
 178         /**
 179          * Set the master wait position and wait for ALL replica DBs to catch up to it
 180          *
 181          * @param DBMasterPos|bool $pos Master position or false
 182          * @param int|null $timeout Max seconds to wait; default is mWaitTimeout
 183          * @return bool Success (able to connect and no timeouts reached)
 184          */
 185         public function waitForAll( $pos, $timeout = null );
 186
 187         /**
 188          * Get any open connection to a given server index, local or foreign
 189          *
 190          * Use CONN_TRX_AUTOCOMMIT to only look for connections opened with that flag
 191          *
 192          * @param int $i Server index or DB_MASTER/DB_REPLICA
 193          * @param int $flags Bitfield of CONN_* class constants
 194          * @return Database|bool False if no such connection is open
 195          */
 196         public function getAnyOpenConnection( $i, $flags = 0 );
 197
 198         /**
 199          * Get a connection handle by server index
 200          *
 201          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 202          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 203          * can be used to check such flags beforehand.
 204          *
 205          * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must also
 206          * call ILoadBalancer::reuseConnection() on the handle when finished using it.
 207          * In all other cases, this is not necessary, though not harmful either.
 208          *
 209          * @param int $i Server index (overrides $groups) or DB_MASTER/DB_REPLICA
 210          * @param array|string|bool $groups Query group(s), or false for the generic reader
 211          * @param string|bool $domain Domain ID, or false for the current domain
 212          * @param int $flags Bitfield of CONN_* class constants
 213          *
 214          * @note This method throws DBAccessError if ILoadBalancer::disable() was called
 215          *
 216          * @throws DBError
 217          * @return Database
 218          */
 219         public function getConnection( $i, $groups = [], $domain = false, $flags = 0 );
 220
 221         /**
 222          * Mark a foreign connection as being available for reuse under a different DB domain
 223          *
 224          * This mechanism is reference-counted, and must be called the same number of times
 225          * as getConnection() to work.
 226          *
 227          * @param IDatabase $conn
 228          * @throws InvalidArgumentException
 229          */
 230         public function reuseConnection( IDatabase $conn );
 231
 232         /**
 233          * Get a database connection handle reference
 234          *
 235          * The handle's methods simply wrap those of a Database handle
 236          *
 237          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 238          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 239          * can be used to check such flags beforehand.
 240          *
 241          * @see ILoadBalancer::getConnection() for parameter information
 242          *
 243          * @param int $i Server index or DB_MASTER/DB_REPLICA
 244          * @param array|string|bool $groups Query group(s), or false for the generic reader
 245          * @param string|bool $domain Domain ID, or false for the current domain
 246          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 247          * @return DBConnRef
 248          */
 249         public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 250
 251         /**
 252          * Get a database connection handle reference without connecting yet
 253          *
 254          * The handle's methods simply wrap those of a Database handle
 255          *
 256          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 257          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 258          * can be used to check such flags beforehand.
 259          *
 260          * @see ILoadBalancer::getConnection() for parameter information
 261          *
 262          * @param int $i Server index or DB_MASTER/DB_REPLICA
 263          * @param array|string|bool $groups Query group(s), or false for the generic reader
 264          * @param string|bool $domain Domain ID, or false for the current domain
 265          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 266          * @return DBConnRef
 267          */
 268         public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 269
 270         /**
 271          * Get a maintenance database connection handle reference for migrations and schema changes
 272          *
 273          * The handle's methods simply wrap those of a Database handle
 274          *
 275          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 276          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 277          * can be used to check such flags beforehand.
 278          *
 279          * @see ILoadBalancer::getConnection() for parameter information
 280          *
 281          * @param int $db Server index or DB_MASTER/DB_REPLICA
 282          * @param array|string|bool $groups Query group(s), or false for the generic reader
 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 MaintainableDBConnRef
 286          */
 287         public function getMaintenanceConnectionRef( $db, $groups = [], $domain = false, $flags = 0 );
 288
 289         /**
 290          * Open a connection to the server given by the specified index
 291          *
 292          * The index must be an actual index into the array. If a connection to the server is
 293          * already open and not considered an "in use" foreign connection, this simply returns it.
