3 * Database load balancing interface
5 * This program is free software; you can redistribute it and/or modify
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7 * the Free Software Foundation; either version 2 of the License, or
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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.
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23 namespace Wikimedia\Rdbms
;
27 use InvalidArgumentException
;
30 * Database cluster connection, tracking, load balancing, and transaction manager interface
32 * A "cluster" is considered to be one master database and zero or more replica databases.
33 * Typically, the replica DBs replicate from the master asynchronously. The first node in the
34 * "servers" configuration array is always considered the "master". However, this class can still
35 * be used when all or some of the "replica" DBs are multi-master peers of the master or even
36 * when all the DBs are non-replicating clones of each other holding read-only data. Thus, the
37 * role of "master" is in some cases merely nominal.
39 * By default, each DB server uses DBO_DEFAULT for its 'flags' setting, unless explicitly set
40 * otherwise in configuration. DBO_DEFAULT behavior depends on whether 'cliMode' is set:
41 * - In CLI mode, the flag has no effect with regards to LoadBalancer.
42 * - In non-CLI mode, the flag causes implicit transactions to be used; the first query on
43 * a database starts a transaction on that database. The transactions are meant to remain
44 * pending until either commitMasterChanges() or rollbackMasterChanges() is called. The
45 * application must have some point where it calls commitMasterChanges() near the end of
47 * Every iteration of beginMasterChanges()/commitMasterChanges() is called a "transaction round".
48 * Rounds are useful on the master DB connections because they make single-DB (and by and large
49 * multi-DB) updates in web requests all-or-nothing. Also, transactions on replica DBs are useful
50 * when REPEATABLE-READ or SERIALIZABLE isolation is used because all foriegn keys and constraints
51 * hold across separate queries in the DB transaction since the data appears within a consistent
52 * point-in-time snapshot.
54 * The typical caller will use LoadBalancer::getConnection( DB_* ) to yield a live database
55 * connection handle. The choice of which DB server to use is based on pre-defined loads for
56 * weighted random selection, adjustments thereof by LoadMonitor, and the amount of replication
57 * lag on each DB server. Lag checks might cause problems in certain setups, so they should be
58 * tuned in the server configuration maps as follows:
59 * - Master + N Replica(s): set 'max lag' to an appropriate threshold for avoiding any database
60 * lagged by this much or more. If all DBs are this lagged, then the load balancer considers
61 * the cluster to be read-only.
62 * - Galera Cluster: Seconds_Behind_Master will be 0, so there probably is nothing to tune.
63 * Note that lag is still possible depending on how wsrep-sync-wait is set server-side.
64 * - Read-only archive clones: set 'is static' in the server configuration maps. This will
65 * treat all such DBs as having 0 lag.
66 * - Externally updated dataset clones: set 'is static' in the server configuration maps.
67 * This will treat all such DBs as having 0 lag.
68 * - SQL load balancing proxy: any proxy should handle lag checks on its own, so the 'max lag'
69 * parameter should probably be set to INF in the server configuration maps. This will make
70 * the load balancer ignore whatever it detects as the lag of the logical replica is (which
71 * would probably just randomly bounce around).
73 * If using a SQL proxy service, it would probably be best to have two proxy hosts for the
74 * load balancer to talk to. One would be the 'host' of the master server entry and another for
75 * the (logical) replica server entry. The proxy could map the load balancer's "replica" DB to
76 * any number of physical replica DBs.
