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[mediawiki.git] / includes / libs / rdbms / loadbalancer / ILoadBalancer.php

<?php
/**
 * Database load balancing interface
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup Database
 * @author Aaron Schulz
 */

/**
 * Database cluster connection, tracking, load balancing, and transaction manager interface
 *
 * A "cluster" is considered to be one master database and zero or more replica databases.
 * Typically, the replica DBs replicate from the master asynchronously. The first node in the
 * "servers" configuration array is always considered the "master". However, this class can still
 * be used when all or some of the "replica" DBs are multi-master peers of the master or even
 * when all the DBs are non-replicating clones of each other holding read-only data. Thus, the
 * role of "master" is in some cases merely nominal.
 *
 * By default, each DB server uses DBO_DEFAULT for its 'flags' setting, unless explicitly set
 * otherwise in configuration. DBO_DEFAULT behavior depends on whether 'cliMode' is set:
 *   - In CLI mode, the flag has no effect with regards to LoadBalancer.
 *   - In non-CLI mode, the flag causes implicit transactions to be used; the first query on
 *     a database starts a transaction on that database. The transactions are meant to remain
 *     pending until either commitMasterChanges() or rollbackMasterChanges() is called. The
 *     application must have some point where it calls commitMasterChanges() near the end of
 *     the PHP request.
 * Every iteration of beginMasterChanges()/commitMasterChanges() is called a "transaction round".
 * Rounds are useful on the master DB connections because they make single-DB (and by and large
 * multi-DB) updates in web requests all-or-nothing. Also, transactions on replica DBs are useful
 * when REPEATABLE-READ or SERIALIZABLE isolation is used because all foriegn keys and constraints
 * hold across separate queries in the DB transaction since the data appears within a consistent
 * point-in-time snapshot.
 *
 * The typical caller will use LoadBalancer::getConnection( DB_* ) to yield a live database
 * connection handle. The choice of which DB server to use is based on pre-defined loads for
 * weighted random selection, adjustments thereof by LoadMonitor, and the amount of replication
 * lag on each DB server. Lag checks might cause problems in certain setups, so they should be
 * tuned in the server configuration maps as follows:
 *   - Master + N Replica(s): set 'max lag' to an appropriate threshold for avoiding any database
 *      lagged by this much or more. If all DBs are this lagged, then the load balancer considers
 *      the cluster to be read-only.
 *   - Galera Cluster: Seconds_Behind_Master will be 0, so there probably is nothing to tune.
 *      Note that lag is still possible depending on how wsrep-sync-wait is set server-side.
 *   - Read-only archive clones: set 'is static' in the server configuration maps. This will
 *      treat all such DBs as having 0 lag.
 *   - SQL load balancing proxy: any proxy should handle lag checks on its own, so the 'max lag'
 *      parameter should probably be set to INF in the server configuration maps. This will make
 *      the load balancer ignore whatever it detects as the lag of the logical replica is (which
 *      would probably just randomly bounce around).
 *
 * If using a SQL proxy service, it would probably be best to have two proxy hosts for the
 * load balancer to talk to. One would be the 'host' of the master server entry and another for
 * the (logical) replica server entry. The proxy could map the load balancer's "replica" DB to
 * any number of physical replica DBs.
 *
 * @since 1.28
 * @ingroup Database
 */
interface ILoadBalancer {
        /** @var integer Request a replica DB connection */
        const DB_REPLICA = -1;
        /** @var integer Request a master DB connection */
        const DB_MASTER = -2;

        /** @var string Domain specifier when no specific database needs to be selected */
        const DOMAIN_ANY = '';

        /**
         * Construct a manager of IDatabase connection objects
         *
         * @param array $params Parameter map with keys:
         *  - servers : Required. Array of server info structures.
         *  - localDomain: A DatabaseDomain or domain ID string.
         *  - loadMonitor : Name of a class used to fetch server lag and load.
         *  - readOnlyReason : Reason the master DB is read-only if so [optional]
         *  - waitTimeout : Maximum time to wait for replicas for consistency [optional]
         *  - srvCache : BagOStuff object for server cache [optional]
         *  - memCache : BagOStuff object for cluster memory cache [optional]
         *  - wanCache : WANObjectCache object [optional]
         *  - hostname : The name of the current server [optional]
         *  - cliMode: Whether the execution context is a CLI script. [optional]
         *  - profiler : Class name or instance with profileIn()/profileOut() methods. [optional]
         *  - trxProfiler: TransactionProfiler instance. [optional]
         *  - replLogger: PSR-3 logger instance. [optional]
         *  - connLogger: PSR-3 logger instance. [optional]
         *  - queryLogger: PSR-3 logger instance. [optional]
         *  - perfLogger: PSR-3 logger instance. [optional]
         *  - errorLogger : Callback that takes an Exception and logs it. [optional]
         * @throws InvalidArgumentException
         */
        public function __construct( array $params );

