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 Return null on connection failure instead of throwing an exception */
  90         const CONN_SILENCE_ERRORS = 2;
  91
  92         /** @var string Manager of ILoadBalancer instances is running post-commit callbacks */
  93         const STAGE_POSTCOMMIT_CALLBACKS = 'stage-postcommit-callbacks';
  94         /** @var string Manager of ILoadBalancer instances is running post-rollback callbacks */
  95         const STAGE_POSTROLLBACK_CALLBACKS = 'stage-postrollback-callbacks';
  96
  97         /**
  98          * Construct a manager of IDatabase connection objects
  99          *
 100          * @param array $params Parameter map with keys:
 101          *  - servers : Required. Array of server info structures.
 102          *  - localDomain: A DatabaseDomain or domain ID string.
 103          *  - loadMonitor : Name of a class used to fetch server lag and load.
 104          *  - readOnlyReason : Reason the master DB is read-only if so [optional]
 105          *  - waitTimeout : Maximum time to wait for replicas for consistency [optional]
 106          *  - maxLag: Try to avoid DB replicas with lag above this many seconds [optional]
 107          *  - srvCache : BagOStuff object for server cache [optional]
 108          *  - wanCache : WANObjectCache object [optional]
 109          *  - chronologyCallback: Callback to run before the first connection attempt [optional]
 110          *  - hostname : The name of the current server [optional]
 111          *  - cliMode: Whether the execution context is a CLI script. [optional]
 112          *  - profiler : Class name or instance with profileIn()/profileOut() methods. [optional]
 113          *  - trxProfiler: TransactionProfiler instance. [optional]
 114          *  - replLogger: PSR-3 logger instance. [optional]
 115          *  - connLogger: PSR-3 logger instance. [optional]
 116          *  - queryLogger: PSR-3 logger instance. [optional]
 117          *  - perfLogger: PSR-3 logger instance. [optional]
 118          *  - errorLogger : Callback that takes an Exception and logs it. [optional]
 119          *  - deprecationLogger: Callback to log a deprecation warning. [optional]
 120          *  - roundStage: STAGE_POSTCOMMIT_* class constant; for internal use [optional]
 121          *  - ownerId: integer ID of an LBFactory instance that manages this instance [optional]
 122          * @throws InvalidArgumentException
 123          */
 124         public function __construct( array $params );
 125
 126         /**
 127          * Get the local (and default) database domain ID of connection handles
 128          *
 129          * @see DatabaseDomain
 130          * @return string Database domain ID; this specifies DB name, schema, and table prefix
 131          * @since 1.31
 132          */
 133         public function getLocalDomainID();
 134
 135         /**
 136          * @param DatabaseDomain|string|bool $domain Database domain
 137          * @return string Value of $domain if it is foreign or the local domain otherwise
 138          * @since 1.32
 139          */
 140         public function resolveDomainID( $domain );
 141
 142         /**
 143          * Close all connection and redefine the local domain for testing or schema creation
 144          *
 145          * @param DatabaseDomain|string $domain
 146          * @since 1.33
 147          */
 148         public function redefineLocalDomain( $domain );
 149
 150         /**
 151          * Get the index of the reader connection, which may be a replica DB
 152          *
 153          * This takes into account load ratios and lag times. It should return a consistent
 154          * index during the life time of the load balancer. This initially checks replica DBs
 155          * for connectivity to avoid returning an unusable server. This means that connections
 156          * might be attempted by calling this method (usally one at the most but possibly more).
 157          * Subsequent calls with the same $group will not need to make new connection attempts
 158          * since the acquired connection for each group is preserved.
 159          *
 160          * @param string|bool $group Query group, or false for the generic group
 161          * @param string|bool $domain Domain ID, or false for the current domain
 162          * @throws DBError
 163          * @return bool|int|string
 164          */
 165         public function getReaderIndex( $group = false, $domain = false );
 166
 167         /**
 168          * Set the master position to reach before the next generic group DB handle query
 169          *
 170          * If a generic replica DB connection is already open then this immediately waits
 171          * for that DB to catch up to the specified replication position. Otherwise, it will
 172          * do so once such a connection is opened.
 173          *
 174          * If a timeout happens when waiting, then getLaggedReplicaMode()/laggedReplicaUsed()
 175          * will return true.