81 interface ILoadBalancer
{
82 /** @var int Request a replica DB connection */
83 const DB_REPLICA
= -1;
84 /** @var int Request a master DB connection */
87 /** @var string Domain specifier when no specific database needs to be selected */
88 const DOMAIN_ANY
= '';
89 /** @var string The generic query group */
90 const GROUP_GENERIC
= '';
92 /** @var int DB handle should have DBO_TRX disabled and the caller will leave it as such */
93 const CONN_TRX_AUTOCOMMIT
= 1;
94 /** @var int Return null on connection failure instead of throwing an exception */
95 const CONN_SILENCE_ERRORS
= 2;
96 /** @var int Caller is requesting the master DB server for possibly writes */
97 const CONN_INTENT_WRITABLE
= 4;
99 /** @var string Manager of ILoadBalancer instances is running post-commit callbacks */
100 const STAGE_POSTCOMMIT_CALLBACKS
= 'stage-postcommit-callbacks';
101 /** @var string Manager of ILoadBalancer instances is running post-rollback callbacks */
102 const STAGE_POSTROLLBACK_CALLBACKS
= 'stage-postrollback-callbacks';
105 * Construct a manager of IDatabase connection objects
107 * @param array $params Parameter map with keys:
108 * - servers : List of server info structures
109 * - localDomain: A DatabaseDomain or domain ID string
110 * - loadMonitor : Name of a class used to fetch server lag and load
111 * - readOnlyReason : Reason the master DB is read-only if so [optional]
112 * - waitTimeout : Maximum time to wait for replicas for consistency [optional]
113 * - maxLag: Try to avoid DB replicas with lag above this many seconds [optional]
114 * - srvCache : BagOStuff object for server cache [optional]
115 * - wanCache : WANObjectCache object [optional]
116 * - chronologyCallback: Callback to run before the first connection attempt [optional]
117 * - defaultGroup: Default query group; the generic group if not specified [optional]
118 * - hostname : The name of the current server [optional]
119 * - cliMode: Whether the execution context is a CLI script [optional]
120 * - profiler : Class name or instance with profileIn()/profileOut() methods [optional]
121 * - trxProfiler: TransactionProfiler instance [optional]
122 * - replLogger: PSR-3 logger instance [optional]
123 * - connLogger: PSR-3 logger instance [optional]
124 * - queryLogger: PSR-3 logger instance [optional]
125 * - perfLogger: PSR-3 logger instance [optional]
126 * - errorLogger : Callback that takes an Exception and logs it [optional]
127 * - deprecationLogger: Callback to log a deprecation warning [optional]
128 * - roundStage: STAGE_POSTCOMMIT_* class constant; for internal use [optional]
129 * - ownerId: integer ID of an LBFactory instance that manages this instance [optional]
130 * @throws InvalidArgumentException
132 public function __construct( array $params );
135 * Get the local (and default) database domain ID of connection handles
137 * @see DatabaseDomain
138 * @return string Database domain ID; this specifies DB name, schema, and table prefix
141 public function getLocalDomainID();
144 * @param DatabaseDomain|string|bool $domain Database domain
145 * @return string Value of $domain if it is foreign or the local domain otherwise
148 public function resolveDomainID( $domain );
151 * Close all connection and redefine the local domain for testing or schema creation
153 * @param DatabaseDomain|string $domain
156 public function redefineLocalDomain( $domain );
159 * Indicate whether the tables on this domain are only temporary tables for testing
161 * In "temporary tables mode", the ILoadBalancer::CONN_TRX_AUTOCOMMIT flag is ignored
164 * @param string $domain
165 * @return bool Whether "temporary tables mode" was active
168 public function setTempTablesOnlyMode( $value, $domain );
171 * Get the server index of the reader connection for a given group
173 * This takes into account load ratios and lag times. It should return a consistent
174 * index during the life time of the load balancer. This initially checks replica DBs
175 * for connectivity to avoid returning an unusable server. This means that connections
176 * might be attempted by calling this method (usally one at the most but possibly more).
177 * Subsequent calls with the same $group will not need to make new connection attempts
178 * since the acquired connection for each group is preserved.
180 * @param string|bool $group Query group or false for the generic group
181 * @param string|bool $domain DB domain ID or false for the local domain
182 * @return int|bool Returns false if no live handle can be obtained
184 public function getReaderIndex( $group = false, $domain = false );
187 * Set the master position to reach before the next generic group DB handle query
189 * If a generic replica DB connection is already open then this immediately waits
190 * for that DB to catch up to the specified replication position. Otherwise, it will
191 * do so once such a connection is opened.
193 * If a timeout happens when waiting, then getLaggedReplicaMode()/laggedReplicaUsed()
196 * @param DBMasterPos|bool $pos Master position or false
198 public function waitFor( $pos );
201 * Set the master wait position and wait for a generic replica DB to catch up to it
203 * This can be used a faster proxy for waitForAll()
205 * @param DBMasterPos|bool $pos Master position or false
206 * @param int|null $timeout Max seconds to wait; default is mWaitTimeout
207 * @return bool Success (able to connect and no timeouts reached)
209 public function waitForOne( $pos, $timeout = null );
212 * Set the master wait position and wait for ALL replica DBs to catch up to it
214 * @param DBMasterPos|bool $pos Master position or false
215 * @param int|null $timeout Max seconds to wait; default is mWaitTimeout
216 * @return bool Success (able to connect and no timeouts reached)
218 public function waitForAll( $pos, $timeout = null );
221 * Get any open connection to a given server index, local or foreign
223 * Use CONN_TRX_AUTOCOMMIT to only look for connections opened with that flag.