        /**
         * Get the index of the reader connection, which may be a replica DB
         *
         * This takes into account load ratios and lag times. It should
         * always return a consistent index during a given invocation.
         *
         * Side effect: opens connections to databases
         * @param string|bool $group Query group, or false for the generic reader
         * @param string|bool $domain Domain ID, or false for the current domain
         * @throws DBError
         * @return bool|int|string
         */
        public function getReaderIndex( $group = false, $domain = false );

        /**
         * Set the master wait position
         *
         * If a DB_REPLICA connection has been opened already, then wait immediately.
         * Otherwise sets a variable telling it to wait if such a connection is opened.
         *
         * @param DBMasterPos $pos
         */
        public function waitFor( $pos );

        /**
         * Set the master wait position and wait for a "generic" replica DB to catch up to it
         *
         * This can be used a faster proxy for waitForAll()
         *
         * @param DBMasterPos $pos
         * @param int $timeout Max seconds to wait; default is mWaitTimeout
         * @return bool Success (able to connect and no timeouts reached)
         */
        public function waitForOne( $pos, $timeout = null );

        /**
         * Set the master wait position and wait for ALL replica DBs to catch up to it
         *
         * @param DBMasterPos $pos
         * @param int $timeout Max seconds to wait; default is mWaitTimeout
         * @return bool Success (able to connect and no timeouts reached)
         */
        public function waitForAll( $pos, $timeout = null );

        /**
         * Get any open connection to a given server index, local or foreign
         *
         * @param int $i Server index or DB_MASTER/DB_REPLICA
         * @return Database|bool False if no such connection is open
         */
        public function getAnyOpenConnection( $i );

        /**
         * Get a connection by index
         *
         * @param int $i Server index or DB_MASTER/DB_REPLICA
         * @param array|string|bool $groups Query group(s), or false for the generic reader
         * @param string|bool $domain Domain ID, or false for the current domain
         *
         * @throws DBError
         * @return Database
         */
        public function getConnection( $i, $groups = [], $domain = false );

        /**
         * Mark a foreign connection as being available for reuse under a different DB domain
         *
         * This mechanism is reference-counted, and must be called the same number of times
         * as getConnection() to work.
         *
         * @param IDatabase $conn
         * @throws InvalidArgumentException
         */
        public function reuseConnection( $conn );

        /**
         * Get a database connection handle reference
         *
         * The handle's methods simply wrap those of a Database handle
         *
         * @see ILoadBalancer::getConnection() for parameter information
         *
         * @param int $i Server index or DB_MASTER/DB_REPLICA
         * @param array|string|bool $groups Query group(s), or false for the generic reader
         * @param string|bool $domain Domain ID, or false for the current domain
         * @return DBConnRef
         */
        public function getConnectionRef( $i, $groups = [], $domain = false );

        /**
         * Get a database connection handle reference without connecting yet
         *
         * The handle's methods simply wrap those of a Database handle
         *
         * @see ILoadBalancer::getConnection() for parameter information
         *
         * @param int $i Server index or DB_MASTER/DB_REPLICA
         * @param array|string|bool $groups Query group(s), or false for the generic reader
         * @param string|bool $domain Domain ID, or false for the current domain
         * @return DBConnRef
         */
        public function getLazyConnectionRef( $i, $groups = [], $domain = false );

        /**
         * Get a maintenance database connection handle reference for migrations and schema changes
         *
         * The handle's methods simply wrap those of a Database handle
         *
         * @see ILoadBalancer::getConnection() for parameter information
         *
         * @param int $db Server index or DB_MASTER/DB_REPLICA
         * @param array|string|bool $groups Query group(s), or false for the generic reader
         * @param string|bool $domain Domain ID, or false for the current domain
         * @return MaintainableDBConnRef
         */
        public function getMaintenanceConnectionRef( $db, $groups = [], $domain = false );

        /**
         * Open a connection to the server given by the specified index
         * Index must be an actual index into the array.
         * If the server is already open, returns it.
         *
         * On error, returns false, and the connection which caused the
         * error will be available via $this->mErrorConnection.
         *
         * @note If disable() was called on this LoadBalancer, this method will throw a DBAccessError.
         *
         * @param int $i Server index or DB_MASTER/DB_REPLICA
         * @param string|bool $domain Domain ID, or false for the current domain
         * @return Database|bool Returns false on errors
         * @throws DBAccessError
         */
        public function openConnection( $i, $domain = false );

