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  *   - Externally updated dataset clones: set 'is static' in the server configuration maps.
  66  *      This will treat all such DBs as having 0 lag.
  67  *   - SQL load balancing proxy: any proxy should handle lag checks on its own, so the 'max lag'
  68  *      parameter should probably be set to INF in the server configuration maps. This will make
  69  *      the load balancer ignore whatever it detects as the lag of the logical replica is (which
  70  *      would probably just randomly bounce around).
  71  *
  72  * If using a SQL proxy service, it would probably be best to have two proxy hosts for the
  73  * load balancer to talk to. One would be the 'host' of the master server entry and another for
  74  * the (logical) replica server entry. The proxy could map the load balancer's "replica" DB to
  75  * any number of physical replica DBs.
  76  *
  77  * @since 1.28
  78  * @ingroup Database
  79  */
  80 interface ILoadBalancer {
  81         /** @var int Request a replica DB connection */
  82         const DB_REPLICA = -1;
  83         /** @var int Request a master DB connection */
  84         const DB_MASTER = -2;
  85
  86         /** @var string Domain specifier when no specific database needs to be selected */
  87         const DOMAIN_ANY = '';
  88
  89         /** @var int DB handle should have DBO_TRX disabled and the caller will leave it as such */
  90         const CONN_TRX_AUTOCOMMIT = 1;
  91         /** @var int Return null on connection failure instead of throwing an exception */
  92         const CONN_SILENCE_ERRORS = 2;
  93
  94         /** @var string Manager of ILoadBalancer instances is running post-commit callbacks */
  95         const STAGE_POSTCOMMIT_CALLBACKS = 'stage-postcommit-callbacks';
  96         /** @var string Manager of ILoadBalancer instances is running post-rollback callbacks */
  97         const STAGE_POSTROLLBACK_CALLBACKS = 'stage-postrollback-callbacks';
  98
  99         /**
 100          * Construct a manager of IDatabase connection objects
 101          *
 102          * @param array $params Parameter map with keys:
 103          *  - servers : Required. Array of server info structures.
 104          *  - localDomain: A DatabaseDomain or domain ID string.
 105          *  - loadMonitor : Name of a class used to fetch server lag and load.
 106          *  - readOnlyReason : Reason the master DB is read-only if so [optional]
 107          *  - waitTimeout : Maximum time to wait for replicas for consistency [optional]
 108          *  - maxLag: Try to avoid DB replicas with lag above this many seconds [optional]
 109          *  - srvCache : BagOStuff object for server cache [optional]
 110          *  - wanCache : WANObjectCache object [optional]
 111          *  - chronologyCallback: Callback to run before the first connection attempt [optional]
 112          *  - hostname : The name of the current server [optional]
 113          *  - cliMode: Whether the execution context is a CLI script. [optional]
 114          *  - profiler : Class name or instance with profileIn()/profileOut() methods. [optional]
 115          *  - trxProfiler: TransactionProfiler instance. [optional]
 116          *  - replLogger: PSR-3 logger instance. [optional]
 117          *  - connLogger: PSR-3 logger instance. [optional]
 118          *  - queryLogger: PSR-3 logger instance. [optional]
 119          *  - perfLogger: PSR-3 logger instance. [optional]
 120          *  - errorLogger : Callback that takes an Exception and logs it. [optional]
 121          *  - deprecationLogger: Callback to log a deprecation warning. [optional]
 122          *  - roundStage: STAGE_POSTCOMMIT_* class constant; for internal use [optional]
 123          *  - ownerId: integer ID of an LBFactory instance that manages this instance [optional]
 124          * @throws InvalidArgumentException
 125          */
 126         public function __construct( array $params );
 127
 128         /**
 129          * Get the local (and default) database domain ID of connection handles
 130          *
 131          * @see DatabaseDomain
 132          * @return string Database domain ID; this specifies DB name, schema, and table prefix
 133          * @since 1.31
 134          */
 135         public function getLocalDomainID();
 136
 137         /**
 138          * @param DatabaseDomain|string|bool $domain Database domain
 139          * @return string Value of $domain if it is foreign or the local domain otherwise
 140          * @since 1.32
 141          */
 142         public function resolveDomainID( $domain );
 143
 144         /**
 145          * Close all connection and redefine the local domain for testing or schema creation
 146          *
 147          * @param DatabaseDomain|string $domain
 148          * @since 1.33
 149          */
 150         public function redefineLocalDomain( $domain );
 151
 152         /**
 153          * Get the server index of the reader connection for a given group
 154          *
 155          * This takes into account load ratios and lag times. It should return a consistent
 156          * index during the life time of the load balancer. This initially checks replica DBs
 157          * for connectivity to avoid returning an unusable server. This means that connections
 158          * might be attempted by calling this method (usally one at the most but possibly more).
