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<?php /** * 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 */ namespace Wikimedia\Rdbms; /** * This class is a delegate to ILBFactory for a given database cluster * * ILoadBalancer tracks the database connections and transactions for a given database cluster. * A "cluster" is considered to be the set of database servers that manage a given dataset. * Within a given cluster, each database server can have one of the following roles: * - sole-primary: the server used by this datacenter for writes from the application * - co-primary: one of several servers used by this datacenter for writes from the application, * relying on asynchronous replication to synchronize their copies of the dataset * - replica: a server used only for reads from the application, relying on asynchronous * replication to apply writes from the primary server or co-primary servers * - static clone: a server that only accepts reads from the application, does not replicate, * and has a copy of the final dataset, which must be static (all the servers reject writes) * * Single-datacenter database clusters consist of either: * - A sole-primary server and zero or more replica servers * - A set of static clone servers * * Multi-datacenter database clusters either consist of either: * - A sole-primary server and zero or more replica servers in the "primary datacenter" * (the one datacenter meant to handle requests/jobs that mutate the database), and zero or * more replica servers in each "secondary datacenter" (all the other datacenters) * - A co-primary server and zero or more replica servers within each datacenter * - A set of static clone servers in each datacenter * * The term "primary" refers to the server used by this datacenter for handling writes, * whether it is a sole-primary or co-primary. * * The "servers" configuration array contains the list of database servers to use for operations * originating from the the local datacenter. The first entry must refer to the server to use for * write and read-for-write operations (e.g. the "writer server"): * - If there is a primary server, then the first entry must refer to it, even if the primary * server resides in a remote datacenter * - If there are co-primary servers, then the first entry must refer to the one in the local * datacenter * - If the servers are static clones, then the first entry can refer to any of them, since the * concept of a "writer server" is merely nominal * * On an infrastructure level, circular replication setups can have more than one database server * act as a replication "source" within the same datacenter, provided that no more than one of the * servers are writable at any time, namely the "writer server". The other source servers will be * treated as replicas by the load balancer, but can be quickly promoted to the "writer server" by * the site admin as needed. * * Likewise, Galera Cluster setups still require the choice of a single "writer server" for each * datacenter. Limiting the number of servers that initiate transactions helps reduce the rate of * aborted transactions due to wsrep conflicts. * * 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 commitPrimaryChanges() or rollbackPrimaryChanges() is called. The * application must have some point where it calls commitPrimaryChanges() near the end of * the PHP request. * Every iteration of beginPrimaryChanges()/commitPrimaryChanges() is called a "transaction round". * Rounds are useful on the primary 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 foreign 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 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: * - Sole-primary + N Replica(s): set 'max lag' to an appropriate threshold for avoiding any * replica database lagged by this much or more. If all replicas are this lagged, then the * load balancer considers the cluster to be read-only. * - Per-datacenter co-primary + N Replica(s): set 'max lag' to an appropriate threshold for * avoiding any replica database lagged by this much or more. If all replicas are this * lagged, then the load balancer considers the cluster to be read-only. * - Read-only archive clones: set 'is static' in the server configuration maps. This will * treat all such DBs as having 0 lag. * - Externally updated dataset 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 "writer 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 { /** * Request a replica DB connection. Can't be used as a binary flag with bitwise operators! */ public const DB_REPLICA = -1; /** * Request a primary, write-enabled DB connection. Can't be used as a binary flag with bitwise * operators! * @since 1.36 */ public const DB_PRIMARY = -2; /** Domain specifier when no specific database needs to be selected */ public const DOMAIN_ANY = ''; /** The generic query group */ public const GROUP_GENERIC = ''; /** Yield a tracked autocommit-mode handle (reuse existing ones) */ public const CONN_TRX_AUTOCOMMIT = 1; /** * Yield an untracked, low-timeout, autocommit-mode handle (to gauge server health) * @internal */ public const CONN_UNTRACKED_GAUGE = 2; /** * Yield null on connection failure instead of throwing an exception * @internal */ public const CONN_SILENCE_ERRORS = 4; /** * Get the name of the overall cluster of database servers managing the dataset * * Note that the cluster might contain servers in multiple datacenters. * The load balancer instance only needs to be aware of the local replica servers, * along with either the sole-primary server or the local co-primary server. * * This is useful for identifying a cluster or replicated dataset, even when: * - The primary server is sometimes swapped with