 294          *
 295          * Avoid using CONN_TRX_AUTOCOMMIT for databases with ATTR_DB_LEVEL_LOCKING (e.g. sqlite) in
 296          * order to avoid deadlocks. ILoadBalancer::getServerAttributes() can be used to check
 297          * such flags beforehand.
 298          *
 299          * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must also
 300          * call ILoadBalancer::reuseConnection() on the handle when finished using it.
 301          * In all other cases, this is not necessary, though not harmful either.
 302          *
 303          * @note This method throws DBAccessError if ILoadBalancer::disable() was called
 304          *
 305          * @param int $i Server index (does not support DB_MASTER/DB_REPLICA)
 306          * @param string|bool $domain Domain ID, or false for the current domain
 307          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 308          * @return Database|bool Returns false on errors
 309          * @throws DBAccessError
 310          */
 311         public function openConnection( $i, $domain = false, $flags = 0 );
 312
 313         /**
 314          * @return int
 315          */
 316         public function getWriterIndex();
 317
 318         /**
 319          * Returns true if the specified index is a valid server index
 320          *
 321          * @param int $i
 322          * @return bool
 323          */
 324         public function haveIndex( $i );
 325
 326         /**
 327          * Returns true if the specified index is valid and has non-zero load
 328          *
 329          * @param int $i
 330          * @return bool
 331          */
 332         public function isNonZeroLoad( $i );
 333
 334         /**
 335          * Get the number of defined servers (not the number of open connections)
 336          *
 337          * @return int
 338          */
 339         public function getServerCount();
 340
 341         /**
 342          * Get the host name or IP address of the server with the specified index
 343          *
 344          * @param int $i
 345          * @return string Readable name if available or IP/host otherwise
 346          */
 347         public function getServerName( $i );
 348
 349         /**
 350          * Return the server info structure for a given index, or false if the index is invalid.
 351          * @param int $i
 352          * @return array|bool
 353          */
 354         public function getServerInfo( $i );
 355
 356         /**
 357          * Get DB type of the server with the specified index
 358          *
 359          * @param int $i
 360          * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes
 361          * @since 1.30
 362          */
 363         public function getServerType( $i );
 364
 365         /**
 366          * @param int $i Server index
 367          * @return array (Database::ATTRIBUTE_* constant => value) for all such constants
 368          * @since 1.31
 369          */
 370         public function getServerAttributes( $i );
 371
 372         /**
 373          * Get the current master position for chronology control purposes
 374          * @return DBMasterPos|bool Returns false if not applicable
 375          */
 376         public function getMasterPos();
 377
 378         /**
 379          * Disable this load balancer. All connections are closed, and any attempt to
 380          * open a new connection will result in a DBAccessError.
 381          */
 382         public function disable();
 383
 384         /**
 385          * Close all open connections
 386          */
 387         public function closeAll();
 388
 389         /**
 390          * Close a connection
 391          *
 392          * Using this function makes sure the LoadBalancer knows the connection is closed.
 393          * If you use $conn->close() directly, the load balancer won't update its state.
 394          *
 395          * @param IDatabase $conn
 396          */
 397         public function closeConnection( IDatabase $conn );
 398
 399         /**
 400          * Commit transactions on all open connections
 401          * @param string $fname Caller name
 402          * @throws DBExpectedError
 403          */
 404         public function commitAll( $fname = __METHOD__ );
 405
 406         /**
 407          * Run pre-commit callbacks and defer execution of post-commit callbacks
 408          *
 409          * Use this only for mutli-database commits
 410          *
 411          * @return int Number of pre-commit callbacks run (since 1.32)
 412          */
 413         public function finalizeMasterChanges();
 414
 415         /**
 416          * Perform all pre-commit checks for things like replication safety
 417          *
 418          * Use this only for mutli-database commits
 419          *
 420          * @param array $options Includes:
 421          *   - maxWriteDuration : max write query duration time in seconds
 422          * @throws DBTransactionError
 423          */
 424         public function approveMasterChanges( array $options );
 425
 426         /**
 427          * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
 428          *
 429          * The DBO_TRX setting will be reverted to the default in each of these methods:
 430          *   - commitMasterChanges()
 431          *   - rollbackMasterChanges()
 432          *   - commitAll()
 433          * This allows for custom transaction rounds from any outer transaction scope.
