3 * Database load balancing interface
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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
;
26 use InvalidArgumentException
;
29 * Database cluster connection, tracking, load balancing, and transaction manager interface
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.
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
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.
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).
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.
78 interface ILoadBalancer
{
79 /** @var int Request a replica DB connection */
80 const DB_REPLICA
= -1;
81 /** @var int Request a master DB connection */
84 /** @var string Domain specifier when no specific database needs to be selected */
85 const DOMAIN_ANY
= '';
87 /** @var int DB handle should have DBO_TRX disabled and the caller will leave it as such */
88 const CONN_TRX_AUTOCOMMIT
= 1;
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';
96 * Construct a manager of IDatabase connection objects
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
121 public function __construct( array $params );
124 * Get the local (and default) database domain ID of connection handles
126 * @see DatabaseDomain
127 * @return string Database domain ID; this specifies DB name, schema, and table prefix
130 public function getLocalDomainID();
133 * @param DatabaseDomain|string|bool $domain Database domain
134 * @return string Value of $domain if provided or the local domain otherwise
137 public function resolveDomainID( $domain );
140 * Close all connection and redefine the local domain for testing or schema creation
142 * @param DatabaseDomain|string $domain
145 public function redefineLocalDomain( $domain );
148 * Get the index of the reader connection, which may be a replica DB
150 * This takes into account load ratios and lag times. It should
151 * always return a consistent index during a given invocation.
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
157 * @return bool|int|string
159 public function getReaderIndex( $group = false, $domain = false );
162 * Set the master wait position
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.
167 * This only applies to connections to the generic replica DB for this request.
168 * If a timeout happens when waiting, then getLaggedReplicaMode()/laggedReplicaUsed()
171 * @param DBMasterPos|bool $pos Master position or false
173 public function waitFor( $pos );
176 * Set the master wait position and wait for a "generic" replica DB to catch up to it
178 * This can be used a faster proxy for waitForAll()
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)
184 public function waitForOne( $pos, $timeout = null );
187 * Set the master wait position and wait for ALL replica DBs to catch up to it
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)
193 public function waitForAll( $pos, $timeout = null );
196 * Get any open connection to a given server index, local or foreign
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.
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
205 public function getAnyOpenConnection( $i, $flags = 0 );
208 * Get a connection handle by server index
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.
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.
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
224 * @note This method throws DBAccessError if ILoadBalancer::disable() was called
229 public function getConnection( $i, $groups = [], $domain = false, $flags = 0 );
232 * Mark a foreign connection as being available for reuse under a different DB domain
234 * This mechanism is reference-counted, and must be called the same number of times
235 * as getConnection() to work.
237 * @param IDatabase $conn
238 * @throws InvalidArgumentException
240 public function reuseConnection( IDatabase
$conn );
243 * Get a database connection handle reference
245 * The handle's methods simply wrap those of a Database handle
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.
252 * @see ILoadBalancer::getConnection() for parameter information
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)
260 public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
263 * Get a database connection handle reference without connecting yet
265 * The handle's methods simply wrap those of a Database handle
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.
272 * @see ILoadBalancer::getConnection() for parameter information
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)
280 public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
283 * Get a maintenance database connection handle reference for migrations and schema changes
285 * The handle's methods simply wrap those of a Database handle
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.
292 * @see ILoadBalancer::getConnection() for parameter information
294 * @param int $db 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
300 public function getMaintenanceConnectionRef( $db, $groups = [], $domain = false, $flags = 0 );
303 * Open a connection to the server given by the specified index
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.
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.
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.
317 * @note This method throws DBAccessError if ILoadBalancer::disable() was called
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
325 public function openConnection( $i, $domain = false, $flags = 0 );
330 public function getWriterIndex();
333 * Returns true if the specified index is a valid server index
338 public function haveIndex( $i );
341 * Returns true if the specified index is valid and has non-zero load
346 public function isNonZeroLoad( $i );
349 * Get the number of defined servers (not the number of open connections)
353 public function getServerCount();
356 * Get the host name or IP address of the server with the specified index
359 * @return string Readable name if available or IP/host otherwise
361 public function getServerName( $i );
364 * Return the server info structure for a given index, or false if the index is invalid.
369 public function getServerInfo( $i );
372 * Get DB type of the server with the specified index
375 * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes
378 public function getServerType( $i );
381 * @param int $i Server index
382 * @return array (Database::ATTRIBUTE_* constant => value) for all such constants
385 public function getServerAttributes( $i );
388 * Get the current master position for chronology control purposes
389 * @return DBMasterPos|bool Returns false if not applicable
391 public function getMasterPos();
394 * Disable this load balancer. All connections are closed, and any attempt to
395 * open a new connection will result in a DBAccessError.
397 public function disable();
400 * Close all open connections
402 public function closeAll();
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.