another one * - The cluster/dataset is replicated among multiple datacenters, with one "primary" * datacenter having the writable primary server and the other datacenters having a * read-only replica in the "primary" server slot * - The dataset is replicated among multiple datacenters, via circular replication, * with each datacenter having its own "co-primary" server * * @return string * @since 1.36 */ public function getClusterName(): string; /** * Get the local (and default) database domain ID of connection handles * * @see DatabaseDomain * @return string Database domain ID; this specifies DB name, schema, and table prefix * @since 1.31 */ public function getLocalDomainID(): string; /** * @param DatabaseDomain|string|false $domain Database domain * @return string Value of $domain if it is foreign or the local domain otherwise * @since 1.32 */ public function resolveDomainID( $domain ): string; /** * Indicate whether the tables on this domain are only temporary tables for testing * * In "temporary tables mode", the CONN_TRX_AUTOCOMMIT flag is ignored * * @param bool $value * @param string $domain * @return bool Whether "temporary tables mode" was active * @since 1.34 */ public function setTempTablesOnlyMode( $value, $domain ); /** * Get the specific server index of the reader connection for a given group * * This takes into account load ratios and lag times. It should return a consistent * index during the life time of the load balancer. This initially checks replica DBs * for connectivity to avoid returning an unusable server. This means that connections * might be attempted by calling this method (usually one at the most but possibly more). * Subsequent calls with the same $group will not need to make new connection attempts * since the acquired connection for each group is preserved. * * @param string|false $group Query group or false for the generic group * @return int|false Specific server index, or false if no DB handle can be obtained */ public function getReaderIndex( $group = false ); /** * Get a lazy-connecting database handle for a specific or virtual (DB_PRIMARY/DB_REPLICA) server index * * The server index, $i, can be one of the following: * - DB_REPLICA: a server index will be selected by the load balancer based on read * weight, connectivity, and replication lag. Note that the primary server might be * configured with read weight. If $groups is empty then it means "the generic group", * in which case all servers defined with read weight will be considered. Additional * query groups can be configured, having their own list of server indexes and read * weights. If a query group list is provided in $groups, then each recognized group * will be tried, left-to-right, until server index selection succeeds or all groups * have been tried, in which case the generic group will be tried. * - DB_PRIMARY: the primary server index will be used; the same as ServerInfo::WRITER_INDEX. * The value of $groups should be [] when using this server index. * - Specific server index: a positive integer can be provided to use the server with * that index. An error will be thrown in no such server index is recognized. This * server selection method is usually only useful for internal load balancing logic. * The value of $groups should be [] when using a specific server index. * * Handle sharing is very useful when callers get DB_PRIMARY handles that are transaction * round aware (the default). All such callers will operate within a single transaction as * a consequence. The same applies to DB_REPLICA that are samely query grouped (the default) * and transaction round aware (the default). * * Use CONN_TRX_AUTOCOMMIT to use a separate pool of only autocommit handles. This flag is * ignored for databases with ATTR_DB_LEVEL_LOCKING (e.g. sqlite) in order to avoid deadlocks. * getServerAttributes() can be used to check this attribute beforehand. Avoid using begin() * and commit() on such handles. If handle methods like startAtomic() and endAtomic() must be * used on the handles, callers should at least make sure that the atomic sections are closed * on failure via try/catch and cancelAtomic(). * * Use CONN_UNTRACKED_GAUGE to get a new, untracked, handle, that uses a low connection timeout, a low * read timeout, and autocommit mode. This flag is intended for use only be internal callers. * * CONN_UNTRACKED_GAUGE and CONN_TRX_AUTOCOMMIT are incompatible. * * @see ILoadBalancer::getServerAttributes() * * @param int $i Specific (overrides $groups) or virtual (DB_PRIMARY/DB_REPLICA) server index * @param string[]|string $groups Query group(s) in preference order; [] for the default group * @param string|false $domain DB domain ID or false for the local domain * @param int $flags Bitfield of CONN_* class constants * @return IDatabase|false This returns false on failure if CONN_SILENCE_ERRORS is set */ public function getConnection( $i, $groups = [], $domain = false, $flags = 0 ); /** * Get a DB handle for a specific server index * * This is an internal utility method for methods like LoadBalancer::getConnectionInternal() * and DBConnRef to create the underlying connection to a concrete server. * * The following is the responsibility of the caller: * * - translate any virtual server indexes (DB_PRIMARY/DB_REPLICA) to a real server index. * - enforce read-only mode on primary DB handle if there is high replication lag. * * @see ILoadBalancer::getConnection() * * @internal Only for use within ILoadBalancer/ILoadMonitor * @param int $i Specific server index * @param string $domain Resolved DB domain * @param int $flags Bitfield of class CONN_* constants * @return IDatabaseForOwner|false This returns false on failure if CONN_SILENCE_ERRORS is