3 * Database load balancing interface
5 * This program is free software; you can redistribute it and/or modify
6 * it under the terms of the GNU General Public License as published by
7 * the Free Software Foundation; either version 2 of the License, or
8 * (at your option) any later version.
10 * This program is distributed in the hope that it will be useful,
11 * but WITHOUT ANY WARRANTY; without even the implied warranty of
12 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
13 * GNU General Public License for more details.
15 * You should have received a copy of the GNU General Public License along
16 * with this program; if not, write to the Free Software Foundation, Inc.,
17 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
18 * http://www.gnu.org/copyleft/gpl.html
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 integer Request a replica DB connection */
80 const DB_REPLICA
= -1;
81 /** @var integer Request a master DB connection */
84 /** @var string Domain specifier when no specific database needs to be selected */
85 const DOMAIN_ANY
= '';
88 * Construct a manager of IDatabase connection objects
90 * @param array $params Parameter map with keys:
91 * - servers : Required. Array of server info structures.
92 * - localDomain: A DatabaseDomain or domain ID string.
93 * - loadMonitor : Name of a class used to fetch server lag and load.
94 * - readOnlyReason : Reason the master DB is read-only if so [optional]
95 * - waitTimeout : Maximum time to wait for replicas for consistency [optional]
96 * - srvCache : BagOStuff object for server cache [optional]
97 * - memCache : BagOStuff object for cluster memory cache [optional]
98 * - wanCache : WANObjectCache object [optional]
99 * - chronologyProtector: ChronologyProtector object [optional]
100 * - hostname : The name of the current server [optional]
101 * - cliMode: Whether the execution context is a CLI script. [optional]
102 * - profiler : Class name or instance with profileIn()/profileOut() methods. [optional]
103 * - trxProfiler: TransactionProfiler instance. [optional]
104 * - replLogger: PSR-3 logger instance. [optional]
105 * - connLogger: PSR-3 logger instance. [optional]
106 * - queryLogger: PSR-3 logger instance. [optional]
107 * - perfLogger: PSR-3 logger instance. [optional]
108 * - errorLogger : Callback that takes an Exception and logs it. [optional]
109 * @throws InvalidArgumentException
111 public function __construct( array $params );
114 * Get the index of the reader connection, which may be a replica DB
116 * This takes into account load ratios and lag times. It should
117 * always return a consistent index during a given invocation.
119 * Side effect: opens connections to databases
120 * @param string|bool $group Query group, or false for the generic reader
121 * @param string|bool $domain Domain ID, or false for the current domain
123 * @return bool|int|string
125 public function getReaderIndex( $group = false, $domain = false );
128 * Set the master wait position
130 * If a DB_REPLICA connection has been opened already, then wait immediately.
131 * Otherwise sets a variable telling it to wait if such a connection is opened.
133 * This only applies to connections to the generic replica DB for this request.
134 * If a timeout happens when waiting, then getLaggedReplicaMode()/laggedReplicaUsed()
137 * @param DBMasterPos|bool $pos Master position or false
139 public function waitFor( $pos );
142 * Set the master wait position and wait for a "generic" replica DB to catch up to it
144 * This can be used a faster proxy for waitForAll()
146 * @param DBMasterPos|bool $pos Master position or false
147 * @param int $timeout Max seconds to wait; default is mWaitTimeout
148 * @return bool Success (able to connect and no timeouts reached)
150 public function waitForOne( $pos, $timeout = null );
153 * Set the master wait position and wait for ALL replica DBs to catch up to it
155 * @param DBMasterPos|bool $pos Master position or false
156 * @param int $timeout Max seconds to wait; default is mWaitTimeout
157 * @return bool Success (able to connect and no timeouts reached)
159 public function waitForAll( $pos, $timeout = null );
162 * Get any open connection to a given server index, local or foreign
164 * @param int $i Server index or DB_MASTER/DB_REPLICA
165 * @return Database|bool False if no such connection is open
167 public function getAnyOpenConnection( $i );
170 * Get a connection by index
172 * @param int $i Server index or DB_MASTER/DB_REPLICA
173 * @param array|string|bool $groups Query group(s), or false for the generic reader
174 * @param string|bool $domain Domain ID, or false for the current domain
179 public function getConnection( $i, $groups = [], $domain = false );
182 * Mark a foreign connection as being available for reuse under a different DB domain
184 * This mechanism is reference-counted, and must be called the same number of times
185 * as getConnection() to work.