 159          * Subsequent calls with the same $group will not need to make new connection attempts
 160          * since the acquired connection for each group is preserved.
 161          *
 162          * @param string|bool $group Query group, or false for the generic group
 163          * @param string|bool $domain Domain ID, or false for the current domain
 164          * @throws DBError
 165          * @return bool|int|string
 166          */
 167         public function getReaderIndex( $group = false, $domain = false );
 168
 169         /**
 170          * Set the master position to reach before the next generic group DB handle query
 171          *
 172          * If a generic replica DB connection is already open then this immediately waits
 173          * for that DB to catch up to the specified replication position. Otherwise, it will
 174          * do so once such a connection is opened.
 175          *
 176          * If a timeout happens when waiting, then getLaggedReplicaMode()/laggedReplicaUsed()
 177          * will return true.
 178          *
 179          * @param DBMasterPos|bool $pos Master position or false
 180          */
 181         public function waitFor( $pos );
 182
 183         /**
 184          * Set the master wait position and wait for a generic replica DB to catch up to it
 185          *
 186          * This can be used a faster proxy for waitForAll()
 187          *
 188          * @param DBMasterPos|bool $pos Master position or false
 189          * @param int|null $timeout Max seconds to wait; default is mWaitTimeout
 190          * @return bool Success (able to connect and no timeouts reached)
 191          */
 192         public function waitForOne( $pos, $timeout = null );
 193
 194         /**
 195          * Set the master wait position and wait for ALL replica DBs to catch up to it
 196          *
 197          * @param DBMasterPos|bool $pos Master position or false
 198          * @param int|null $timeout Max seconds to wait; default is mWaitTimeout
 199          * @return bool Success (able to connect and no timeouts reached)
 200          */
 201         public function waitForAll( $pos, $timeout = null );
 202
 203         /**
 204          * Get any open connection to a given server index, local or foreign
 205          *
 206          * Use CONN_TRX_AUTOCOMMIT to only look for connections opened with that flag.
 207          * Avoid the use of begin() or startAtomic() on any such connections.
 208          *
 209          * @param int $i Server index or DB_MASTER/DB_REPLICA
 210          * @param int $flags Bitfield of CONN_* class constants
 211          * @return Database|bool False if no such connection is open
 212          */
 213         public function getAnyOpenConnection( $i, $flags = 0 );
 214
 215         /**
 216          * Get a connection handle by server index
 217          *
 218          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 219          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 220          * can be used to check such flags beforehand.
 221          *
 222          * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must
 223          * also call ILoadBalancer::reuseConnection() on the handle when finished using it.
 224          * In all other cases, this is not necessary, though not harmful either.
 225          * Avoid the use of begin() or startAtomic() on any such connections.
