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: Try to avoid DB replicas with lag above this many seconds [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          * Close all connection and redefine the local domain for testing or schema creation
 141          *
 142          * @param DatabaseDomain|string $domain
 143          * @since 1.33
 144          */
 145         public function redefineLocalDomain( $domain );
 146
 147         /**
 148          * Get the index of the reader connection, which may be a replica DB
 149          *
 150          * This takes into account load ratios and lag times. It should
 151          * always return a consistent index during a given invocation.
 152          *
 153          * Side effect: opens connections to databases
 154          * @param string|bool $group Query group, or false for the generic reader
 155          * @param string|bool $domain Domain ID, or false for the current domain
 156          * @throws DBError
 157          * @return bool|int|string
 158          */
 159         public function getReaderIndex( $group = false, $domain = false );
 160
 161         /**
 162          * Set the master wait position
 163          *
 164          * If a DB_REPLICA connection has been opened already, then wait immediately.
 165          * Otherwise sets a variable telling it to wait if such a connection is opened.
 166          *
 167          * This only applies to connections to the generic replica DB for this request.
 168          * If a timeout happens when waiting, then getLaggedReplicaMode()/laggedReplicaUsed()
 169          * will return true.
 170          *
 171          * @param DBMasterPos|bool $pos Master position or false
 172          */
 173         public function waitFor( $pos );
 174
 175         /**
 176          * Set the master wait position and wait for a "generic" replica DB to catch up to it
 177          *
 178          * This can be used a faster proxy for waitForAll()
 179          *
 180          * @param DBMasterPos|bool $pos Master position or false
 181          * @param int|null $timeout Max seconds to wait; default is mWaitTimeout
 182          * @return bool Success (able to connect and no timeouts reached)
 183          */
 184         public function waitForOne( $pos, $timeout = null );
 185
 186         /**
 187          * Set the master wait position and wait for ALL replica DBs to catch up to it
 188          *
 189          * @param DBMasterPos|bool $pos Master position or false
 190          * @param int|null $timeout Max seconds to wait; default is mWaitTimeout
 191          * @return bool Success (able to connect and no timeouts reached)
 192          */
 193         public function waitForAll( $pos, $timeout = null );
 194
 195         /**
 196          * Get any open connection to a given server index, local or foreign
 197          *
 198          * Use CONN_TRX_AUTOCOMMIT to only look for connections opened with that flag.
 199          * Avoid the use of begin() or startAtomic() on any such connections.
 200          *
 201          * @param int $i Server index or DB_MASTER/DB_REPLICA
 202          * @param int $flags Bitfield of CONN_* class constants
 203          * @return Database|bool False if no such connection is open
 204          */
 205         public function getAnyOpenConnection( $i, $flags = 0 );
 206
 207         /**
 208          * Get a connection handle by server index
 209          *
 210          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 211          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 212          * can be used to check such flags beforehand.
 213          *
 214          * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must
 215          * also call ILoadBalancer::reuseConnection() on the handle when finished using it.
 216          * In all other cases, this is not necessary, though not harmful either.
 217          * Avoid the use of begin() or startAtomic() on any such connections.
 218          *
 219          * @param int $i Server index (overrides $groups) or DB_MASTER/DB_REPLICA
 220          * @param array|string|bool $groups Query group(s), or false for the generic reader
 221          * @param string|bool $domain Domain ID, or false for the current domain
 222          * @param int $flags Bitfield of CONN_* class constants
 223          *
 224          * @note This method throws DBAccessError if ILoadBalancer::disable() was called
 225          *
 226          * @throws DBError
 227          * @return Database
 228          */
 229         public function getConnection( $i, $groups = [], $domain = false, $flags = 0 );
 230
 231         /**
 232          * Mark a foreign connection as being available for reuse under a different DB domain
 233          *
 234          * This mechanism is reference-counted, and must be called the same number of times
 235          * as getConnection() to work.
 236          *
 237          * @param IDatabase $conn
 238          * @throws InvalidArgumentException
 239          */
 240         public function reuseConnection( IDatabase $conn );
 241
 242         /**
 243          * Get a database connection handle reference
 244          *
 245          * The handle's methods simply wrap those of a Database handle
 246          *
 247          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 248          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 249          * can be used to check such flags beforehand. Avoid the use of begin() or startAtomic()
 250          * on any CONN_TRX_AUTOCOMMIT connections.