 176          *
 177          * @param DBMasterPos|bool $pos Master position or false
 178          */
 179         public function waitFor( $pos );
 180
 181         /**
 182          * Set the master wait position and wait for a generic replica DB to catch up to it
 183          *
 184          * This can be used a faster proxy for waitForAll()
 185          *
 186          * @param DBMasterPos|bool $pos Master position or false
 187          * @param int|null $timeout Max seconds to wait; default is mWaitTimeout
 188          * @return bool Success (able to connect and no timeouts reached)
 189          */
 190         public function waitForOne( $pos, $timeout = null );
 191
 192         /**
 193          * Set the master wait position and wait for ALL replica DBs to catch up to it
 194          *
 195          * @param DBMasterPos|bool $pos Master position or false
 196          * @param int|null $timeout Max seconds to wait; default is mWaitTimeout
 197          * @return bool Success (able to connect and no timeouts reached)
 198          */
 199         public function waitForAll( $pos, $timeout = null );
 200
 201         /**
 202          * Get any open connection to a given server index, local or foreign
 203          *
 204          * Use CONN_TRX_AUTOCOMMIT to only look for connections opened with that flag.
 205          * Avoid the use of begin() or startAtomic() on any such connections.
 206          *
 207          * @param int $i Server index or DB_MASTER/DB_REPLICA
 208          * @param int $flags Bitfield of CONN_* class constants
 209          * @return Database|bool False if no such connection is open
 210          */
 211         public function getAnyOpenConnection( $i, $flags = 0 );
 212
 213         /**
 214          * Get a connection handle by server index
 215          *
 216          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 217          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 218          * can be used to check such flags beforehand.
 219          *
 220          * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must
 221          * also call ILoadBalancer::reuseConnection() on the handle when finished using it.
 222          * In all other cases, this is not necessary, though not harmful either.
 223          * Avoid the use of begin() or startAtomic() on any such connections.
 224          *
 225          * @param int $i Server index (overrides $groups) or DB_MASTER/DB_REPLICA
 226          * @param array|string|bool $groups Query group(s), or false for the generic reader
 227          * @param string|bool $domain Domain ID, or false for the current domain
 228          * @param int $flags Bitfield of CONN_* class constants
 229          *
 230          * @note This method throws DBAccessError if ILoadBalancer::disable() was called
 231          *
 232          * @throws DBError If any error occurs that prevents the yielding of a (live) IDatabase
 233          * @return IDatabase|bool This returns false on failure if CONN_SILENCE_ERRORS is set
 234          */
 235         public function getConnection( $i, $groups = [], $domain = false, $flags = 0 );
 236
 237         /**
 238          * Mark a foreign connection as being available for reuse under a different DB domain
 239          *
 240          * This mechanism is reference-counted, and must be called the same number of times
 241          * as getConnection() to work.
 242          *
 243          * @param IDatabase $conn
 244          * @throws InvalidArgumentException
 245          */
 246         public function reuseConnection( IDatabase $conn );
 247
 248         /**
 249          * Get a database connection handle reference
 250          *
 251          * The handle's methods simply wrap those of a Database handle
 252          *
 253          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 254          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 255          * can be used to check such flags beforehand. Avoid the use of begin() or startAtomic()
 256          * on any CONN_TRX_AUTOCOMMIT connections.
 257          *
 258          * @see ILoadBalancer::getConnection() for parameter information
 259          *
 260          * @param int $i Server index or DB_MASTER/DB_REPLICA
 261          * @param array|string|bool $groups Query group(s), or false for the generic reader
 262          * @param string|bool $domain Domain ID, or false for the current domain
 263          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 264          * @return DBConnRef
 265          */
 266         public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 267
 268         /**
 269          * Get a database connection handle reference without connecting yet
 270          *
 271          * The handle's methods simply wrap those of a Database handle
 272          *
 273          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 274          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 275          * can be used to check such flags beforehand. Avoid the use of begin() or startAtomic()
 276          * on any CONN_TRX_AUTOCOMMIT connections.