224 * Avoid the use of transaction methods like IDatabase::begin() or IDatabase::startAtomic()
225 * on any such connections.
227 * @param int $i Server index or DB_MASTER/DB_REPLICA
228 * @param int $flags Bitfield of CONN_* class constants
229 * @return Database|bool False if no such connection is open
231 public function getAnyOpenConnection( $i, $flags = 0 );
234 * Get a live handle for a real or virtual (DB_MASTER/DB_REPLICA) server index
236 * The server index, $i, can be one of the following:
237 * - DB_REPLICA: a server index will be selected by the load balancer based on read
238 * weight, connectivity, and replication lag. Note that the master server might be
239 * configured with read weight. If $groups is empty then it means "the generic group",
240 * in which case all servers defined with read weight will be considered. Additional
241 * query groups can be configured, having their own list of server indexes and read
242 * weights. If a query group list is provided in $groups, then each recognized group
243 * will be tried, left-to-right, until server index selection succeeds or all groups
244 * have been tried, in which case the generic group will be tried.
245 * - DB_MASTER: the master server index will be used; the same as getWriterIndex().
246 * The value of $groups should be [] when using this server index.
247 * - Specific server index: a positive integer can be provided to use the server with
248 * that index. An error will be thrown in no such server index is recognized. This
249 * server selection method is usually only useful for internal load balancing logic.
250 * The value of $groups should be [] when using a specific server index.
252 * Handles acquired by this method, getConnectionRef(), getLazyConnectionRef(), and
253 * getMaintenanceConnectionRef() use the same set of shared connection pools. Callers that
254 * get a *local* DB domain handle for the same server will share one handle for all of those
255 * callers using CONN_TRX_AUTOCOMMIT (via $flags) and one handle for all of those callers not
256 * using CONN_TRX_AUTOCOMMIT. Callers that get a *foreign* DB domain handle (via $domain) will
257 * share any handle that has the right CONN_TRX_AUTOCOMMIT mode and is already on the right
258 * DB domain. Otherwise, one of the "free for reuse" handles will be claimed or a new handle
259 * will be made if there are none.
261 * Handle sharing is particularly useful when callers get local DB domain (the default),
262 * transaction round aware (the default), DB_MASTER handles. All such callers will operate
263 * within a single database transaction as a consequence. Handle sharing is also useful when
264 * callers get local DB domain (the default), transaction round aware (the default), samely
265 * query grouped (the default), DB_REPLICA handles. All such callers will operate within a
266 * single database transaction as a consequence.
268 * Calling functions that use $domain must call reuseConnection() once the last query of the
269 * function is executed. This lets the load balancer share this handle with other callers
270 * requesting connections on different database domains.
272 * Use CONN_TRX_AUTOCOMMIT to use a separate pool of only auto-commit handles. This flag
273 * is ignored for databases with ATTR_DB_LEVEL_LOCKING (e.g. sqlite) in order to avoid
274 * deadlocks. getServerAttributes() can be used to check such attributes beforehand. Avoid
275 * using IDatabase::begin() and IDatabase::commit() on such handles. If it is not possible
276 * to avoid using methods like IDatabase::startAtomic() and IDatabase::endAtomic(), callers
277 * should at least make sure that the atomic sections are closed on failure via try/catch
278 * and IDatabase::cancelAtomic().
280 * @see ILoadBalancer::reuseConnection()
281 * @see ILoadBalancer::getServerAttributes()
283 * @param int $i Server index (overrides $groups) or DB_MASTER/DB_REPLICA
284 * @param string[]|string $groups Query group(s) in preference order; [] for the default group
285 * @param string|bool $domain DB domain ID or false for the local domain
286 * @param int $flags Bitfield of CONN_* class constants
288 * @note This method throws DBAccessError if ILoadBalancer::disable() was called
290 * @return IDatabase|bool This returns false on failure if CONN_SILENCE_ERRORS is set
291 * @throws DBError If no live handle can be obtained and CONN_SILENCE_ERRORS is not set
292 * @throws DBAccessError If disable() was previously called
293 * @throws InvalidArgumentException
295 public function getConnection( $i, $groups = [], $domain = false, $flags = 0 );
298 * Get a live handle for a server index
300 * This is a simpler version of getConnection() that does not accept virtual server
301 * indexes (e.g. DB_MASTER/DB_REPLICA), does not assure that master DB handles have
302 * read-only mode when there is high replication lag, and can only trigger attempts
303 * to connect to a single server (the one with the specified server index).