        /**
         * @return int
         */
        public function getWriterIndex();

        /**
         * Returns true if the specified index is a valid server index
         *
         * @param string $i
         * @return bool
         */
        public function haveIndex( $i );

        /**
         * Returns true if the specified index is valid and has non-zero load
         *
         * @param string $i
         * @return bool
         */
        public function isNonZeroLoad( $i );

        /**
         * Get the number of defined servers (not the number of open connections)
         *
         * @return int
         */
        public function getServerCount();

        /**
         * Get the host name or IP address of the server with the specified index
         * Prefer a readable name if available.
         * @param string $i
         * @return string
         */
        public function getServerName( $i );

        /**
         * Return the server info structure for a given index, or false if the index is invalid.
         * @param int $i
         * @return array|bool
         */
        public function getServerInfo( $i );

        /**
         * Sets the server info structure for the given index. Entry at index $i
         * is created if it doesn't exist
         * @param int $i
         * @param array $serverInfo
         */
        public function setServerInfo( $i, array $serverInfo );

        /**
         * Get the current master position for chronology control purposes
         * @return DBMasterPos|bool Returns false if not applicable
         */
        public function getMasterPos();

        /**
         * Disable this load balancer. All connections are closed, and any attempt to
         * open a new connection will result in a DBAccessError.
         */
        public function disable();

        /**
         * Close all open connections
         */
        public function closeAll();

        /**
         * Close a connection
         *
         * Using this function makes sure the LoadBalancer knows the connection is closed.
         * If you use $conn->close() directly, the load balancer won't update its state.
         *
         * @param IDatabase $conn
         */
        public function closeConnection( IDatabase $conn );

        /**
         * Commit transactions on all open connections
         * @param string $fname Caller name
         * @throws DBExpectedError
         */
        public function commitAll( $fname = __METHOD__ );

        /**
         * Perform all pre-commit callbacks that remain part of the atomic transactions
         * and disable any post-commit callbacks until runMasterPostTrxCallbacks()
         *
         * Use this only for mutli-database commits
         */
        public function finalizeMasterChanges();

        /**
         * Perform all pre-commit checks for things like replication safety
         *
         * Use this only for mutli-database commits
         *
         * @param array $options Includes:
         *   - maxWriteDuration : max write query duration time in seconds
         * @throws DBTransactionError
         */
        public function approveMasterChanges( array $options );

        /**
         * Flush any master transaction snapshots and set DBO_TRX (if DBO_DEFAULT is set)
         *
         * The DBO_TRX setting will be reverted to the default in each of these methods:
         *   - commitMasterChanges()
         *   - rollbackMasterChanges()
         *   - commitAll()
         * This allows for custom transaction rounds from any outer transaction scope.
         *
         * @param string $fname
         * @throws DBExpectedError
         */
        public function beginMasterChanges( $fname = __METHOD__ );

        /**
         * Issue COMMIT on all master connections where writes where done
         * @param string $fname Caller name
         * @throws DBExpectedError
         */
        public function commitMasterChanges( $fname = __METHOD__ );

        /**
         * Issue all pending post-COMMIT/ROLLBACK callbacks
         *
         * Use this only for mutli-database commits
         *
         * @param integer $type IDatabase::TRIGGER_* constant
         * @return Exception|null The first exception or null if there were none
         */
        public function runMasterPostTrxCallbacks( $type );

        /**
         * Issue ROLLBACK only on master, only if queries were done on connection
         * @param string $fname Caller name
         * @throws DBExpectedError
         */
        public function rollbackMasterChanges( $fname = __METHOD__ );

        /**
         * Suppress all pending post-COMMIT/ROLLBACK callbacks
         *
         * Use this only for mutli-database commits
         *
         * @return Exception|null The first exception or null if there were none
         */
        public function suppressTransactionEndCallbacks();

        /**
         * Commit all replica DB transactions so as to flush any REPEATABLE-READ or SSI snapshot
         *
         * @param string $fname Caller name
         */
        public function flushReplicaSnapshots( $fname = __METHOD__ );

        /**
         * @return bool Whether a master connection is already open
         */
        public function hasMasterConnection();

        /**
         * Determine if there are pending changes in a transaction by this thread
         * @return bool
         */
        public function hasMasterChanges();

        /**
         * Get the timestamp of the latest write query done by this thread
         * @return float|bool UNIX timestamp or false
         */
        public function lastMasterChangeTimestamp();

