3 * Database load balancing interface
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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: Avoid replica DB servers with more lag than this [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 index of the reader connection, which may be a replica DB
126 * This takes into account load ratios and lag times. It should
127 * always return a consistent index during a given invocation.
129 * Side effect: opens connections to databases
130 * @param string|bool $group Query group, or false for the generic reader
131 * @param string|bool $domain Domain ID, or false for the current domain
133 * @return bool|int|string
135 public function getReaderIndex( $group = false, $domain = false );
138 * Set the master wait position
140 * If a DB_REPLICA connection has been opened already, then wait immediately.
141 * Otherwise sets a variable telling it to wait if such a connection is opened.
143 * This only applies to connections to the generic replica DB for this request.
144 * If a timeout happens when waiting, then getLaggedReplicaMode()/laggedReplicaUsed()
147 * @param DBMasterPos|bool $pos Master position or false
149 public function waitFor( $pos );
152 * Set the master wait position and wait for a "generic" replica DB to catch up to it
154 * This can be used a faster proxy for waitForAll()
156 * @param DBMasterPos|bool $pos Master position or false
157 * @param int $timeout Max seconds to wait; default is mWaitTimeout
158 * @return bool Success (able to connect and no timeouts reached)
160 public function waitForOne( $pos, $timeout = null );
163 * Set the master wait position and wait for ALL replica DBs to catch up to it
165 * @param DBMasterPos|bool $pos Master position or false
166 * @param int $timeout Max seconds to wait; default is mWaitTimeout
167 * @return bool Success (able to connect and no timeouts reached)
169 public function waitForAll( $pos, $timeout = null );
172 * Get any open connection to a given server index, local or foreign
174 * Use CONN_TRX_AUTOCOMMIT to only look for connections opened with that flag
176 * @param int $i Server index or DB_MASTER/DB_REPLICA
177 * @param int $flags Bitfield of CONN_* class constants
178 * @return Database|bool False if no such connection is open
180 public function getAnyOpenConnection( $i, $flags = 0 );
183 * Get a connection handle by server index
185 * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
186 * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
187 * can be used to check such flags beforehand.
189 * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must also
190 * call ILoadBalancer::reuseConnection() on the handle when finished using it.
191 * In all other cases, this is not necessary, though not harmful either.
193 * @param int $i Server index (overrides $groups) or DB_MASTER/DB_REPLICA
194 * @param array|string|bool $groups Query group(s), or false for the generic reader
195 * @param string|bool $domain Domain ID, or false for the current domain
196 * @param int $flags Bitfield of CONN_* class constants
198 * @note This method throws DBAccessError if ILoadBalancer::disable() was called
203 public function getConnection( $i, $groups = [], $domain = false, $flags = 0 );
206 * Mark a foreign connection as being available for reuse under a different DB domain
208 * This mechanism is reference-counted, and must be called the same number of times
209 * as getConnection() to work.
211 * @param IDatabase $conn
212 * @throws InvalidArgumentException
214 public function reuseConnection( IDatabase
$conn );
217 * Get a database connection handle reference
219 * The handle's methods simply wrap those of a Database handle
221 * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
222 * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
223 * can be used to check such flags beforehand.
225 * @see ILoadBalancer::getConnection() for parameter information
227 * @param int $i Server index 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 (e.g. CONN_TRX_AUTOCOMMIT)
233 public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
236 * Get a database connection handle reference without connecting yet
238 * The handle's methods simply wrap those of a Database handle
240 * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
241 * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
242 * can be used to check such flags beforehand.
244 * @see ILoadBalancer::getConnection() for parameter information
246 * @param int $i Server index or DB_MASTER/DB_REPLICA
247 * @param array|string|bool $groups Query group(s), or false for the generic reader
248 * @param string|bool $domain Domain ID, or false for the current domain
249 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
252 public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
255 * Get a maintenance database connection handle reference for migrations and schema changes
257 * The handle's methods simply wrap those of a Database handle
259 * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING
260 * (e.g. sqlite) in order to avoid deadlocks. ILoadBalancer::getServerAttributes()
261 * can be used to check such flags beforehand.
263 * @see ILoadBalancer::getConnection() for parameter information
265 * @param int $db Server index or DB_MASTER/DB_REPLICA
266 * @param array|string|bool $groups Query group(s), or false for the generic reader
267 * @param string|bool $domain Domain ID, or false for the current domain
268 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
269 * @return MaintainableDBConnRef
271 public function getMaintenanceConnectionRef( $db, $groups = [], $domain = false, $flags = 0 );
274 * Open a connection to the server given by the specified index
276 * The index must be an actual index into the array. If a connection to the server is
277 * already open and not considered an "in use" foreign connection, this simply returns it.