305 * @see ILoadBalancer::getConnection()
307 * @param int $i Specific server index
308 * @param string $domain Resolved DB domain
309 * @param int $flags Bitfield of class CONN_* constants
310 * @return IDatabase|bool
312 public function getServerConnection( $i, $domain, $flags = 0 );
315 * Mark a live handle as being available for reuse under a different database domain
317 * This mechanism is reference-counted, and must be called the same number of times as
318 * getConnection() to work. Never call this on handles acquired via getConnectionRef(),
319 * getLazyConnectionRef(), and getMaintenanceConnectionRef(), as they already manage
320 * the logic of calling this method when they fall out of scope in PHP.
322 * @see ILoadBalancer::getConnection()
324 * @param IDatabase $conn
325 * @throws LogicException
327 public function reuseConnection( IDatabase
$conn );
330 * Get a live database handle reference for a real or virtual (DB_MASTER/DB_REPLICA) server index
332 * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
333 * (e.g. sqlite) in order to avoid deadlocks. getServerAttributes()
334 * can be used to check such flags beforehand. Avoid the use of begin() or startAtomic()
335 * on any CONN_TRX_AUTOCOMMIT connections.
337 * @see ILoadBalancer::getConnection() for parameter information
339 * @param int $i Server index or DB_MASTER/DB_REPLICA
340 * @param string[]|string $groups Query group(s) in preference order; [] for the default group
341 * @param string|bool $domain DB domain ID or false for the local domain
342 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
345 public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
348 * Get a database handle reference for a real or virtual (DB_MASTER/DB_REPLICA) server index
350 * The handle's methods simply proxy to those of an underlying IDatabase handle which
351 * takes care of the actual connection and query logic.
353 * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
354 * (e.g. sqlite) in order to avoid deadlocks. getServerAttributes()
355 * can be used to check such flags beforehand. Avoid the use of begin() or startAtomic()
356 * on any CONN_TRX_AUTOCOMMIT connections.
358 * @see ILoadBalancer::getConnection() for parameter information
360 * @param int $i Server index or DB_MASTER/DB_REPLICA
361 * @param string[]|string $groups Query group(s) in preference order; [] for the default group
362 * @param string|bool $domain DB domain ID or false for the local domain
363 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
366 public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
369 * Get a live database handle for a real or virtual (DB_MASTER/DB_REPLICA) server index
370 * that can be used for data migrations and schema changes
372 * The handle's methods simply proxy to those of an underlying IDatabase handle which
373 * takes care of the actual connection and query logic.
375 * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
376 * (e.g. sqlite) in order to avoid deadlocks. getServerAttributes()
377 * can be used to check such flags beforehand. Avoid the use of begin() or startAtomic()
378 * on any CONN_TRX_AUTOCOMMIT connections.
380 * @see ILoadBalancer::getConnection() for parameter information
382 * @param int $i Server index or DB_MASTER/DB_REPLICA
383 * @param string[]|string $groups Query group(s) in preference order; [] for the default group
384 * @param string|bool $domain DB domain ID or false for the local domain
385 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
386 * @return MaintainableDBConnRef
388 public function getMaintenanceConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
391 * Get the server index of the master server
395 public function getWriterIndex();
398 * Get the number of servers defined in configuration
402 public function getServerCount();
405 * Whether there are any replica servers configured
407 * This counts both servers using streaming replication from the master server and
408 * servers that just have a clone of the static dataset found on the master server
413 public function hasReplicaServers();
416 * Whether any replica servers use streaming replication from the master server
418 * Generally this is one less than getServerCount(), though it might otherwise
419 * return a lower number if some of the servers are configured with "is static".
420 * That flag is used when both the server has no active replication setup and the
421 * dataset is either read-only or occasionally updated out-of-band. For example,
422 * a script might import a new geographic information dataset each week by writing
423 * it to each server and later directing the application to use the new version.
425 * It is possible for some replicas to be configured with "is static" but not
426 * others, though it generally should either be set for all or none of the replicas.
428 * If this returns zero, this means that there is generally no reason to execute
429 * replication wait logic for session consistency and lag reduction.
