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_AUTO
= 1;
91 * Construct a manager of IDatabase connection objects
93 * @param array $params Parameter map with keys:
94 * - servers : Required. Array of server info structures.
95 * - localDomain: A DatabaseDomain or domain ID string.
96 * - loadMonitor : Name of a class used to fetch server lag and load.
97 * - readOnlyReason : Reason the master DB is read-only if so [optional]
98 * - waitTimeout : Maximum time to wait for replicas for consistency [optional]
99 * - maxLag: Avoid replica DB servers with more lag than this [optional]
100 * - srvCache : BagOStuff object for server cache [optional]
101 * - wanCache : WANObjectCache object [optional]
102 * - chronologyProtector: ChronologyProtector object [optional]
103 * - hostname : The name of the current server [optional]
104 * - cliMode: Whether the execution context is a CLI script. [optional]
105 * - profiler : Class name or instance with profileIn()/profileOut() methods. [optional]
106 * - trxProfiler: TransactionProfiler instance. [optional]
107 * - replLogger: PSR-3 logger instance. [optional]
108 * - connLogger: PSR-3 logger instance. [optional]
109 * - queryLogger: PSR-3 logger instance. [optional]
110 * - perfLogger: PSR-3 logger instance. [optional]
111 * - errorLogger : Callback that takes an Exception and logs it. [optional]
112 * @throws InvalidArgumentException
114 public function __construct( array $params );
117 * Get the index of the reader connection, which may be a replica DB
119 * This takes into account load ratios and lag times. It should
120 * always return a consistent index during a given invocation.
122 * Side effect: opens connections to databases
123 * @param string|bool $group Query group, or false for the generic reader
124 * @param string|bool $domain Domain ID, or false for the current domain
126 * @return bool|int|string
128 public function getReaderIndex( $group = false, $domain = false );
131 * Set the master wait position
133 * If a DB_REPLICA connection has been opened already, then wait immediately.
134 * Otherwise sets a variable telling it to wait if such a connection is opened.
136 * This only applies to connections to the generic replica DB for this request.
137 * If a timeout happens when waiting, then getLaggedReplicaMode()/laggedReplicaUsed()
140 * @param DBMasterPos|bool $pos Master position or false
142 public function waitFor( $pos );
145 * Set the master wait position and wait for a "generic" replica DB to catch up to it
147 * This can be used a faster proxy for waitForAll()
149 * @param DBMasterPos|bool $pos Master position or false
150 * @param int $timeout Max seconds to wait; default is mWaitTimeout
151 * @return bool Success (able to connect and no timeouts reached)
153 public function waitForOne( $pos, $timeout = null );
156 * Set the master wait position and wait for ALL replica DBs to catch up to it
158 * @param DBMasterPos|bool $pos Master position or false
159 * @param int $timeout Max seconds to wait; default is mWaitTimeout
160 * @return bool Success (able to connect and no timeouts reached)
162 public function waitForAll( $pos, $timeout = null );
165 * Get any open connection to a given server index, local or foreign
167 * @param int $i Server index or DB_MASTER/DB_REPLICA
168 * @return Database|bool False if no such connection is open
170 public function getAnyOpenConnection( $i );
173 * Get a connection handle by server index
175 * Avoid using CONN_TRX_AUTO with sqlite (e.g. check getServerType() first)
177 * If the caller uses $domain or sets CONN_TRX_AUTO in $flags, then it must also
178 * call ILoadBalancer::reuseConnection() on the handle when finished using it.
179 * In all other cases, this is not necessary, though not harmful either.
181 * @param int $i Server index or DB_MASTER/DB_REPLICA
182 * @param array|string|bool $groups Query group(s), or false for the generic reader
183 * @param string|bool $domain Domain ID, or false for the current domain
184 * @param int $flags Bitfield of CONN_* class constants
186 * @note This method throws DBAccessError if ILoadBalancer::disable() was called
191 public function getConnection( $i, $groups = [], $domain = false, $flags = 0 );
194 * Mark a foreign connection as being available for reuse under a different DB domain
196 * This mechanism is reference-counted, and must be called the same number of times
197 * as getConnection() to work.