279 * Avoid using CONN_TRX_AUTOCOMMIT for databases with ATTR_DB_LEVEL_LOCKING (e.g. sqlite) in
280 * order to avoid deadlocks. ILoadBalancer::getServerAttributes() can be used to check
281 * such flags beforehand.
283 * If the caller uses $domain or sets CONN_TRX_AUTOCOMMIT in $flags, then it must also
284 * call ILoadBalancer::reuseConnection() on the handle when finished using it.
285 * In all other cases, this is not necessary, though not harmful either.
287 * @note This method throws DBAccessError if ILoadBalancer::disable() was called
289 * @param int $i Server index (does not support DB_MASTER/DB_REPLICA)
290 * @param string|bool $domain Domain ID, or false for the current domain
291 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT)
292 * @return Database|bool Returns false on errors
293 * @throws DBAccessError
295 public function openConnection( $i, $domain = false, $flags = 0 );
300 public function getWriterIndex();
303 * Returns true if the specified index is a valid server index
308 public function haveIndex( $i );
311 * Returns true if the specified index is valid and has non-zero load
316 public function isNonZeroLoad( $i );
319 * Get the number of defined servers (not the number of open connections)
323 public function getServerCount();
326 * Get the host name or IP address of the server with the specified index
329 * @return string Readable name if available or IP/host otherwise
331 public function getServerName( $i );
334 * Return the server info structure for a given index, or false if the index is invalid.
338 public function getServerInfo( $i );
341 * Get DB type of the server with the specified index
344 * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes
347 public function getServerType( $i );
350 * @param int $i Server index
351 * @return array (Database::ATTRIBUTE_* constant => value) for all such constants
354 public function getServerAttributes( $i );
357 * Get the current master position for chronology control purposes
358 * @return DBMasterPos|bool Returns false if not applicable
360 public function getMasterPos();
363 * Disable this load balancer. All connections are closed, and any attempt to
364 * open a new connection will result in a DBAccessError.
366 public function disable();
369 * Close all open connections
371 public function closeAll();
376 * Using this function makes sure the LoadBalancer knows the connection is closed.
377 * If you use $conn->close() directly, the load balancer won't update its state.
379 * @param IDatabase $conn
381 public function closeConnection( IDatabase
$conn );
384 * Commit transactions on all open connections
385 * @param string $fname Caller name
386 * @throws DBExpectedError
388 public function commitAll( $fname = __METHOD__
);
391 * Run pre-commit callbacks and defer execution of post-commit callbacks
393 * Use this only for mutli-database commits
395 * @return int Number of pre-commit callbacks run (since 1.32)
397 public function finalizeMasterChanges();
400 * Perform all pre-commit checks for things like replication safety
402 * Use this only for mutli-database commits
404 * @param array $options Includes:
405 * - maxWriteDuration : max write query duration time in seconds
406 * @throws DBTransactionError
408 public function approveMasterChanges( array $options );
411 * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
413 * The DBO_TRX setting will be reverted to the default in each of these methods:
414 * - commitMasterChanges()
415 * - rollbackMasterChanges()
417 * This allows for custom transaction rounds from any outer transaction scope.
419 * @param string $fname
420 * @throws DBExpectedError
422 public function beginMasterChanges( $fname = __METHOD__
);
425 * Issue COMMIT on all master connections where writes where done
426 * @param string $fname Caller name
427 * @throws DBExpectedError
429 public function commitMasterChanges( $fname = __METHOD__
);
432 * Consume and run all pending post-COMMIT/ROLLBACK callbacks and commit dangling transactions
434 * @return Exception|null The first exception or null if there were none
436 public function runMasterTransactionIdleCallbacks();
439 * Run all recurring post-COMMIT/ROLLBACK listener callbacks
441 * @return Exception|null The first exception or null if there were none
443 public function runMasterTransactionListenerCallbacks();
446 * Issue ROLLBACK only on master, only if queries were done on connection
447 * @param string $fname Caller name
448 * @throws DBExpectedError
450 public function rollbackMasterChanges( $fname = __METHOD__
);
453 * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
455 * @param string $fname Caller name
457 public function flushReplicaSnapshots( $fname = __METHOD__
);
460 * Commit all master DB transactions so as to flush any REPEATABLE-READ or SSI snapshots
462 * An error will be thrown if a connection has pending writes or callbacks
464 * @param string $fname Caller name
466 public function flushMasterSnapshots( $fname = __METHOD__
);
469 * @return bool Whether a master connection is already open
471 public function hasMasterConnection();
474 * Whether there are pending changes or callbacks in a transaction by this thread
477 public function hasMasterChanges();
480 * Get the timestamp of the latest write query done by this thread
481 * @return float|bool UNIX timestamp or false
483 public function lastMasterChangeTimestamp();
486 * Check if this load balancer object had any recent or still