 434          *
 435          * @param string $fname
 436          * @throws DBExpectedError
 437          */
 438         public function beginMasterChanges( $fname = __METHOD__ );
 439
 440         /**
 441          * Issue COMMIT on all master connections where writes where done
 442          * @param string $fname Caller name
 443          * @throws DBExpectedError
 444          */
 445         public function commitMasterChanges( $fname = __METHOD__ );
 446
 447         /**
 448          * Consume and run all pending post-COMMIT/ROLLBACK callbacks and commit dangling transactions
 449          *
 450          * @return Exception|null The first exception or null if there were none
 451          */
 452         public function runMasterTransactionIdleCallbacks();
 453
 454         /**
 455          * Run all recurring post-COMMIT/ROLLBACK listener callbacks
 456          *
 457          * @return Exception|null The first exception or null if there were none
 458          */
 459         public function runMasterTransactionListenerCallbacks();
 460
 461         /**
 462          * Issue ROLLBACK only on master, only if queries were done on connection
 463          * @param string $fname Caller name
 464          * @throws DBExpectedError
 465          */
 466         public function rollbackMasterChanges( $fname = __METHOD__ );
 467
 468         /**
 469          * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
 470          *
 471          * @param string $fname Caller name
 472          */
 473         public function flushReplicaSnapshots( $fname = __METHOD__ );
 474
 475         /**
 476          * Commit all master DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
 477          *
 478          * An error will be thrown if a connection has pending writes or callbacks
 479          *
 480          * @param string $fname Caller name
 481          */
 482         public function flushMasterSnapshots( $fname = __METHOD__ );
 483
 484         /**
 485          * @return bool Whether a master connection is already open
 486          */
 487         public function hasMasterConnection();
 488
 489         /**
 490          * Whether there are pending changes or callbacks in a transaction by this thread
 491          * @return bool
 492          */
 493         public function hasMasterChanges();
 494
 495         /**
 496          * Get the timestamp of the latest write query done by this thread
 497          * @return float|bool UNIX timestamp or false
 498          */
 499         public function lastMasterChangeTimestamp();
 500
 501         /**
 502          * Check if this load balancer object had any recent or still
 503          * pending writes issued against it by this PHP thread
 504          *
 505          * @param float|null $age How many seconds ago is "recent" [defaults to mWaitTimeout]
 506          * @return bool
 507          */
 508         public function hasOrMadeRecentMasterChanges( $age = null );
 509
 510         /**
 511          * Get the list of callers that have pending master changes
 512          *
 513          * @return string[] List of method names
 514          */
 515         public function pendingMasterChangeCallers();
 516
 517         /**
 518          * @note This method will trigger a DB connection if not yet done
 519          * @param string|bool $domain Domain ID, or false for the current domain
 520          * @return bool Whether the database for generic connections this request is highly "lagged"
 521          */
 522         public function getLaggedReplicaMode( $domain = false );
 523
 524         /**
 525          * Checks whether the database for generic connections this request was both:
 526          *   - a) Already choosen due to a prior connection attempt
 527          *   - b) Considered highly "lagged"
 528          *
 529          * @note This method will never cause a new DB connection
 530          * @return bool
 531          */
 532         public function laggedReplicaUsed();
 533
 534         /**
 535          * @note This method may trigger a DB connection if not yet done
 536          * @param string|bool $domain Domain ID, or false for the current domain
 537          * @param IDatabase|null $conn DB master connection; used to avoid loops [optional]
 538          * @return string|bool Reason the master is read-only or false if it is not
 539          */
 540         public function getReadOnlyReason( $domain = false, IDatabase $conn = null );
 541
 542         /**
 543          * Disables/enables lag checks
 544          * @param null|bool $mode
 545          * @return bool
 546          */
 547         public function allowLagged( $mode = null );
 548
 549         /**
 550          * @return bool
 551          */
 552         public function pingAll();
 553
 554         /**
 555          * Call a function with each open connection object
 556          * @param callable $callback
 557          * @param array $params
 558          */
 559         public function forEachOpenConnection( $callback, array $params = [] );
 560
 561         /**
 562          * Call a function with each open connection object to a master
 563          * @param callable $callback
 564          * @param array $params
 565          */
 566         public function forEachOpenMasterConnection( $callback, array $params = [] );
 567
 568         /**
 569          * Call a function with each open replica DB connection object
 570          * @param callable $callback
 571          * @param array $params
 572          */
 573         public function forEachOpenReplicaConnection( $callback, array $params = [] );
 574
 575         /**
 576          * Get the hostname and lag time of the most-lagged replica DB
 577          *
 578          * This is useful for maintenance scripts that need to throttle their updates.