199 * @param IDatabase $conn
200 * @throws InvalidArgumentException
202 public function reuseConnection( IDatabase
$conn );
205 * Get a database connection handle reference
207 * The handle's methods simply wrap those of a Database handle
209 * Avoid using CONN_TRX_AUTO with sqlite (e.g. check getServerType() first)
211 * @see ILoadBalancer::getConnection() for parameter information
213 * @param int $i Server index or DB_MASTER/DB_REPLICA
214 * @param array|string|bool $groups Query group(s), or false for the generic reader
215 * @param string|bool $domain Domain ID, or false for the current domain
216 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTO)
219 public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
222 * Get a database connection handle reference without connecting yet
224 * The handle's methods simply wrap those of a Database handle
226 * Avoid using CONN_TRX_AUTO with sqlite (e.g. check getServerType() first)
228 * @see ILoadBalancer::getConnection() for parameter information
230 * @param int $i Server index or DB_MASTER/DB_REPLICA
231 * @param array|string|bool $groups Query group(s), or false for the generic reader
232 * @param string|bool $domain Domain ID, or false for the current domain
233 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTO)
236 public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
239 * Get a maintenance database connection handle reference for migrations and schema changes
241 * The handle's methods simply wrap those of a Database handle
243 * Avoid using CONN_TRX_AUTO with sqlite (e.g. check getServerType() first)
245 * @see ILoadBalancer::getConnection() for parameter information
247 * @param int $db Server index or DB_MASTER/DB_REPLICA
248 * @param array|string|bool $groups Query group(s), or false for the generic reader
249 * @param string|bool $domain Domain ID, or false for the current domain
250 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTO)
251 * @return MaintainableDBConnRef
253 public function getMaintenanceConnectionRef( $db, $groups = [], $domain = false, $flags = 0 );
256 * Open a connection to the server given by the specified index
258 * The index must be an actual index into the array. If a connection to the server is
259 * already open and not considered an "in use" foreign connection, this simply returns it.
261 * Avoid using CONN_TRX_AUTO with sqlite (e.g. check getServerType() first)
263 * If the caller uses $domain or sets CONN_TRX_AUTO in $flags, then it must also
264 * call ILoadBalancer::reuseConnection() on the handle when finished using it.
265 * In all other cases, this is not necessary, though not harmful either.
267 * @note This method throws DBAccessError if ILoadBalancer::disable() was called
269 * @param int $i Server index (does not support DB_MASTER/DB_REPLICA)
270 * @param string|bool $domain Domain ID, or false for the current domain
271 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTO)
272 * @return Database|bool Returns false on errors
273 * @throws DBAccessError
275 public function openConnection( $i, $domain = false, $flags = 0 );
280 public function getWriterIndex();
283 * Returns true if the specified index is a valid server index
288 public function haveIndex( $i );
291 * Returns true if the specified index is valid and has non-zero load
296 public function isNonZeroLoad( $i );
299 * Get the number of defined servers (not the number of open connections)
303 public function getServerCount();
306 * Get the host name or IP address of the server with the specified index
309 * @return string Readable name if available or IP/host otherwise
311 public function getServerName( $i );
314 * Get DB type of the server with the specified index
317 * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes
320 public function getServerType( $i );
323 * Get the current master position for chronology control purposes
324 * @return DBMasterPos|bool Returns false if not applicable
326 public function getMasterPos();
329 * Disable this load balancer. All connections are closed, and any attempt to
330 * open a new connection will result in a DBAccessError.
332 public function disable();
335 * Close all open connections
337 public function closeAll();
342 * Using this function makes sure the LoadBalancer knows the connection is closed.
343 * If you use $conn->close() directly, the load balancer won't update its state.
345 * @param IDatabase $conn
347 public function closeConnection( IDatabase
$conn );
350 * Commit transactions on all open connections
351 * @param string $fname Caller name
352 * @throws DBExpectedError
354 public function commitAll( $fname = __METHOD__
);
357 * Perform all pre-commit callbacks that remain part of the atomic transactions
358 * and disable any post-commit callbacks until runMasterPostTrxCallbacks()
360 * Use this only for mutli-database commits
362 public function finalizeMasterChanges();
365 * Perform all pre-commit checks for things like replication safety
367 * Use this only for mutli-database commits
369 * @param array $options Includes:
370 * - maxWriteDuration : max write query duration time in seconds
371 * @throws DBTransactionError
373 public function approveMasterChanges( array $options );
376 * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
378 * The DBO_TRX setting will be reverted to the default in each of these methods:
379 * - commitMasterChanges()
380 * - rollbackMasterChanges()
382 * This allows for custom transaction rounds from any outer transaction scope.