 226          *
 227          * @param int $i Server index (overrides $groups) or DB_MASTER/DB_REPLICA
 228          * @param array|string|bool $groups Query group(s), or false for the generic reader
 229          * @param string|bool $domain Domain ID, or false for the current domain
 230          * @param int $flags Bitfield of CONN_* class constants
 231          *
 232          * @note This method throws DBAccessError if ILoadBalancer::disable() was called
 233          *
 234          * @throws DBError If any error occurs that prevents the yielding of a (live) IDatabase
 235          * @return IDatabase|bool This returns false on failure if CONN_SILENCE_ERRORS is set
 236          */
 237         public function getConnection( $i, $groups = [], $domain = false, $flags = 0 );
 238
 239         /**
 240          * Mark a foreign connection as being available for reuse under a different DB domain
 241          *
 242          * This mechanism is reference-counted, and must be called the same number of times
 243          * as getConnection() to work.
 244          *
 245          * @param IDatabase $conn
 246          * @throws InvalidArgumentException
 247          */
 248         public function reuseConnection( IDatabase $conn );
 249
 250         /**
 251          * Get a database connection handle reference
 252          *
 253          * The handle's methods simply wrap those of a Database handle
 254          *
 255          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 256          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 257          * can be used to check such flags beforehand. Avoid the use of begin() or startAtomic()
 258          * on any CONN_TRX_AUTOCOMMIT connections.
 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 getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 269
 270         /**
 271          * Get a database connection handle reference without connecting yet
 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. Avoid the use of begin() or startAtomic()
 278          * on any CONN_TRX_AUTOCOMMIT connections.
 279          *
 280          * @see ILoadBalancer::getConnection() for parameter information
 281          *
 282          * @param int $i Server index or DB_MASTER/DB_REPLICA
 283          * @param array|string|bool $groups Query group(s), or false for the generic reader
 284          * @param string|bool $domain Domain ID, or false for the current domain
 285          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 286          * @return DBConnRef
 287          */
 288         public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 289
 290         /**
 291          * Get a maintenance database connection handle reference for migrations and schema changes
 292          *
 293          * The handle's methods simply wrap those of a Database handle
 294          *
 295          * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
 296          * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
 297          * can be used to check such flags beforehand. Avoid the use of begin() or startAtomic()
 298          * on any CONN_TRX_AUTOCOMMIT connections.
 299          *
 300          * @see ILoadBalancer::getConnection() for parameter information
 301          *
 302          * @param int $i Server index or DB_MASTER/DB_REPLICA
 303          * @param array|string|bool $groups Query group(s), or false for the generic reader
 304          * @param string|bool $domain Domain ID, or false for the current domain
 305          * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
 306          * @return MaintainableDBConnRef
 307          */
 308         public function getMaintenanceConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
 309
 310         /**
 311          * Get the server index of the master server
 312          *
 313          * @return int
 314          */
 315         public function getWriterIndex();
 316
 317         /**
 318          * Returns true if the specified index is a valid server index
 319          *
 320          * @param int $i
 321          * @return bool
 322          */
 323         public function haveIndex( $i );
 324
 325         /**
 326          * Returns true if the specified index is valid and has non-zero load
 327          *
 328          * @param int $i
 329          * @return bool
 330          */
 331         public function isNonZeroLoad( $i );
 332
 333         /**
 334          * Get the number of servers defined in configuration
 335          *
 336          * @return int
 337          */
 338         public function getServerCount();
 339
 340         /**
 341          * Whether there are any replica servers configured
 342          *
 343          * This counts both servers using streaming replication from the master server and
 344          * servers that just have a clone of the static dataset found on the master server
 345          *
 346          * @return int
 347          * @since 1.34
 348          */
 349         public function hasReplicaServers();
 350
 351         /**
 352          * Whether any replica servers use streaming replication from the master server
 353          *
 354          * Generally this is one less than getServerCount(), though it might otherwise
 355          * return a lower number if some of the servers are configured with "is static".
 356          * That flag is used when both the server has no active replication setup and the
 357          * dataset is either read-only or occasionally updated out-of-band. For example,
 358          * a script might import a new geographic information dataset each week by writing
 359          * it to each server and later directing the application to use the new version.
