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 by index
175 * Avoid using CONN_TRX_AUTO with sqlite (e.g. check getServerType() first)
177 * @param int $i Server index or DB_MASTER/DB_REPLICA
178 * @param array|string|bool $groups Query group(s), or false for the generic reader
179 * @param string|bool $domain Domain ID, or false for the current domain
180 * @param int $flags Bitfield of CONN_* class constants
185 public function getConnection( $i, $groups = [], $domain = false, $flags = 0 );
188 * Mark a foreign connection as being available for reuse under a different DB domain
190 * This mechanism is reference-counted, and must be called the same number of times
191 * as getConnection() to work.
193 * @param IDatabase $conn
194 * @throws InvalidArgumentException
196 public function reuseConnection( $conn );
199 * Get a database connection handle reference
201 * The handle's methods simply wrap those of a Database handle
203 * Avoid using CONN_TRX_AUTO with sqlite (e.g. check getServerType() first)
205 * @see ILoadBalancer::getConnection() for parameter information
207 * @param int $i Server index or DB_MASTER/DB_REPLICA
208 * @param array|string|bool $groups Query group(s), or false for the generic reader
209 * @param string|bool $domain Domain ID, or false for the current domain
210 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTO)
213 public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
216 * Get a database connection handle reference without connecting yet
218 * The handle's methods simply wrap those of a Database handle
220 * Avoid using CONN_TRX_AUTO with sqlite (e.g. check getServerType() first)
222 * @see ILoadBalancer::getConnection() for parameter information
224 * @param int $i Server index or DB_MASTER/DB_REPLICA
225 * @param array|string|bool $groups Query group(s), or false for the generic reader
226 * @param string|bool $domain Domain ID, or false for the current domain
227 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTO)
230 public function getLazyConnectionRef( $i, $groups = [], $domain = false, $flags = 0 );
233 * Get a maintenance database connection handle reference for migrations and schema changes
235 * The handle's methods simply wrap those of a Database handle
237 * Avoid using CONN_TRX_AUTO with sqlite (e.g. check getServerType() first)
239 * @see ILoadBalancer::getConnection() for parameter information
241 * @param int $db Server index or DB_MASTER/DB_REPLICA
242 * @param array|string|bool $groups Query group(s), or false for the generic reader
243 * @param string|bool $domain Domain ID, or false for the current domain
244 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTO)
245 * @return MaintainableDBConnRef
247 public function getMaintenanceConnectionRef( $db, $groups = [], $domain = false, $flags = 0 );
250 * Open a connection to the server given by the specified index
252 * The index must be an actual index into the array. If a connection to the server is
253 * already open and not considered an "in use" foreign connection, this simply returns it.
255 * Avoid using CONN_TRX_AUTO with sqlite (e.g. check getServerType() first)
257 * @note If disable() was called on this LoadBalancer, this method will throw a DBAccessError.
259 * @param int $i Server index (does not support DB_MASTER/DB_REPLICA)
260 * @param string|bool $domain Domain ID, or false for the current domain
261 * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTO)
262 * @return Database|bool Returns false on errors
263 * @throws DBAccessError
265 public function openConnection( $i, $domain = false, $flags = 0 );
270 public function getWriterIndex();
273 * Returns true if the specified index is a valid server index
278 public function haveIndex( $i );
281 * Returns true if the specified index is valid and has non-zero load
286 public function isNonZeroLoad( $i );
289 * Get the number of defined servers (not the number of open connections)
293 public function getServerCount();
296 * Get the host name or IP address of the server with the specified index
299 * @return string Readable name if available or IP/host otherwise
301 public function getServerName( $i );
304 * Get DB type of the server with the specified index
307 * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes
310 public function getServerType( $i );
313 * Get the current master position for chronology control purposes
314 * @return DBMasterPos|bool Returns false if not applicable
316 public function getMasterPos();
319 * Disable this load balancer. All connections are closed, and any attempt to
320 * open a new connection will result in a DBAccessError.
322 public function disable();
325 * Close all open connections
327 public function closeAll();
332 * Using this function makes sure the LoadBalancer knows the connection is closed.
333 * If you use $conn->close() directly, the load balancer won't update its state.
335 * @param IDatabase $conn
337 public function closeConnection( IDatabase
$conn );
340 * Commit transactions on all open connections
341 * @param string $fname Caller name
342 * @throws DBExpectedError
344 public function commitAll( $fname = __METHOD__
);
347 * Perform all pre-commit callbacks that remain part of the atomic transactions
348 * and disable any post-commit callbacks until runMasterPostTrxCallbacks()
350 * Use this only for mutli-database commits
352 public function finalizeMasterChanges();
355 * Perform all pre-commit checks for things like replication safety
357 * Use this only for mutli-database commits
359 * @param array $options Includes:
360 * - maxWriteDuration : max write query duration time in seconds
361 * @throws DBTransactionError
363 public function approveMasterChanges( array $options );
366 * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
368 * The DBO_TRX setting will be reverted to the default in each of these methods:
369 * - commitMasterChanges()
370 * - rollbackMasterChanges()
372 * This allows for custom transaction rounds from any outer transaction scope.