187 * @param IDatabase $conn
188 * @throws InvalidArgumentException
190 public function reuseConnection( $conn );
193 * Get a database connection handle reference
195 * The handle's methods simply wrap those of a Database handle
197 * @see ILoadBalancer::getConnection() for parameter information
199 * @param int $i Server index or DB_MASTER/DB_REPLICA
200 * @param array|string|bool $groups Query group(s), or false for the generic reader
201 * @param string|bool $domain Domain ID, or false for the current domain
204 public function getConnectionRef( $i, $groups = [], $domain = false );
207 * Get a database connection handle reference without connecting yet
209 * The handle's methods simply wrap those of a Database handle
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
218 public function getLazyConnectionRef( $i, $groups = [], $domain = false );
221 * Get a maintenance database connection handle reference for migrations and schema changes
223 * The handle's methods simply wrap those of a Database handle
225 * @see ILoadBalancer::getConnection() for parameter information
227 * @param int $db 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 * @return MaintainableDBConnRef
232 public function getMaintenanceConnectionRef( $db, $groups = [], $domain = false );
235 * Open a connection to the server given by the specified index
236 * Index must be an actual index into the array.
237 * If the server is already open, returns it.
239 * @note If disable() was called on this LoadBalancer, this method will throw a DBAccessError.
241 * @param int $i Server index or DB_MASTER/DB_REPLICA
242 * @param string|bool $domain Domain ID, or false for the current domain
243 * @return Database|bool Returns false on errors
244 * @throws DBAccessError
246 public function openConnection( $i, $domain = false );
251 public function getWriterIndex();
254 * Returns true if the specified index is a valid server index
259 public function haveIndex( $i );
262 * Returns true if the specified index is valid and has non-zero load
267 public function isNonZeroLoad( $i );
270 * Get the number of defined servers (not the number of open connections)
274 public function getServerCount();
277 * Get the host name or IP address of the server with the specified index
278 * Prefer a readable name if available.
282 public function getServerName( $i );
285 * Return the server info structure for a given index, or false if the index is invalid.
289 * @deprecated Since 1.30, no alternative
291 public function getServerInfo( $i );
294 * Sets the server info structure for the given index. Entry at index $i
295 * is created if it doesn't exist
297 * @param array $serverInfo
299 * @deprecated Since 1.30, construct new object
301 public function setServerInfo( $i, array $serverInfo );
304 * Get the current master position for chronology control purposes
305 * @return DBMasterPos|bool Returns false if not applicable
307 public function getMasterPos();
310 * Disable this load balancer. All connections are closed, and any attempt to
311 * open a new connection will result in a DBAccessError.
313 public function disable();
316 * Close all open connections
318 public function closeAll();
323 * Using this function makes sure the LoadBalancer knows the connection is closed.
324 * If you use $conn->close() directly, the load balancer won't update its state.
326 * @param IDatabase $conn
328 public function closeConnection( IDatabase
$conn );
331 * Commit transactions on all open connections
332 * @param string $fname Caller name
333 * @throws DBExpectedError
335 public function commitAll( $fname = __METHOD__
);
338 * Perform all pre-commit callbacks that remain part of the atomic transactions
339 * and disable any post-commit callbacks until runMasterPostTrxCallbacks()
341 * Use this only for mutli-database commits
343 public function finalizeMasterChanges();
346 * Perform all pre-commit checks for things like replication safety
348 * Use this only for mutli-database commits
350 * @param array $options Includes:
351 * - maxWriteDuration : max write query duration time in seconds
352 * @throws DBTransactionError
354 public function approveMasterChanges( array $options );
357 * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
359 * The DBO_TRX setting will be reverted to the default in each of these methods:
360 * - commitMasterChanges()
361 * - rollbackMasterChanges()
363 * This allows for custom transaction rounds from any outer transaction scope.