 360          *
 361          * It is possible for some replicas to be configured with "is static" but not
 362          * others, though it generally should either be set for all or none of the replicas.
 363          *
 364          * If this returns zero, this means that there is generally no reason to execute
 365          * replication wait logic for session consistency and lag reduction.
 366          *
 367          * @return int
 368          * @since 1.34
 369          */
 370         public function hasStreamingReplicaServers();
 371
 372         /**
 373          * Get the host name or IP address of the server with the specified index
 374          *
 375          * @param int $i
 376          * @return string Readable name if available or IP/host otherwise
 377          */
 378         public function getServerName( $i );
 379
 380         /**
 381          * Return the server info structure for a given index, or false if the index is invalid.
 382          * @param int $i
 383          * @return array|bool
 384          * @since 1.31
 385          */
 386         public function getServerInfo( $i );
 387
 388         /**
 389          * Get DB type of the server with the specified index
 390          *
 391          * @param int $i
 392          * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes
 393          * @since 1.30
 394          */
 395         public function getServerType( $i );
 396
 397         /**
 398          * @param int $i Server index
 399          * @return array (Database::ATTRIBUTE_* constant => value) for all such constants
 400          * @since 1.31
 401          */
 402         public function getServerAttributes( $i );
 403
 404         /**
 405          * Get the current master position for chronology control purposes
 406          * @return DBMasterPos|bool Returns false if not applicable
 407          */
 408         public function getMasterPos();
 409
 410         /**
 411          * Disable this load balancer. All connections are closed, and any attempt to
 412          * open a new connection will result in a DBAccessError.
 413          */
 414         public function disable();
 415
 416         /**
 417          * Close all open connections
 418          */
 419         public function closeAll();
 420
 421         /**
 422          * Close a connection
 423          *
 424          * Using this function makes sure the LoadBalancer knows the connection is closed.
 425          * If you use $conn->close() directly, the load balancer won't update its state.
 426          *
 427          * @param IDatabase $conn
 428          */
 429         public function closeConnection( IDatabase $conn );
 430
 431         /**
 432          * Commit transactions on all open connections
 433          * @param string $fname Caller name
 434          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 435          * @throws DBExpectedError
 436          */
 437         public function commitAll( $fname = __METHOD__, $owner = null );
 438
 439         /**
 440          * Run pre-commit callbacks and defer execution of post-commit callbacks
 441          *
 442          * Use this only for mutli-database commits
 443          *
 444          * @param string $fname Caller name
 445          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 446          * @return int Number of pre-commit callbacks run (since 1.32)
 447          */
 448         public function finalizeMasterChanges( $fname = __METHOD__, $owner = null );
 449
 450         /**
 451          * Perform all pre-commit checks for things like replication safety
 452          *
 453          * Use this only for mutli-database commits
 454          *
 455          * @param array $options Includes:
 456          *   - maxWriteDuration : max write query duration time in seconds
 457          * @param string $fname Caller name
 458          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 459          * @throws DBTransactionError
 460          */
 461         public function approveMasterChanges( array $options, $fname, $owner = null );
 462
 463         /**
 464          * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
 465          *
 466          * The DBO_TRX setting will be reverted to the default in each of these methods:
 467          *   - commitMasterChanges()
 468          *   - rollbackMasterChanges()
 469          *   - commitAll()
 470          * This allows for custom transaction rounds from any outer transaction scope.