 579          * May attempt to open connections to replica DBs on the default DB. If there is
 580          * no lag, the maximum lag will be reported as -1.
 581          *
 582          * @param bool|string $domain Domain ID, or false for the default database
 583          * @return array ( host, max lag, index of max lagged host )
 584          */
 585         public function getMaxLag( $domain = false );
 586
 587         /**
 588          * Get an estimate of replication lag (in seconds) for each server
 589          *
 590          * Results are cached for a short time in memcached/process cache
 591          *
 592          * Values may be "false" if replication is too broken to estimate
 593          *
 594          * @param string|bool $domain
 595          * @return int[] Map of (server index => float|int|bool)
 596          */
 597         public function getLagTimes( $domain = false );
 598
 599         /**
 600          * Get the lag in seconds for a given connection, or zero if this load
 601          * balancer does not have replication enabled.
 602          *
 603          * This should be used in preference to Database::getLag() in cases where
 604          * replication may not be in use, since there is no way to determine if
 605          * replication is in use at the connection level without running
 606          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
 607          * function instead of Database::getLag() avoids a fatal error in this
 608          * case on many installations.
 609          *
 610          * @param IDatabase $conn
 611          * @return int|bool Returns false on error
 612          */
 613         public function safeGetLag( IDatabase $conn );
 614
 615         /**
 616          * Wait for a replica DB to reach a specified master position
 617          *
 618          * This will connect to the master to get an accurate position if $pos is not given
 619          *
 620          * @param IDatabase $conn Replica DB
 621          * @param DBMasterPos|bool $pos Master position; default: current position
 622          * @param int $timeout Timeout in seconds [optional]
 623          * @return bool Success
 624          */
 625         public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = 10 );
 626
 627         /**
 628          * Set a callback via IDatabase::setTransactionListener() on
 629          * all current and future master connections of this load balancer
 630          *
 631          * @param string $name Callback name
 632          * @param callable|null $callback
 633          */
 634         public function setTransactionListener( $name, callable $callback = null );
 635
 636         /**
 637          * Set a new table prefix for the existing local domain ID for testing
 638          *
 639          * @param string $prefix
 640          */
 641         public function setDomainPrefix( $prefix );
 642
 643         /**
 644          * Make certain table names use their own database, schema, and table prefix
 645          * when passed into SQL queries pre-escaped and without a qualified database name
 646          *
 647          * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
 648          * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
 649          *
 650          * Calling this twice will completely clear any old table aliases. Also, note that
 651          * callers are responsible for making sure the schemas and databases actually exist.
 652          *
 653          * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
 654          */
 655         public function setTableAliases( array $aliases );
 656
 657         /**
 658          * Convert certain index names to alternative names before querying the DB
 659          *
 660          * Note that this applies to indexes regardless of the table they belong to.
 661          *
 662          * This can be employed when an index was renamed X => Y in code, but the new Y-named
 663          * indexes were not yet built on all DBs. After all the Y-named ones are added by the DBA,
 664          * the aliases can be removed, and then the old X-named indexes dropped.
 665          *
 666          * @param string[] $aliases
 667          * @return mixed
 668          * @since 1.31
 669          */
 670         public function setIndexAliases( array $aliases );
 671 }