 277          *
 278          * @see ILoadBalancer::getConnection() for parameter information
 279          *
 280          * @param int $i Server index or DB_MASTER/DB_REPLICA
 281          * @param array|string|bool $groups Query group(s), or false for the generic reader
 282          * @param string|bool $domain Domain ID, or false for the current domain
 283          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 284          * @return DBConnRef
 285          */
 286         public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 287
 288         /**
 289          * Get a maintenance database connection handle reference for migrations and schema changes
 290          *
 291          * The handle's methods simply wrap those of a Database handle
 292          *
 293          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 294          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 295          * can be used to check such flags beforehand. Avoid the use of begin() or startAtomic()
 296          * on any CONN_TRX_AUTOCOMMIT connections.
 297          *
 298          * @see ILoadBalancer::getConnection() for parameter information
 299          *
 300          * @param int $i Server index or DB_MASTER/DB_REPLICA
 301          * @param array|string|bool $groups Query group(s), or false for the generic reader
 302          * @param string|bool $domain Domain ID, or false for the current domain
 303          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 304          * @return MaintainableDBConnRef
 305          */
 306         public function getMaintenanceConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 307
 308         /**
 309          * @return int
 310          */
 311         public function getWriterIndex();
 312
 313         /**
 314          * Returns true if the specified index is a valid server index
 315          *
 316          * @param int $i
 317          * @return bool
 318          */
 319         public function haveIndex( $i );
 320
 321         /**
 322          * Returns true if the specified index is valid and has non-zero load
 323          *
 324          * @param int $i
 325          * @return bool
 326          */
 327         public function isNonZeroLoad( $i );
 328
 329         /**
 330          * Get the number of defined servers (not the number of open connections)
 331          *
 332          * @return int
 333          */
 334         public function getServerCount();
 335
 336         /**
 337          * Get the host name or IP address of the server with the specified index
 338          *
 339          * @param int $i
 340          * @return string Readable name if available or IP/host otherwise
 341          */
 342         public function getServerName( $i );
 343
 344         /**
 345          * Return the server info structure for a given index, or false if the index is invalid.
 346          * @param int $i
 347          * @return array|bool
 348          * @since 1.31
 349          */
 350         public function getServerInfo( $i );
 351
 352         /**
 353          * Get DB type of the server with the specified index
 354          *
 355          * @param int $i
 356          * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes
 357          * @since 1.30
 358          */
 359         public function getServerType( $i );
 360
 361         /**
 362          * @param int $i Server index
 363          * @return array (Database::ATTRIBUTE_* constant => value) for all such constants
 364          * @since 1.31
 365          */
 366         public function getServerAttributes( $i );
 367
 368         /**
 369          * Get the current master position for chronology control purposes
 370          * @return DBMasterPos|bool Returns false if not applicable
 371          */
 372         public function getMasterPos();
 373
 374         /**
 375          * Disable this load balancer. All connections are closed, and any attempt to
 376          * open a new connection will result in a DBAccessError.
 377          */
 378         public function disable();
 379
 380         /**
 381          * Close all open connections
 382          */
 383         public function closeAll();
 384
 385         /**
 386          * Close a connection
 387          *
 388          * Using this function makes sure the LoadBalancer knows the connection is closed.
 389          * If you use $conn->close() directly, the load balancer won't update its state.
 390          *
 391          * @param IDatabase $conn
 392          */
 393         public function closeConnection( IDatabase $conn );
 394
 395         /**
 396          * Commit transactions on all open connections
 397          * @param string $fname Caller name
 398          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 399          * @throws DBExpectedError
 400          */
 401         public function commitAll( $fname = __METHOD__, $owner = null );
 402
 403         /**
 404          * Run pre-commit callbacks and defer execution of post-commit callbacks
 405          *
 406          * Use this only for mutli-database commits
 407          *
 408          * @param string $fname Caller name
 409          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 410          * @return int Number of pre-commit callbacks run (since 1.32)
 411          */
 412         public function finalizeMasterChanges( $fname = __METHOD__, $owner = null );
 413
 414         /**
 415          * Perform all pre-commit checks for things like replication safety
 416          *
 417          * Use this only for mutli-database commits
 418          *
 419          * @param array $options Includes:
 420          *   - maxWriteDuration : max write query duration time in seconds
 421          * @param string $fname Caller name
 422          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 423          * @throws DBTransactionError
 424          */
 425         public function approveMasterChanges( array $options, $fname, $owner = null );
 426
 427         /**
 428          * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
 429          *
 430          * The DBO_TRX setting will be reverted to the default in each of these methods:
 431          *   - commitMasterChanges()
 432          *   - rollbackMasterChanges()
 433          *   - commitAll()
 434          * This allows for custom transaction rounds from any outer transaction scope.