434 public function hasStreamingReplicaServers();
437 * Get the host name or IP address of the server with the specified index
440 * @return string Readable name if available or IP/host otherwise
442 public function getServerName( $i );
445 * Return the server info structure for a given index or false if the index is invalid.
450 public function getServerInfo( $i );
453 * Get DB type of the server with the specified index
456 * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes
459 public function getServerType( $i );
462 * @param int $i Server index
463 * @return array (Database::ATTRIBUTE_* constant => value) for all such constants
466 public function getServerAttributes( $i );
469 * Get the current master replication position
471 * @return DBMasterPos|bool Returns false if not applicable
474 public function getMasterPos();
477 * Get the highest DB replication position for chronology control purposes
479 * If there is only a master server then this returns false. If replication is present
480 * and correctly configured, then this returns the highest replication position of any
481 * server with an open connection. That position can later be passed to waitFor() on a
482 * new load balancer instance to make sure that queries on the new connections see data
483 * at least as up-to-date as queries (prior to this method call) on the old connections.
485 * This can be useful for implementing session consistency, where the session
486 * will be resumed accross multiple HTTP requests or CLI script instances.
488 * @return DBMasterPos|bool Replication position or false if not applicable
491 public function getReplicaResumePos();
494 * Disable this load balancer. All connections are closed, and any attempt to
495 * open a new connection will result in a DBAccessError.
497 public function disable();
500 * Close all open connections
502 public function closeAll();
507 * Using this function makes sure the LoadBalancer knows the connection is closed.
508 * If you use $conn->close() directly, the load balancer won't update its state.
510 * @param IDatabase $conn
512 public function closeConnection( IDatabase
$conn );
515 * Commit transactions on all open connections
516 * @param string $fname Caller name
517 * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
518 * @throws DBExpectedError
520 public function commitAll( $fname = __METHOD__
, $owner = null );
523 * Run pre-commit callbacks and defer execution of post-commit callbacks
525 * Use this only for mutli-database commits
527 * @param string $fname Caller name
528 * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
529 * @return int Number of pre-commit callbacks run (since 1.32)
531 public function finalizeMasterChanges( $fname = __METHOD__
, $owner = null );
534 * Perform all pre-commit checks for things like replication safety
536 * Use this only for mutli-database commits
538 * @param array $options Includes:
539 * - maxWriteDuration : max write query duration time in seconds
540 * @param string $fname Caller name
541 * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
542 * @throws DBTransactionError
544 public function approveMasterChanges( array $options, $fname, $owner = null );
547 * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
549 * The DBO_TRX setting will be reverted to the default in each of these methods:
550 * - commitMasterChanges()
551 * - rollbackMasterChanges()
553 * This allows for custom transaction rounds from any outer transaction scope.
555 * @param string $fname Caller name
556 * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
557 * @throws DBExpectedError
559 public function beginMasterChanges( $fname = __METHOD__
, $owner = null );
562 * Issue COMMIT on all open master connections to flush changes and view snapshots
563 * @param string $fname Caller name
564 * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
565 * @throws DBExpectedError
567 public function commitMasterChanges( $fname = __METHOD__
, $owner = null );
570 * Consume and run all pending post-COMMIT/ROLLBACK callbacks and commit dangling transactions
572 * @param string $fname Caller name
573 * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
574 * @return Exception|null The first exception or null if there were none
576 public function runMasterTransactionIdleCallbacks( $fname = __METHOD__
, $owner = null );
579 * Run all recurring post-COMMIT/ROLLBACK listener callbacks
581 * @param string $fname Caller name
582 * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
583 * @return Exception|null The first exception or null if there were none
585 public function runMasterTransactionListenerCallbacks( $fname = __METHOD__
, $owner = null );
588 * Issue ROLLBACK only on master, only if queries were done on connection
589 * @param string $fname Caller name
590 * @param int|null $owner ID of the calling instance (e.g. the LBFactory ID)
591 * @throws DBExpectedError
593 public function rollbackMasterChanges( $fname = __METHOD__
, $owner = null );
596 * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
598 * @param string $fname Caller name
600 public function flushReplicaSnapshots( $fname = __METHOD__
);
603 * Commit all master DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
605 * An error will be thrown if a connection has pending writes or callbacks
607 * @param string $fname Caller name
609 public function flushMasterSnapshots( $fname = __METHOD__
);