 471          *
 472          * @param string $fname Caller name
 473          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 474          * @throws DBExpectedError
 475          */
 476         public function beginMasterChanges( $fname = __METHOD__, $owner = null );
 477
 478         /**
 479          * Issue COMMIT on all open master connections to flush changes and view snapshots
 480          * @param string $fname Caller name
 481          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 482          * @throws DBExpectedError
 483          */
 484         public function commitMasterChanges( $fname = __METHOD__, $owner = null );
 485
 486         /**
 487          * Consume and run all pending post-COMMIT/ROLLBACK callbacks and commit dangling transactions
 488          *
 489          * @param string $fname Caller name
 490          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 491          * @return Exception|null The first exception or null if there were none
 492          */
 493         public function runMasterTransactionIdleCallbacks( $fname = __METHOD__, $owner = null );
 494
 495         /**
 496          * Run all recurring post-COMMIT/ROLLBACK listener callbacks
 497          *
 498          * @param string $fname Caller name
 499          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 500          * @return Exception|null The first exception or null if there were none
 501          */
 502         public function runMasterTransactionListenerCallbacks( $fname = __METHOD__, $owner = null );
 503
 504         /**
 505          * Issue ROLLBACK only on master, only if queries were done on connection
 506          * @param string $fname Caller name
 507          * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
 508          * @throws DBExpectedError
 509          */
 510         public function rollbackMasterChanges( $fname = __METHOD__, $owner = null );
 511
 512         /**
 513          * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
 514          *
 515          * @param string $fname Caller name
 516          */
 517         public function flushReplicaSnapshots( $fname = __METHOD__ );
 518
 519         /**
 520          * Commit all master DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
 521          *
 522          * An error will be thrown if a connection has pending writes or callbacks
 523          *
 524          * @param string $fname Caller name
 525          */
 526         public function flushMasterSnapshots( $fname = __METHOD__ );
 527
 528         /**
 529          * @return bool Whether a master connection is already open
 530          */
 531         public function hasMasterConnection();
 532
 533         /**
 534          * Whether there are pending changes or callbacks in a transaction by this thread
 535          * @return bool
 536          */
 537         public function hasMasterChanges();
 538
 539         /**
 540          * Get the timestamp of the latest write query done by this thread
 541          * @return float|bool UNIX timestamp or false
 542          */
 543         public function lastMasterChangeTimestamp();
 544
 545         /**
 546          * Check if this load balancer object had any recent or still
 547          * pending writes issued against it by this PHP thread
 548          *
 549          * @param float|null $age How many seconds ago is "recent" [defaults to mWaitTimeout]
 550          * @return bool
 551          */
 552         public function hasOrMadeRecentMasterChanges( $age = null );
 553
 554         /**
 555          * Get the list of callers that have pending master changes
 556          *
 557          * @return string[] List of method names
 558          */
 559         public function pendingMasterChangeCallers();
 560
 561         /**
 562          * @note This method will trigger a DB connection if not yet done
 563          * @param string|bool $domain Domain ID, or false for the current domain
 564          * @return bool Whether the database for generic connections this request is highly "lagged"
 565          */
 566         public function getLaggedReplicaMode( $domain = false );
 567
 568         /**
 569          * Checks whether the database for generic connections this request was both:
 570          *   - a) Already choosen due to a prior connection attempt
 571          *   - b) Considered highly "lagged"
 572          *
 573          * @note This method will never cause a new DB connection
 574          * @return bool
 575          */
 576         public function laggedReplicaUsed();
 577
 578         /**
 579          * @note This method may trigger a DB connection if not yet done
 580          * @param string|bool $domain Domain ID, or false for the current domain
 581          * @param IDatabase|null $conn DB master connection; used to avoid loops [optional]
 582          * @return string|bool Reason the master is read-only or false if it is not
 583          */
 584         public function getReadOnlyReason( $domain = false, IDatabase $conn = null );
 585
 586         /**
 587          * Disables/enables lag checks
 588          * @param null|bool $mode
 589          * @return bool
 590          */
 591         public function allowLagged( $mode = null );
 592
 593         /**
 594          * @return bool
 595          */
 596         public function pingAll();
 597
 598         /**
 599          * Call a function with each open connection object
 600          * @param callable $callback
 601          * @param array $params
 602          */
 603         public function forEachOpenConnection( $callback, array $params = [] );
 604
 605         /**
 606          * Call a function with each open connection object to a master
 607          * @param callable $callback
 608          * @param array $params
 609          */
 610         public function forEachOpenMasterConnection( $callback, array $params = [] );
 611
 612         /**
 613          * Call a function with each open replica DB connection object
 614          * @param callable $callback
 615          * @param array $params
 616          */
 617         public function forEachOpenReplicaConnection( $callback, array $params = [] );
 618
 619         /**
 620          * Get the hostname and lag time of the most-lagged replica server
 621          *
 622          * This is useful for maintenance scripts that need to throttle their updates.