365 * @param string $fname
366 * @throws DBExpectedError
368 public function beginMasterChanges( $fname = __METHOD__
);
371 * Issue COMMIT on all master connections where writes where done
372 * @param string $fname Caller name
373 * @throws DBExpectedError
375 public function commitMasterChanges( $fname = __METHOD__
);
378 * Issue all pending post-COMMIT/ROLLBACK callbacks
380 * Use this only for mutli-database commits
382 * @param int $type IDatabase::TRIGGER_* constant
383 * @return Exception|null The first exception or null if there were none
385 public function runMasterPostTrxCallbacks( $type );
388 * Issue ROLLBACK only on master, only if queries were done on connection
389 * @param string $fname Caller name
390 * @throws DBExpectedError
392 public function rollbackMasterChanges( $fname = __METHOD__
);
395 * Suppress all pending post-COMMIT/ROLLBACK callbacks
397 * Use this only for mutli-database commits
399 * @return Exception|null The first exception or null if there were none
401 public function suppressTransactionEndCallbacks();
404 * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshot
406 * @param string $fname Caller name
408 public function flushReplicaSnapshots( $fname = __METHOD__
);
411 * @return bool Whether a master connection is already open
413 public function hasMasterConnection();
416 * Determine if there are pending changes in a transaction by this thread
419 public function hasMasterChanges();
422 * Get the timestamp of the latest write query done by this thread
423 * @return float|bool UNIX timestamp or false
425 public function lastMasterChangeTimestamp();
428 * Check if this load balancer object had any recent or still
429 * pending writes issued against it by this PHP thread
431 * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
434 public function hasOrMadeRecentMasterChanges( $age = null );
437 * Get the list of callers that have pending master changes
439 * @return string[] List of method names
441 public function pendingMasterChangeCallers();
444 * @note This method will trigger a DB connection if not yet done
445 * @param string|bool $domain Domain ID, or false for the current domain
446 * @return bool Whether the database for generic connections this request is highly "lagged"
448 public function getLaggedReplicaMode( $domain = false );
451 * Checks whether the database for generic connections this request was both:
452 * - a) Already choosen due to a prior connection attempt
453 * - b) Considered highly "lagged"
455 * @note This method will never cause a new DB connection
458 public function laggedReplicaUsed();
461 * @note This method may trigger a DB connection if not yet done
462 * @param string|bool $domain Domain ID, or false for the current domain
463 * @param IDatabase|null $conn DB master connection; used to avoid loops [optional]
464 * @return string|bool Reason the master is read-only or false if it is not
466 public function getReadOnlyReason( $domain = false, IDatabase
$conn = null );
469 * Disables/enables lag checks
470 * @param null|bool $mode
473 public function allowLagged( $mode = null );
478 public function pingAll();
481 * Call a function with each open connection object
482 * @param callable $callback
483 * @param array $params
485 public function forEachOpenConnection( $callback, array $params = [] );
488 * Call a function with each open connection object to a master
489 * @param callable $callback
490 * @param array $params
492 public function forEachOpenMasterConnection( $callback, array $params = [] );
495 * Call a function with each open replica DB connection object
496 * @param callable $callback
497 * @param array $params
499 public function forEachOpenReplicaConnection( $callback, array $params = [] );
502 * Get the hostname and lag time of the most-lagged replica DB
504 * This is useful for maintenance scripts that need to throttle their updates.
505 * May attempt to open connections to replica DBs on the default DB. If there is
506 * no lag, the maximum lag will be reported as -1.
508 * @param bool|string $domain Domain ID, or false for the default database
509 * @return array ( host, max lag, index of max lagged host )
511 public function getMaxLag( $domain = false );
514 * Get an estimate of replication lag (in seconds) for each server
516 * Results are cached for a short time in memcached/process cache
518 * Values may be "false" if replication is too broken to estimate
520 * @param string|bool $domain
521 * @return int[] Map of (server index => float|int|bool)
523 public function getLagTimes( $domain = false );
526 * Get the lag in seconds for a given connection, or zero if this load
527 * balancer does not have replication enabled.
529 * This should be used in preference to Database::getLag() in cases where
530 * replication may not be in use, since there is no way to determine if
531 * replication is in use at the connection level without running
532 * potentially restricted queries such as SHOW SLAVE STATUS. Using this
533 * function instead of Database::getLag() avoids a fatal error in this
534 * case on many installations.
536 * @param IDatabase $conn
537 * @return int|bool Returns false on error
539 public function safeGetLag( IDatabase
$conn );
542 * Wait for a replica DB to reach a specified master position
544 * This will connect to the master to get an accurate position if $pos is not given
546 * @param IDatabase $conn Replica DB
547 * @param DBMasterPos|bool $pos Master position; default: current position
548 * @param int $timeout Timeout in seconds [optional]
549 * @return bool Success
551 public function safeWaitForMasterPos( IDatabase
$conn, $pos = false, $timeout = 10 );
554 * Set a callback via IDatabase::setTransactionListener() on
555 * all current and future master connections of this load balancer
557 * @param string $name Callback name
558 * @param callable|null $callback
560 public function setTransactionListener( $name, callable
$callback = null );
563 * Set a new table prefix for the existing local domain ID for testing
565 * @param string $prefix
567 public function setDomainPrefix( $prefix );
570 * Make certain table names use their own database, schema, and table prefix
571 * when passed into SQL queries pre-escaped and without a qualified database name
573 * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
574 * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
576 * Calling this twice will completely clear any old table aliases. Also, note that
577 * callers are responsible for making sure the schemas and databases actually exist.
579 * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
581 public function setTableAliases( array $aliases );