612 * @return bool Whether a master connection is already open
614 public function hasMasterConnection();
617 * Whether there are pending changes or callbacks in a transaction by this thread
620 public function hasMasterChanges();
623 * Get the timestamp of the latest write query done by this thread
624 * @return float|bool UNIX timestamp or false
626 public function lastMasterChangeTimestamp();
629 * Check if this load balancer object had any recent or still
630 * pending writes issued against it by this PHP thread
632 * @param float|null $age How many seconds ago is "recent" [defaults to mWaitTimeout]
635 public function hasOrMadeRecentMasterChanges( $age = null );
638 * Get the list of callers that have pending master changes
640 * @return string[] List of method names
642 public function pendingMasterChangeCallers();
645 * @note This method will trigger a DB connection if not yet done
646 * @param string|bool $domain DB domain ID or false for the local domain
647 * @return bool Whether the database for generic connections this request is highly "lagged"
649 public function getLaggedReplicaMode( $domain = false );
652 * Checks whether the database for generic connections this request was both:
653 * - a) Already choosen due to a prior connection attempt
654 * - b) Considered highly "lagged"
656 * @note This method will never cause a new DB connection
659 public function laggedReplicaUsed();
662 * @note This method may trigger a DB connection if not yet done
663 * @param string|bool $domain DB domain ID or false for the local domain
664 * @param IDatabase|null $conn DB master connection; used to avoid loops [optional]
665 * @return string|bool Reason the master is read-only or false if it is not
667 public function getReadOnlyReason( $domain = false, IDatabase
$conn = null );
670 * Disables/enables lag checks
671 * @param null|bool $mode
674 public function allowLagged( $mode = null );
679 public function pingAll();
682 * Call a function with each open connection object
683 * @param callable $callback
684 * @param array $params
686 public function forEachOpenConnection( $callback, array $params = [] );
689 * Call a function with each open connection object to a master
690 * @param callable $callback
691 * @param array $params
693 public function forEachOpenMasterConnection( $callback, array $params = [] );
696 * Call a function with each open replica DB connection object
697 * @param callable $callback
698 * @param array $params
700 public function forEachOpenReplicaConnection( $callback, array $params = [] );
703 * Get the hostname and lag time of the most-lagged replica server
705 * This is useful for maintenance scripts that need to throttle their updates.
706 * May attempt to open connections to replica DBs on the default DB. If there is
707 * no lag, the maximum lag will be reported as -1.
709 * @param bool|string $domain Domain ID or false for the default database
710 * @return array ( host, max lag, index of max lagged host )
712 public function getMaxLag( $domain = false );
715 * Get an estimate of replication lag (in seconds) for each server
717 * Results are cached for a short time in memcached/process cache
719 * Values may be "false" if replication is too broken to estimate
721 * @param string|bool $domain
722 * @return int[] Map of (server index => float|int|bool)
724 public function getLagTimes( $domain = false );
727 * Wait for a replica DB to reach a specified master position
729 * If $conn is not a replica server connection, then this will return true.
730 * Otherwise, if $pos is not provided, this will connect to the master server
731 * to get an accurate position.
733 * @param IDatabase $conn Replica DB
734 * @param DBMasterPos|bool $pos Master position; default: current position
735 * @param int $timeout Timeout in seconds [optional]
736 * @return bool Success
739 public function waitForMasterPos( IDatabase
$conn, $pos = false, $timeout = 10 );
742 * Set a callback via IDatabase::setTransactionListener() on
743 * all current and future master connections of this load balancer
745 * @param string $name Callback name
746 * @param callable|null $callback
748 public function setTransactionListener( $name, callable
$callback = null );
751 * Set a new table prefix for the existing local domain ID for testing
753 * @param string $prefix
756 public function setLocalDomainPrefix( $prefix );
759 * Make certain table names use their own database, schema, and table prefix
760 * when passed into SQL queries pre-escaped and without a qualified database name
762 * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
763 * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
765 * Calling this twice will completely clear any old table aliases. Also, note that
766 * callers are responsible for making sure the schemas and databases actually exist.
768 * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
770 public function setTableAliases( array $aliases );
773 * Convert certain index names to alternative names before querying the DB
775 * Note that this applies to indexes regardless of the table they belong to.
777 * This can be employed when an index was renamed X => Y in code, but the new Y-named
778 * indexes were not yet built on all DBs. After all the Y-named ones are added by the DBA,
779 * the aliases can be removed, and then the old X-named indexes dropped.
781 * @param string[] $aliases
784 public function setIndexAliases( array $aliases );