 435          *
 436          * @param string $fname Caller name
 437          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 438          * @throws DBExpectedError
 439          */
 440         public function beginMasterChanges( $fname = __METHOD__, $owner = null );
 441
 442         /**
 443          * Issue COMMIT on all open master connections to flush changes and view snapshots
 444          * @param string $fname Caller name
 445          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 446          * @throws DBExpectedError
 447          */
 448         public function commitMasterChanges( $fname = __METHOD__, $owner = null );
 449
 450         /**
 451          * Consume and run all pending post-COMMIT/ROLLBACK callbacks and commit dangling transactions
 452          *
 453          * @param string $fname Caller name
 454          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 455          * @return Exception|null The first exception or null if there were none
 456          */
 457         public function runMasterTransactionIdleCallbacks( $fname = __METHOD__, $owner = null );
 458
 459         /**
 460          * Run all recurring post-COMMIT/ROLLBACK listener callbacks
 461          *
 462          * @param string $fname Caller name
 463          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 464          * @return Exception|null The first exception or null if there were none
 465          */
 466         public function runMasterTransactionListenerCallbacks( $fname = __METHOD__, $owner = null );
 467
 468         /**
 469          * Issue ROLLBACK only on master, only if queries were done on connection
 470          * @param string $fname Caller name
 471          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 472          * @throws DBExpectedError
 473          */
 474         public function rollbackMasterChanges( $fname = __METHOD__, $owner = null );
 475
 476         /**
 477          * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
 478          *
 479          * @param string $fname Caller name
 480          */
 481         public function flushReplicaSnapshots( $fname = __METHOD__ );
 482
 483         /**
 484          * Commit all master DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
 485          *
 486          * An error will be thrown if a connection has pending writes or callbacks
 487          *
 488          * @param string $fname Caller name
 489          */
 490         public function flushMasterSnapshots( $fname = __METHOD__ );
 491
 492         /**
 493          * @return bool Whether a master connection is already open
 494          */
 495         public function hasMasterConnection();
 496
 497         /**
 498          * Whether there are pending changes or callbacks in a transaction by this thread
 499          * @return bool
 500          */
 501         public function hasMasterChanges();
 502
 503         /**
 504          * Get the timestamp of the latest write query done by this thread
 505          * @return float|bool UNIX timestamp or false
 506          */
 507         public function lastMasterChangeTimestamp();
 508
 509         /**
 510          * Check if this load balancer object had any recent or still
 511          * pending writes issued against it by this PHP thread
 512          *
 513          * @param float|null $age How many seconds ago is "recent" [defaults to mWaitTimeout]
 514          * @return bool
 515          */
 516         public function hasOrMadeRecentMasterChanges( $age = null );
 517
 518         /**
 519          * Get the list of callers that have pending master changes
 520          *
 521          * @return string[] List of method names
 522          */
 523         public function pendingMasterChangeCallers();
 524
 525         /**
 526          * @note This method will trigger a DB connection if not yet done
 527          * @param string|bool $domain Domain ID, or false for the current domain
 528          * @return bool Whether the database for generic connections this request is highly "lagged"
 529          */
 530         public function getLaggedReplicaMode( $domain = false );
 531
 532         /**
 533          * Checks whether the database for generic connections this request was both:
 534          *   - a) Already choosen due to a prior connection attempt
 535          *   - b) Considered highly "lagged"
 536          *
 537          * @note This method will never cause a new DB connection
 538          * @return bool
 539          */
 540         public function laggedReplicaUsed();
 541
 542         /**
 543          * @note This method may trigger a DB connection if not yet done
 544          * @param string|bool $domain Domain ID, or false for the current domain
 545          * @param IDatabase|null $conn DB master connection; used to avoid loops [optional]
 546          * @return string|bool Reason the master is read-only or false if it is not
 547          */
 548         public function getReadOnlyReason( $domain = false, IDatabase $conn = null );
 549
 550         /**
 551          * Disables/enables lag checks
 552          * @param null|bool $mode
 553          * @return bool
 554          */
 555         public function allowLagged( $mode = null );
 556
 557         /**
 558          * @return bool
 559          */
 560         public function pingAll();
 561
 562         /**
 563          * Call a function with each open connection object
 564          * @param callable $callback
 565          * @param array $params
 566          */
 567         public function forEachOpenConnection( $callback, array $params = [] );
 568
 569         /**
 570          * Call a function with each open connection object to a master
 571          * @param callable $callback
 572          * @param array $params
 573          */
 574         public function forEachOpenMasterConnection( $callback, array $params = [] );
 575
 576         /**
 577          * Call a function with each open replica DB connection object
 578          * @param callable $callback
 579          * @param array $params
 580          */
 581         public function forEachOpenReplicaConnection( $callback, array $params = [] );
 582
 583         /**
 584          * Get the hostname and lag time of the most-lagged replica DB
 585          *
 586          * This is useful for maintenance scripts that need to throttle their updates.