 251          *
 252          * @see ILoadBalancer::getConnection() for parameter information
 253          *
 254          * @param int $i Server index or DB_MASTER/DB_REPLICA
 255          * @param array|string|bool $groups Query group(s), or false for the generic reader
 256          * @param string|bool $domain Domain ID, or false for the current domain
 257          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 258          * @return DBConnRef
 259          */
 260         public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 261
 262         /**
 263          * Get a database connection handle reference without connecting yet
 264          *
 265          * The handle's methods simply wrap those of a Database handle
 266          *
 267          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 268          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 269          * can be used to check such flags beforehand. Avoid the use of begin() or startAtomic()
 270          * on any CONN_TRX_AUTOCOMMIT connections.
 271          *
 272          * @see ILoadBalancer::getConnection() for parameter information
 273          *
 274          * @param int $i Server index or DB_MASTER/DB_REPLICA
 275          * @param array|string|bool $groups Query group(s), or false for the generic reader
 276          * @param string|bool $domain Domain ID, or false for the current domain
 277          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 278          * @return DBConnRef
 279          */
 280         public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 281
 282         /**
 283          * Get a maintenance database connection handle reference for migrations and schema changes
 284          *
 285          * The handle's methods simply wrap those of a Database handle
 286          *
 287          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 288          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 289          * can be used to check such flags beforehand. Avoid the use of begin() or startAtomic()
 290          * on any CONN_TRX_AUTOCOMMIT connections.
 291          *
 292          * @see ILoadBalancer::getConnection() for parameter information
 293          *
 294          * @param int $i Server index or DB_MASTER/DB_REPLICA
 295          * @param array|string|bool $groups Query group(s), or false for the generic reader
 296          * @param string|bool $domain Domain ID, or false for the current domain
 297          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 298          * @return MaintainableDBConnRef
 299          */
 300         public function getMaintenanceConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 301
 302         /**
 303          * Open a connection to the server given by the specified index
 304          *
 305          * The index must be an actual index into the array. If a connection to the server is
 306          * already open and not considered an "in use" foreign connection, this simply returns it.
 307          *
 308          * Avoid using CONN_TRX_AUTOCOMMIT for databases with ATTR_DB_LEVEL_LOCKING (e.g. sqlite)
 309          * in order to avoid deadlocks. ILoadBalancer::getServerAttributes() can be used to check
 310          * such flags beforehand.
 311          *
 312          * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must
 313          * also call ILoadBalancer::reuseConnection() on the handle when finished using it.
 314          * In all other cases, this is not necessary, though not harmful either.
 315          * Avoid the use of begin() or startAtomic() on any such connections.
 316          *
 317          * @note This method throws DBAccessError if ILoadBalancer::disable() was called
 318          *
 319          * @param int $i Server index (does not support DB_MASTER/DB_REPLICA)
 320          * @param string|bool $domain Domain ID, or false for the current domain
 321          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 322          * @return Database|bool Returns false on errors
 323          * @throws DBAccessError
 324          */
 325         public function openConnection( $i, $domain = false, $flags = 0 );
 326
 327         /**
 328          * @return int
 329          */
 330         public function getWriterIndex();
 331
 332         /**
 333          * Returns true if the specified index is a valid server index
 334          *
 335          * @param int $i
 336          * @return bool
 337          */
 338         public function haveIndex( $i );
 339
 340         /**
 341          * Returns true if the specified index is valid and has non-zero load
 342          *
 343          * @param int $i
 344          * @return bool
 345          */
 346         public function isNonZeroLoad( $i );
 347
 348         /**
 349          * Get the number of defined servers (not the number of open connections)
 350          *
 351          * @return int
 352          */
 353         public function getServerCount();
 354
 355         /**
 356          * Get the host name or IP address of the server with the specified index
 357          *
 358          * @param int $i
 359          * @return string Readable name if available or IP/host otherwise
 360          */
 361         public function getServerName( $i );
 362
 363         /**
 364          * Return the server info structure for a given index, or false if the index is invalid.
 365          * @param int $i
 366          * @return array|bool
 367          * @since 1.31
 368          */
 369         public function getServerInfo( $i );
 370
 371         /**
 372          * Get DB type of the server with the specified index
 373          *
 374          * @param int $i
 375          * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes
 376          * @since 1.30
 377          */
 378         public function getServerType( $i );
 379
 380         /**
 381          * @param int $i Server index
 382          * @return array (Database::ATTRIBUTE_* constant => value) for all such constants
 383          * @since 1.31
 384          */
 385         public function getServerAttributes( $i );
 386
 387         /**
 388          * Get the current master position for chronology control purposes
 389          * @return DBMasterPos|bool Returns false if not applicable
 390          */
 391         public function getMasterPos();
 392
 393         /**
 394          * Disable this load balancer. All connections are closed, and any attempt to
 395          * open a new connection will result in a DBAccessError.