 623          * May attempt to open connections to replica DBs on the default DB. If there is
 624          * no lag, the maximum lag will be reported as -1.
 625          *
 626          * @param bool|string $domain Domain ID, or false for the default database
 627          * @return array ( host, max lag, index of max lagged host )
 628          */
 629         public function getMaxLag( $domain = false );
 630
 631         /**
 632          * Get an estimate of replication lag (in seconds) for each server
 633          *
 634          * Results are cached for a short time in memcached/process cache
 635          *
 636          * Values may be "false" if replication is too broken to estimate
 637          *
 638          * @param string|bool $domain
 639          * @return int[] Map of (server index => float|int|bool)
 640          */
 641         public function getLagTimes( $domain = false );
 642
 643         /**
 644          * Get the lag in seconds for a given connection, or zero if this load
 645          * balancer does not have replication enabled.
 646          *
 647          * This should be used in preference to Database::getLag() in cases where
 648          * replication may not be in use, since there is no way to determine if
 649          * replication is in use at the connection level without running
 650          * potentially restricted queries such as SHOW SLAVE STATUS. Using this
 651          * function instead of Database::getLag() avoids a fatal error in this
 652          * case on many installations.
 653          *
 654          * @param IDatabase $conn
 655          * @return int|bool Returns false on error
 656          */
 657         public function safeGetLag( IDatabase $conn );
 658
 659         /**
 660          * Wait for a replica DB to reach a specified master position
 661          *
 662          * This will connect to the master to get an accurate position if $pos is not given
 663          *
 664          * @param IDatabase $conn Replica DB
 665          * @param DBMasterPos|bool $pos Master position; default: current position
 666          * @param int $timeout Timeout in seconds [optional]
 667          * @return bool Success
 668          */
 669         public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = 10 );
 670
 671         /**
 672          * Set a callback via IDatabase::setTransactionListener() on
 673          * all current and future master connections of this load balancer
 674          *
 675          * @param string $name Callback name
 676          * @param callable|null $callback
 677          */
 678         public function setTransactionListener( $name, callable $callback = null );
 679
 680         /**
 681          * Set a new table prefix for the existing local domain ID for testing
 682          *
 683          * @param string $prefix
 684          * @since 1.33
 685          */
 686         public function setLocalDomainPrefix( $prefix );
 687
 688         /**
 689          * Make certain table names use their own database, schema, and table prefix
 690          * when passed into SQL queries pre-escaped and without a qualified database name
 691          *
 692          * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
 693          * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
 694          *
 695          * Calling this twice will completely clear any old table aliases. Also, note that
 696          * callers are responsible for making sure the schemas and databases actually exist.
 697          *
 698          * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
 699          */
 700         public function setTableAliases( array $aliases );
 701
 702         /**
 703          * Convert certain index names to alternative names before querying the DB
 704          *
 705          * Note that this applies to indexes regardless of the table they belong to.
 706          *
 707          * This can be employed when an index was renamed X => Y in code, but the new Y-named
 708          * indexes were not yet built on all DBs. After all the Y-named ones are added by the DBA,
 709          * the aliases can be removed, and then the old X-named indexes dropped.
 710          *
 711          * @param string[] $aliases
 712          * @since 1.31
 713          */
 714         public function setIndexAliases( array $aliases );
 715 }