        /**
         * Check if this load balancer object had any recent or still
         * pending writes issued against it by this PHP thread
         *
         * @param float $age How many seconds ago is "recent" [defaults to mWaitTimeout]
         * @return bool
         */
        public function hasOrMadeRecentMasterChanges( $age = null );

        /**
         * Get the list of callers that have pending master changes
         *
         * @return string[] List of method names
         */
        public function pendingMasterChangeCallers();

        /**
         * @note This method will trigger a DB connection if not yet done
         * @param string|bool $domain Domain ID, or false for the current domain
         * @return bool Whether the generic connection for reads is highly "lagged"
         */
        public function getLaggedReplicaMode( $domain = false );

        /**
         * @note This method will never cause a new DB connection
         * @return bool Whether any generic connection used for reads was highly "lagged"
         */
        public function laggedReplicaUsed();

        /**
         * @note This method may trigger a DB connection if not yet done
         * @param string|bool $domain Domain ID, or false for the current domain
         * @param IDatabase|null DB master connection; used to avoid loops [optional]
         * @return string|bool Reason the master is read-only or false if it is not
         */
        public function getReadOnlyReason( $domain = false, IDatabase $conn = null );

        /**
         * Disables/enables lag checks
         * @param null|bool $mode
         * @return bool
         */
        public function allowLagged( $mode = null );

        /**
         * @return bool
         */
        public function pingAll();

        /**
         * Call a function with each open connection object
         * @param callable $callback
         * @param array $params
         */
        public function forEachOpenConnection( $callback, array $params = [] );

        /**
         * Call a function with each open connection object to a master
         * @param callable $callback
         * @param array $params
         */
        public function forEachOpenMasterConnection( $callback, array $params = [] );

        /**
         * Call a function with each open replica DB connection object
         * @param callable $callback
         * @param array $params
         */
        public function forEachOpenReplicaConnection( $callback, array $params = [] );

        /**
         * Get the hostname and lag time of the most-lagged replica DB
         *
         * This is useful for maintenance scripts that need to throttle their updates.
         * May attempt to open connections to replica DBs on the default DB. If there is
         * no lag, the maximum lag will be reported as -1.
         *
         * @param bool|string $domain Domain ID, or false for the default database
         * @return array ( host, max lag, index of max lagged host )
         */
        public function getMaxLag( $domain = false );

        /**
         * Get an estimate of replication lag (in seconds) for each server
         *
         * Results are cached for a short time in memcached/process cache
         *
         * Values may be "false" if replication is too broken to estimate
         *
         * @param string|bool $domain
         * @return int[] Map of (server index => float|int|bool)
         */
        public function getLagTimes( $domain = false );

        /**
         * Get the lag in seconds for a given connection, or zero if this load
         * balancer does not have replication enabled.
         *
         * This should be used in preference to Database::getLag() in cases where
         * replication may not be in use, since there is no way to determine if
         * replication is in use at the connection level without running
         * potentially restricted queries such as SHOW SLAVE STATUS. Using this
         * function instead of Database::getLag() avoids a fatal error in this
         * case on many installations.
         *
         * @param IDatabase $conn
         * @return int|bool Returns false on error
         */
        public function safeGetLag( IDatabase $conn );

        /**
         * Wait for a replica DB to reach a specified master position
         *
         * This will connect to the master to get an accurate position if $pos is not given
         *
         * @param IDatabase $conn Replica DB
         * @param DBMasterPos|bool $pos Master position; default: current position
         * @param integer|null $timeout Timeout in seconds [optional]
         * @return bool Success
         */
        public function safeWaitForMasterPos( IDatabase $conn, $pos = false, $timeout = null );

        /**
         * Set a callback via IDatabase::setTransactionListener() on
         * all current and future master connections of this load balancer
         *
         * @param string $name Callback name
         * @param callable|null $callback
         */
        public function setTransactionListener( $name, callable $callback = null );

        /**
         * Set a new table prefix for the existing local domain ID for testing
         *
         * @param string $prefix
         */
        public function setDomainPrefix( $prefix );

        /**
         * Make certain table names use their own database, schema, and table prefix
         * when passed into SQL queries pre-escaped and without a qualified database name
         *
         * For example, "user" can be converted to "myschema.mydbname.user" for convenience.
         * Appearances like `user`, somedb.user, somedb.someschema.user will used literally.
         *
         * Calling this twice will completely clear any old table aliases. Also, note that
         * callers are responsible for making sure the schemas and databases actually exist.
         *
         * @param array[] $aliases Map of (table => (dbname, schema, prefix) map)
         */
        public function setTableAliases( array $aliases );
}