384 * @param string $fname
385 * @throws DBExpectedError
387 public function beginMasterChanges( $fname = __METHOD__
);
390 * Issue COMMIT on all master connections where writes where done
391 * @param string $fname Caller name
392 * @throws DBExpectedError
394 public function commitMasterChanges( $fname = __METHOD__
);
397 * Issue all pending post-COMMIT/ROLLBACK callbacks
399 * Use this only for mutli-database commits
401 * @param int $type IDatabase::TRIGGER_* constant
402 * @return Exception|null The first exception or null if there were none
404 public function runMasterPostTrxCallbacks( $type );
407 * Issue ROLLBACK only on master, only if queries were done on connection
408 * @param string $fname Caller name
409 * @throws DBExpectedError
411 public function rollbackMasterChanges( $fname = __METHOD__
);
414 * Suppress all pending post-COMMIT/ROLLBACK callbacks
416 * Use this only for mutli-database commits
418 * @return Exception|null The first exception or null if there were none
420 public function suppressTransactionEndCallbacks();
423 * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshot
425 * @param string $fname Caller name
427 public function flushReplicaSnapshots( $fname = __METHOD__
);
430 * @return bool Whether a master connection is already open
432 public function hasMasterConnection();
435 * Determine if there are pending changes in a transaction by this thread
438 public function hasMasterChanges();
441 * Get the timestamp of the latest write query done by this thread
442 * @return float|bool UNIX timestamp or false
444 public function lastMasterChangeTimestamp();
447 * Check if this load balancer object had any recent or still
448 * pending writes issued against it by this PHP thread
450 * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
453 public function hasOrMadeRecentMasterChanges( $age = null );
456 * Get the list of callers that have pending master changes
458 * @return string[] List of method names
460 public function pendingMasterChangeCallers();
463 * @note This method will trigger a DB connection if not yet done
464 * @param string|bool $domain Domain ID, or false for the current domain
465 * @return bool Whether the database for generic connections this request is highly "lagged"
467 public function getLaggedReplicaMode( $domain = false );
470 * Checks whether the database for generic connections this request was both:
471 * - a) Already choosen due to a prior connection attempt
472 * - b) Considered highly "lagged"
474 * @note This method will never cause a new DB connection
477 public function laggedReplicaUsed();
480 * @note This method may trigger a DB connection if not yet done
481 * @param string|bool $domain Domain ID, or false for the current domain
482 * @param IDatabase|null $conn DB master connection; used to avoid loops [optional]
483 * @return string|bool Reason the master is read-only or false if it is not
485 public function getReadOnlyReason( $domain = false, IDatabase
$conn = null );
488 * Disables/enables lag checks
489 * @param null|bool $mode
492 public function allowLagged( $mode = null );
497 public function pingAll();
500 * Call a function with each open connection object
501 * @param callable $callback
502 * @param array $params
504 public function forEachOpenConnection( $callback, array $params = [] );
507 * Call a function with each open connection object to a master
508 * @param callable $callback
509 * @param array $params
511 public function forEachOpenMasterConnection( $callback, array $params = [] );
514 * Call a function with each open replica DB connection object
515 * @param callable $callback
516 * @param array $params
518 public function forEachOpenReplicaConnection( $callback, array $params = [] );
521 * Get the hostname and lag time of the most-lagged replica DB
523 * This is useful for maintenance scripts that need to throttle their updates.
524 * May attempt to open connections to replica DBs on the default DB. If there is
525 * no lag, the maximum lag will be reported as -1.
527 * @param bool|string $domain Domain ID, or false for the default database
528 * @return array ( host, max lag, index of max lagged host )
530 public function getMaxLag( $domain = false );
533 * Get an estimate of replication lag (in seconds) for each server
535 * Results are cached for a short time in memcached/process cache
537 * Values may be "false" if replication is too broken to estimate
539 * @param string|bool $domain
540 * @return int[] Map of (server index => float|int|bool)
542 public function getLagTimes( $domain = false );
545 * Get the lag in seconds for a given connection, or zero if this load
546 * balancer does not have replication enabled.
548 * This should be used in preference to Database::getLag() in cases where
549 * replication may not be in use, since there is no way to determine if
550 * replication is in use at the connection level without running
551 * potentially restricted queries such as SHOW SLAVE STATUS. Using this
552 * function instead of Database::getLag() avoids a fatal error in this
553 * case on many installations.
555 * @param IDatabase $conn
556 * @return int|bool Returns false on error
558 public function safeGetLag( IDatabase
$conn );
561 * Wait for a replica DB to reach a specified master position
563 * This will connect to the master to get an accurate position if $pos is not given
565 * @param IDatabase $conn Replica DB
566 * @param DBMasterPos|bool $pos Master position; default: current position
567 * @param int $timeout Timeout in seconds [optional]
568 * @return bool Success
570 public function safeWaitForMasterPos( IDatabase
$conn, $pos = false, $timeout = 10 );
573 * Set a callback via IDatabase::setTransactionListener() on
574 * all current and future master connections of this load balancer
576 * @param string $name Callback name
577 * @param callable|null $callback
579 public function setTransactionListener( $name, callable
$callback = null );
582 * Set a new table prefix for the existing local domain ID for testing
584 * @param string $prefix
586 public function setDomainPrefix( $prefix );
589 * Make certain table names use their own database, schema, and table prefix
590 * when passed into SQL queries pre-escaped and without a qualified database name
592 * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
593 * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
595 * Calling this twice will completely clear any old table aliases. Also, note that
596 * callers are responsible for making sure the schemas and databases actually exist.
598 * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
600 public function setTableAliases( array $aliases );