487 * pending writes issued against it by this PHP thread
489 * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
492 public function hasOrMadeRecentMasterChanges( $age = null );
495 * Get the list of callers that have pending master changes
497 * @return string[] List of method names
499 public function pendingMasterChangeCallers();
502 * @note This method will trigger a DB connection if not yet done
503 * @param string|bool $domain Domain ID, or false for the current domain
504 * @return bool Whether the database for generic connections this request is highly "lagged"
506 public function getLaggedReplicaMode( $domain = false );
509 * Checks whether the database for generic connections this request was both:
510 * - a) Already choosen due to a prior connection attempt
511 * - b) Considered highly "lagged"
513 * @note This method will never cause a new DB connection
516 public function laggedReplicaUsed();
519 * @note This method may trigger a DB connection if not yet done
520 * @param string|bool $domain Domain ID, or false for the current domain
521 * @param IDatabase|null $conn DB master connection; used to avoid loops [optional]
522 * @return string|bool Reason the master is read-only or false if it is not
524 public function getReadOnlyReason( $domain = false, IDatabase
$conn = null );
527 * Disables/enables lag checks
528 * @param null|bool $mode
531 public function allowLagged( $mode = null );
536 public function pingAll();
539 * Call a function with each open connection object
540 * @param callable $callback
541 * @param array $params
543 public function forEachOpenConnection( $callback, array $params = [] );
546 * Call a function with each open connection object to a master
547 * @param callable $callback
548 * @param array $params
550 public function forEachOpenMasterConnection( $callback, array $params = [] );
553 * Call a function with each open replica DB connection object
554 * @param callable $callback
555 * @param array $params
557 public function forEachOpenReplicaConnection( $callback, array $params = [] );
560 * Get the hostname and lag time of the most-lagged replica DB
562 * This is useful for maintenance scripts that need to throttle their updates.
563 * May attempt to open connections to replica DBs on the default DB. If there is
564 * no lag, the maximum lag will be reported as -1.
566 * @param bool|string $domain Domain ID, or false for the default database
567 * @return array ( host, max lag, index of max lagged host )
569 public function getMaxLag( $domain = false );
572 * Get an estimate of replication lag (in seconds) for each server
574 * Results are cached for a short time in memcached/process cache
576 * Values may be "false" if replication is too broken to estimate
578 * @param string|bool $domain
579 * @return int[] Map of (server index => float|int|bool)
581 public function getLagTimes( $domain = false );
584 * Get the lag in seconds for a given connection, or zero if this load
585 * balancer does not have replication enabled.
587 * This should be used in preference to Database::getLag() in cases where
588 * replication may not be in use, since there is no way to determine if
589 * replication is in use at the connection level without running
590 * potentially restricted queries such as SHOW SLAVE STATUS. Using this
591 * function instead of Database::getLag() avoids a fatal error in this
592 * case on many installations.
594 * @param IDatabase $conn
595 * @return int|bool Returns false on error
597 public function safeGetLag( IDatabase
$conn );
600 * Wait for a replica DB to reach a specified master position
602 * This will connect to the master to get an accurate position if $pos is not given
604 * @param IDatabase $conn Replica DB
605 * @param DBMasterPos|bool $pos Master position; default: current position
606 * @param int $timeout Timeout in seconds [optional]
607 * @return bool Success
609 public function safeWaitForMasterPos( IDatabase
$conn, $pos = false, $timeout = 10 );
612 * Set a callback via IDatabase::setTransactionListener() on
613 * all current and future master connections of this load balancer
615 * @param string $name Callback name
616 * @param callable|null $callback
618 public function setTransactionListener( $name, callable
$callback = null );
621 * Set a new table prefix for the existing local domain ID for testing
623 * @param string $prefix
625 public function setDomainPrefix( $prefix );
628 * Make certain table names use their own database, schema, and table prefix
629 * when passed into SQL queries pre-escaped and without a qualified database name
631 * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
632 * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
634 * Calling this twice will completely clear any old table aliases. Also, note that
635 * callers are responsible for making sure the schemas and databases actually exist.
637 * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
639 public function setTableAliases( array $aliases );
642 * Convert certain index names to alternative names before querying the DB
644 * Note that this applies to indexes regardless of the table they belong to.
646 * This can be employed when an index was renamed X => Y in code, but the new Y-named
647 * indexes were not yet built on all DBs. After all the Y-named ones are added by the DBA,
648 * the aliases can be removed, and then the old X-named indexes dropped.
650 * @param string[] $aliases
654 public function setIndexAliases( array $aliases );