374 * @param string $fname
375 * @throws DBExpectedError
377 public function beginMasterChanges( $fname = __METHOD__
);
380 * Issue COMMIT on all master connections where writes where done
381 * @param string $fname Caller name
382 * @throws DBExpectedError
384 public function commitMasterChanges( $fname = __METHOD__
);
387 * Issue all pending post-COMMIT/ROLLBACK callbacks
389 * Use this only for mutli-database commits
391 * @param int $type IDatabase::TRIGGER_* constant
392 * @return Exception|null The first exception or null if there were none
394 public function runMasterPostTrxCallbacks( $type );
397 * Issue ROLLBACK only on master, only if queries were done on connection
398 * @param string $fname Caller name
399 * @throws DBExpectedError
401 public function rollbackMasterChanges( $fname = __METHOD__
);
404 * Suppress all pending post-COMMIT/ROLLBACK callbacks
406 * Use this only for mutli-database commits
408 * @return Exception|null The first exception or null if there were none
410 public function suppressTransactionEndCallbacks();
413 * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshot
415 * @param string $fname Caller name
417 public function flushReplicaSnapshots( $fname = __METHOD__
);
420 * @return bool Whether a master connection is already open
422 public function hasMasterConnection();
425 * Determine if there are pending changes in a transaction by this thread
428 public function hasMasterChanges();
431 * Get the timestamp of the latest write query done by this thread
432 * @return float|bool UNIX timestamp or false
434 public function lastMasterChangeTimestamp();
437 * Check if this load balancer object had any recent or still
438 * pending writes issued against it by this PHP thread
440 * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
443 public function hasOrMadeRecentMasterChanges( $age = null );
446 * Get the list of callers that have pending master changes
448 * @return string[] List of method names
450 public function pendingMasterChangeCallers();
453 * @note This method will trigger a DB connection if not yet done
454 * @param string|bool $domain Domain ID, or false for the current domain
455 * @return bool Whether the database for generic connections this request is highly "lagged"
457 public function getLaggedReplicaMode( $domain = false );
460 * Checks whether the database for generic connections this request was both:
461 * - a) Already choosen due to a prior connection attempt
462 * - b) Considered highly "lagged"
464 * @note This method will never cause a new DB connection
467 public function laggedReplicaUsed();
470 * @note This method may trigger a DB connection if not yet done
471 * @param string|bool $domain Domain ID, or false for the current domain
472 * @param IDatabase|null $conn DB master connection; used to avoid loops [optional]
473 * @return string|bool Reason the master is read-only or false if it is not
475 public function getReadOnlyReason( $domain = false, IDatabase
$conn = null );
478 * Disables/enables lag checks
479 * @param null|bool $mode
482 public function allowLagged( $mode = null );
487 public function pingAll();
490 * Call a function with each open connection object
491 * @param callable $callback
492 * @param array $params
494 public function forEachOpenConnection( $callback, array $params = [] );
497 * Call a function with each open connection object to a master
498 * @param callable $callback
499 * @param array $params
501 public function forEachOpenMasterConnection( $callback, array $params = [] );
504 * Call a function with each open replica DB connection object
505 * @param callable $callback
506 * @param array $params
508 public function forEachOpenReplicaConnection( $callback, array $params = [] );
511 * Get the hostname and lag time of the most-lagged replica DB
513 * This is useful for maintenance scripts that need to throttle their updates.
514 * May attempt to open connections to replica DBs on the default DB. If there is
515 * no lag, the maximum lag will be reported as -1.
517 * @param bool|string $domain Domain ID, or false for the default database
518 * @return array ( host, max lag, index of max lagged host )
520 public function getMaxLag( $domain = false );
523 * Get an estimate of replication lag (in seconds) for each server
525 * Results are cached for a short time in memcached/process cache
527 * Values may be "false" if replication is too broken to estimate
529 * @param string|bool $domain
530 * @return int[] Map of (server index => float|int|bool)
532 public function getLagTimes( $domain = false );
535 * Get the lag in seconds for a given connection, or zero if this load
536 * balancer does not have replication enabled.
538 * This should be used in preference to Database::getLag() in cases where
539 * replication may not be in use, since there is no way to determine if
540 * replication is in use at the connection level without running
541 * potentially restricted queries such as SHOW SLAVE STATUS. Using this
542 * function instead of Database::getLag() avoids a fatal error in this
543 * case on many installations.
545 * @param IDatabase $conn
546 * @return int|bool Returns false on error
548 public function safeGetLag( IDatabase
$conn );
551 * Wait for a replica DB to reach a specified master position
553 * This will connect to the master to get an accurate position if $pos is not given
555 * @param IDatabase $conn Replica DB
556 * @param DBMasterPos|bool $pos Master position; default: current position
557 * @param int $timeout Timeout in seconds [optional]
558 * @return bool Success
560 public function safeWaitForMasterPos( IDatabase
$conn, $pos = false, $timeout = 10 );
563 * Set a callback via IDatabase::setTransactionListener() on
564 * all current and future master connections of this load balancer
566 * @param string $name Callback name
567 * @param callable|null $callback
569 public function setTransactionListener( $name, callable
$callback = null );
572 * Set a new table prefix for the existing local domain ID for testing
574 * @param string $prefix
576 public function setDomainPrefix( $prefix );
579 * Make certain table names use their own database, schema, and table prefix
580 * when passed into SQL queries pre-escaped and without a qualified database name
582 * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
583 * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
585 * Calling this twice will completely clear any old table aliases. Also, note that
586 * callers are responsible for making sure the schemas and databases actually exist.
588 * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
590 public function setTableAliases( array $aliases );