410 * @param IDatabase $conn
412 public function closeConnection( IDatabase
$conn );
415 * Commit transactions on all open connections
416 * @param string $fname Caller name
417 * @throws DBExpectedError
419 public function commitAll( $fname = __METHOD__
);
422 * Run pre-commit callbacks and defer execution of post-commit callbacks
424 * Use this only for mutli-database commits
426 * @return int Number of pre-commit callbacks run (since 1.32)
428 public function finalizeMasterChanges();
431 * Perform all pre-commit checks for things like replication safety
433 * Use this only for mutli-database commits
435 * @param array $options Includes:
436 * - maxWriteDuration : max write query duration time in seconds
437 * @throws DBTransactionError
439 public function approveMasterChanges( array $options );
442 * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
444 * The DBO_TRX setting will be reverted to the default in each of these methods:
445 * - commitMasterChanges()
446 * - rollbackMasterChanges()
448 * This allows for custom transaction rounds from any outer transaction scope.
450 * @param string $fname
451 * @throws DBExpectedError
453 public function beginMasterChanges( $fname = __METHOD__
);
456 * Issue COMMIT on all open master connections to flush changes and view snapshots
457 * @param string $fname Caller name
458 * @throws DBExpectedError
460 public function commitMasterChanges( $fname = __METHOD__
);
463 * Consume and run all pending post-COMMIT/ROLLBACK callbacks and commit dangling transactions
465 * @return Exception|null The first exception or null if there were none
467 public function runMasterTransactionIdleCallbacks();
470 * Run all recurring post-COMMIT/ROLLBACK listener callbacks
472 * @return Exception|null The first exception or null if there were none
474 public function runMasterTransactionListenerCallbacks();
477 * Issue ROLLBACK only on master, only if queries were done on connection
478 * @param string $fname Caller name
479 * @throws DBExpectedError
481 public function rollbackMasterChanges( $fname = __METHOD__
);
484 * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
486 * @param string $fname Caller name
488 public function flushReplicaSnapshots( $fname = __METHOD__
);
491 * Commit all master DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
493 * An error will be thrown if a connection has pending writes or callbacks
495 * @param string $fname Caller name
497 public function flushMasterSnapshots( $fname = __METHOD__
);
500 * @return bool Whether a master connection is already open
502 public function hasMasterConnection();
505 * Whether there are pending changes or callbacks in a transaction by this thread
508 public function hasMasterChanges();
511 * Get the timestamp of the latest write query done by this thread
512 * @return float|bool UNIX timestamp or false
514 public function lastMasterChangeTimestamp();
517 * Check if this load balancer object had any recent or still
518 * pending writes issued against it by this PHP thread
520 * @param float|null $age How many seconds ago is "recent" [defaults to mWaitTimeout]
523 public function hasOrMadeRecentMasterChanges( $age = null );
526 * Get the list of callers that have pending master changes
528 * @return string[] List of method names
530 public function pendingMasterChangeCallers();
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"
537 public function getLaggedReplicaMode( $domain = false );
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"
544 * @note This method will never cause a new DB connection
547 public function laggedReplicaUsed();
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
555 public function getReadOnlyReason( $domain = false, IDatabase
$conn = null );
558 * Disables/enables lag checks
559 * @param null|bool $mode
562 public function allowLagged( $mode = null );
567 public function pingAll();
570 * Call a function with each open connection object
571 * @param callable $callback
572 * @param array $params
574 public function forEachOpenConnection( $callback, array $params = [] );
577 * Call a function with each open connection object to a master
578 * @param callable $callback
579 * @param array $params
581 public function forEachOpenMasterConnection( $callback, array $params = [] );
584 * Call a function with each open replica DB connection object
585 * @param callable $callback
586 * @param array $params
588 public function forEachOpenReplicaConnection( $callback, array $params = [] );
591 * Get the hostname and lag time of the most-lagged replica DB
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.
597 * @param bool|string $domain Domain ID, or false for the default database
598 * @return array ( host, max lag, index of max lagged host )
600 public function getMaxLag( $domain = false );
603 * Get an estimate of replication lag (in seconds) for each server
605 * Results are cached for a short time in memcached/process cache
607 * Values may be "false" if replication is too broken to estimate
609 * @param string|bool $domain
610 * @return int[] Map of (server index => float|int|bool)
612 public function getLagTimes( $domain = false );
615 * Get the lag in seconds for a given connection, or zero if this load
616 * balancer does not have replication enabled.
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.
625 * @param IDatabase $conn
626 * @return int|bool Returns false on error
628 public function safeGetLag( IDatabase
$conn );
631 * Wait for a replica DB to reach a specified master position
633 * This will connect to the master to get an accurate position if $pos is not given
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
640 public function safeWaitForMasterPos( IDatabase
$conn, $pos = false, $timeout = 10 );
643 * Set a callback via IDatabase::setTransactionListener() on
644 * all current and future master connections of this load balancer
646 * @param string $name Callback name
647 * @param callable|null $callback
649 public function setTransactionListener( $name, callable
$callback = null );
652 * Set a new table prefix for the existing local domain ID for testing
654 * @param string $prefix
657 public function setLocalDomainPrefix( $prefix );
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
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.
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.
669 * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
671 public function setTableAliases( array $aliases );
674 * Convert certain index names to alternative names before querying the DB
676 * Note that this applies to indexes regardless of the table they belong to.
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.
682 * @param string[] $aliases
686 public function setIndexAliases( array $aliases );