 396          */
 397         public function disable();
 398
 399         /**
 400          * Close all open connections
 401          */
 402         public function closeAll();
 403
 404         /**
 405          * Close a connection
 406          *
 407          * Using this function makes sure the LoadBalancer knows the connection is closed.
 408          * If you use $conn->close() directly, the load balancer won't update its state.
 409          *
 410          * @param IDatabase $conn
 411          */
 412         public function closeConnection( IDatabase $conn );
 413
 414         /**
 415          * Commit transactions on all open connections
 416          * @param string $fname Caller name
 417          * @throws DBExpectedError
 418          */
 419         public function commitAll( $fname = __METHOD__ );
 420
 421         /**
 422          * Run pre-commit callbacks and defer execution of post-commit callbacks
 423          *
 424          * Use this only for mutli-database commits
 425          *
 426          * @return int Number of pre-commit callbacks run (since 1.32)
 427          */
 428         public function finalizeMasterChanges();
 429
 430         /**
 431          * Perform all pre-commit checks for things like replication safety
 432          *
 433          * Use this only for mutli-database commits
 434          *
 435          * @param array $options Includes:
 436          *   - maxWriteDuration : max write query duration time in seconds
 437          * @throws DBTransactionError
 438          */
 439         public function approveMasterChanges( array $options );
 440
 441         /**
 442          * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
 443          *
 444          * The DBO_TRX setting will be reverted to the default in each of these methods:
 445          *   - commitMasterChanges()
 446          *   - rollbackMasterChanges()
 447          *   - commitAll()
 448          * This allows for custom transaction rounds from any outer transaction scope.
 449          *
 450          * @param string $fname
 451          * @throws DBExpectedError
 452          */
 453         public function beginMasterChanges( $fname = __METHOD__ );
 454
 455         /**
 456          * Issue COMMIT on all open master connections to flush changes and view snapshots
 457          * @param string $fname Caller name
 458          * @throws DBExpectedError
 459          */
 460         public function commitMasterChanges( $fname = __METHOD__ );
 461
 462         /**
 463          * Consume and run all pending post-COMMIT/ROLLBACK callbacks and commit dangling transactions
 464          *
 465          * @return Exception|null The first exception or null if there were none
 466          */
 467         public function runMasterTransactionIdleCallbacks();
 468
 469         /**
 470          * Run all recurring post-COMMIT/ROLLBACK listener callbacks
 471          *
 472          * @return Exception|null The first exception or null if there were none
 473          */
 474         public function runMasterTransactionListenerCallbacks();
 475
 476         /**
 477          * Issue ROLLBACK only on master, only if queries were done on connection
 478          * @param string $fname Caller name
 479          * @throws DBExpectedError
 480          */
 481         public function rollbackMasterChanges( $fname = __METHOD__ );
 482
 483         /**
 484          * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
 485          *
 486          * @param string $fname Caller name
 487          */
 488         public function flushReplicaSnapshots( $fname = __METHOD__ );
 489
 490         /**
 491          * Commit all master DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
 492          *
 493          * An error will be thrown if a connection has pending writes or callbacks
 494          *
 495          * @param string $fname Caller name
 496          */
 497         public function flushMasterSnapshots( $fname = __METHOD__ );
 498
 499         /**
 500          * @return bool Whether a master connection is already open
 501          */
 502         public function hasMasterConnection();
 503
 504         /**
 505          * Whether there are pending changes or callbacks in a transaction by this thread
 506          * @return bool
 507          */
 508         public function hasMasterChanges();
 509
 510         /**
 511          * Get the timestamp of the latest write query done by this thread
 512          * @return float|bool UNIX timestamp or false
 513          */
 514         public function lastMasterChangeTimestamp();
 515
 516         /**
 517          * Check if this load balancer object had any recent or still
 518          * pending writes issued against it by this PHP thread
 519          *
 520          * @param float|null $age How many seconds ago is "recent" [defaults to mWaitTimeout]
 521          * @return bool
 522          */
 523         public function hasOrMadeRecentMasterChanges( $age = null );
 524
 525         /**
 526          * Get the list of callers that have pending master changes
 527          *
 528          * @return string[] List of method names
 529          */
 530         public function pendingMasterChangeCallers();
 531
 532         /**
 533          * @note This method will trigger a DB connection if not yet done
 534          * @param string|bool $domain Domain ID, or false for the current domain
 535          * @return bool Whether the database for generic connections this request is highly "lagged"
 536          */
 537         public function getLaggedReplicaMode( $domain = false );
 538
 539         /**
 540          * Checks whether the database for generic connections this request was both:
 541          *   - a) Already choosen due to a prior connection attempt
 542          *   - b) Considered highly "lagged"
 543          *
 544          * @note This method will never cause a new DB connection
 545          * @return bool
 546          */
 547         public function laggedReplicaUsed();
 548
 549         /**
 550          * @note This method may trigger a DB connection if not yet done