 587          * May attempt to open connections to replica DBs on the default DB. If there is
 588          * no lag, the maximum lag will be reported as -1.
 589          *
 590          * @param bool|string $domain Domain ID, or false for the default database
 591          * @return array ( host, max lag, index of max lagged host )
 592          */
 593         public function getMaxLag( $domain = false );
 594
 595         /**
 596          * Get an estimate of replication lag (in seconds) for each server
 597          *
 598          * Results are cached for a short time in memcached/process cache
 599          *
 600          * Values may be "false" if replication is too broken to estimate
 601          *
 602          * @param string|bool $domain
 603          * @return int[] Map of (server index => float|int|bool)
 604          */
 605         public function getLagTimes( $domain = false );
 606
 607         /**
 608          * Get the lag in seconds for a given connection, or zero if this load
 609          * balancer does not have replication enabled.
 610          *
 611          * This should be used in preference to Database::getLag() in cases where
 612          * replication may not be in use, since there is no way to determine if
 613          * replication is in use at the connection level without running
 614          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
 615          * function instead of Database::getLag() avoids a fatal error in this
 616          * case on many installations.
 617          *
 618          * @param IDatabase $conn
 619          * @return int|bool Returns false on error
 620          */
 621         public function safeGetLag( IDatabase $conn );
 622
 623         /**
 624          * Wait for a replica DB to reach a specified master position
 625          *
 626          * This will connect to the master to get an accurate position if $pos is not given
 627          *
 628          * @param IDatabase $conn Replica DB
 629          * @param DBMasterPos|bool $pos Master position; default: current position
 630          * @param int $timeout Timeout in seconds [optional]
 631          * @return bool Success
 632          */
 633         public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = 10 );
 634
 635         /**
 636          * Set a callback via IDatabase::setTransactionListener() on
 637          * all current and future master connections of this load balancer
 638          *
 639          * @param string $name Callback name
 640          * @param callable|null $callback
 641          */
 642         public function setTransactionListener( $name, callable $callback = null );
 643
 644         /**
 645          * Set a new table prefix for the existing local domain ID for testing
 646          *
 647          * @param string $prefix
 648          * @since 1.33
 649          */
 650         public function setLocalDomainPrefix( $prefix );
 651
 652         /**
 653          * Make certain table names use their own database, schema, and table prefix
 654          * when passed into SQL queries pre-escaped and without a qualified database name
 655          *
 656          * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
 657          * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
 658          *
 659          * Calling this twice will completely clear any old table aliases. Also, note that
 660          * callers are responsible for making sure the schemas and databases actually exist.
 661          *
 662          * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
 663          */
 664         public function setTableAliases( array $aliases );
 665
 666         /**
 667          * Convert certain index names to alternative names before querying the DB
 668          *
 669          * Note that this applies to indexes regardless of the table they belong to.
 670          *
 671          * This can be employed when an index was renamed X => Y in code, but the new Y-named
 672          * indexes were not yet built on all DBs. After all the Y-named ones are added by the DBA,
 673          * the aliases can be removed, and then the old X-named indexes dropped.
 674          *
 675          * @param string[] $aliases
 676          * @since 1.31
 677          */
 678         public function setIndexAliases( array $aliases );
 679 }