 551          * @param string|bool $domain Domain ID, or false for the current domain
 552          * @param IDatabase|null $conn DB master connection; used to avoid loops [optional]
 553          * @return string|bool Reason the master is read-only or false if it is not
 554          */
 555         public function getReadOnlyReason( $domain = false, IDatabase $conn = null );
 556
 557         /**
 558          * Disables/enables lag checks
 559          * @param null|bool $mode
 560          * @return bool
 561          */
 562         public function allowLagged( $mode = null );
 563
 564         /**
 565          * @return bool
 566          */
 567         public function pingAll();
 568
 569         /**
 570          * Call a function with each open connection object
 571          * @param callable $callback
 572          * @param array $params
 573          */
 574         public function forEachOpenConnection( $callback, array $params = [] );
 575
 576         /**
 577          * Call a function with each open connection object to a master
 578          * @param callable $callback
 579          * @param array $params
 580          */
 581         public function forEachOpenMasterConnection( $callback, array $params = [] );
 582
 583         /**
 584          * Call a function with each open replica DB connection object
 585          * @param callable $callback
 586          * @param array $params
 587          */
 588         public function forEachOpenReplicaConnection( $callback, array $params = [] );
 589
 590         /**
 591          * Get the hostname and lag time of the most-lagged replica DB
 592          *
 593          * This is useful for maintenance scripts that need to throttle their updates.
 594          * May attempt to open connections to replica DBs on the default DB. If there is
 595          * no lag, the maximum lag will be reported as -1.
 596          *
 597          * @param bool|string $domain Domain ID, or false for the default database
 598          * @return array ( host, max lag, index of max lagged host )
 599          */
 600         public function getMaxLag( $domain = false );
 601
 602         /**
 603          * Get an estimate of replication lag (in seconds) for each server
 604          *
 605          * Results are cached for a short time in memcached/process cache
 606          *
 607          * Values may be "false" if replication is too broken to estimate
 608          *
 609          * @param string|bool $domain
 610          * @return int[] Map of (server index => float|int|bool)
 611          */
 612         public function getLagTimes( $domain = false );
 613
 614         /**
 615          * Get the lag in seconds for a given connection, or zero if this load
 616          * balancer does not have replication enabled.
 617          *
 618          * This should be used in preference to Database::getLag() in cases where
 619          * replication may not be in use, since there is no way to determine if
 620          * replication is in use at the connection level without running
 621          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
 622          * function instead of Database::getLag() avoids a fatal error in this
 623          * case on many installations.
 624          *
 625          * @param IDatabase $conn
 626          * @return int|bool Returns false on error
 627          */
 628         public function safeGetLag( IDatabase $conn );
 629
 630         /**
 631          * Wait for a replica DB to reach a specified master position
 632          *
 633          * This will connect to the master to get an accurate position if $pos is not given
 634          *
 635          * @param IDatabase $conn Replica DB
 636          * @param DBMasterPos|bool $pos Master position; default: current position
 637          * @param int $timeout Timeout in seconds [optional]
 638          * @return bool Success
 639          */
 640         public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = 10 );
 641
 642         /**
 643          * Set a callback via IDatabase::setTransactionListener() on
 644          * all current and future master connections of this load balancer
 645          *
 646          * @param string $name Callback name
 647          * @param callable|null $callback
 648          */
 649         public function setTransactionListener( $name, callable $callback = null );
 650
 651         /**
 652          * Set a new table prefix for the existing local domain ID for testing
 653          *
 654          * @param string $prefix
 655          * @since 1.33
 656          */
 657         public function setLocalDomainPrefix( $prefix );
 658
 659         /**
 660          * Make certain table names use their own database, schema, and table prefix
 661          * when passed into SQL queries pre-escaped and without a qualified database name
 662          *
 663          * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
 664          * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
 665          *
 666          * Calling this twice will completely clear any old table aliases. Also, note that
 667          * callers are responsible for making sure the schemas and databases actually exist.
 668          *
 669          * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
 670          */
 671         public function setTableAliases( array $aliases );
 672
 673         /**
 674          * Convert certain index names to alternative names before querying the DB
 675          *
 676          * Note that this applies to indexes regardless of the table they belong to.
 677          *
 678          * This can be employed when an index was renamed X => Y in code, but the new Y-named
 679          * indexes were not yet built on all DBs. After all the Y-named ones are added by the DBA,
 680          * the aliases can be removed, and then the old X-named indexes dropped.
 681          *
 682          * @param string[] $aliases
 683          * @since 1.31
 684          */
 685         public function setIndexAliases( array $aliases );
 686 }