set * @throws DBError If no DB handle could be obtained and CONN_SILENCE_ERRORS is not set */ public function getServerConnection( $i, $domain, $flags = 0 ); /** * @deprecated since 1.39, use ILoadBalancer::getConnection() instead. * @param int $i Specific or virtual (DB_PRIMARY/DB_REPLICA) server index * @param string[]|string $groups Query group(s) in preference order; [] for the default group * @param string|false $domain DB domain ID or false for the local domain * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT) * @return DBConnRef */ public function getConnectionRef( $i, $groups = [], $domain = false, $flags = 0 ): DBConnRef; /** * @internal Only to be used by DBConnRef * @param int $i Specific (overrides $groups) or virtual (DB_PRIMARY/DB_REPLICA) server index * @param string[]|string $groups Query group(s) in preference order; [] for the default group * @param string|false $domain DB domain ID or false for the local domain * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT) * @return IDatabase */ public function getConnectionInternal( $i, $groups = [], $domain = false, $flags = 0 ): IDatabase; /** * Get a DB handle, suitable for migrations and schema changes, for a server index * * The DBConnRef methods simply proxy an underlying IDatabase object which * takes care of the actual connection and query logic. * * The CONN_TRX_AUTOCOMMIT flag is ignored for databases with ATTR_DB_LEVEL_LOCKING * (e.g. sqlite) in order to avoid deadlocks. getServerAttributes() * can be used to check such flags beforehand. Avoid the use of begin() or startAtomic() * on any CONN_TRX_AUTOCOMMIT connections. * * @see ILoadBalancer::getConnection() for parameter information * @param int $i Specific or virtual (DB_PRIMARY/DB_REPLICA) server index * @param string[]|string $groups Query group(s) in preference order; [] for the default group * @param string|false $domain DB domain ID or false for the local domain * @param int $flags Bitfield of CONN_* class constants (e.g. CONN_TRX_AUTOCOMMIT) * @return DBConnRef */ public function getMaintenanceConnectionRef( $i, $groups = [], $domain = false, $flags = 0 ): DBConnRef; /** * Get the number of servers defined in configuration * * @return int */ public function getServerCount(); /** * Whether there are any replica servers configured * * This scans the list of servers defined in configuration, checking for: * - Servers that are listed after the primary and not flagged with "is static"; * such servers are assumed to be typical streaming replicas * - Servers that are listed after the primary and flagged with "is static"; * such servers are assumed to have a clone of the static dataset (matching the primary) * * @return bool * @since 1.34 */ public function hasReplicaServers(); /** * Whether any replica servers use streaming replication from the primary server * * This scans the list of servers defined in configuration, checking for: * - Servers that are listed after the primary and not flagged with "is static"; * such servers are assumed to be typical streaming replicas * * It is possible for some replicas to be configured with "is static" but not * others, though it generally should either be set for all or none of the replicas. * * If this returns false, this means that there is generally no reason to execute * replication wait logic for session consistency and lag reduction. * * @return bool * @since 1.34 */ public function hasStreamingReplicaServers(); /** * Get the readable name of the server with the specified index * * @param int $i Specific server index * @return string Readable server name, falling back to the hostname or IP address */ public function getServerName( $i ): string; /** * Return the server configuration map for the server with the specified index * * @param int $i Specific server index * @return array|false Server configuration map; false if the index is invalid * @since 1.31 */ public function getServerInfo( $i ); /** * Get the RDBMS type of the server with the specified index (e.g. "mysql", "sqlite") * * @param int $i Specific server index * @return string One of (mysql,postgres,sqlite,...) or "unknown" for bad indexes * @since 1.30 */ public function getServerType( $i ); /** * Get basic attributes of the server with the specified index without connecting * * @param int $i Specific server index * @return array (Database::ATTRIBUTE_* constant => value) for all such constants * @since 1.31 */ public function getServerAttributes( $i ); /** * Get the current primary replication position * * @return DBPrimaryPos|false Returns false if not applicable * @throws DBError * @since 1.37 */ public function getPrimaryPos(); /** * Whether there are pending changes or callbacks in a transaction by this thread * @return bool * @since 1.37 */ public function hasPrimaryChanges(); /** * Determine whether an explicit transaction is active on any open primary * connection. * @return bool * @since 1.39 */ public function explicitTrxActive(); /** * Check if this load balancer object had any recent or still * pending writes issued against it by this PHP thread * * @param float|null $age How many seconds ago is "recent" [defaults to mWaitTimeout] * @return bool * @since 1.37 */ public function hasOrMadeRecentPrimaryChanges( $age = null ); /** * @note This method may trigger a DB connection if not yet done * @return string|false Reason the primary is read-only or false if it is not */ public function getReadOnlyReason(); /** * @return bool */ public function pingAll(); /** * Get the name and lag time of the most-lagged replica server * * 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. * * @return array{0:string,1:float|int|false,2:int} (host, max lag, index of max lagged host) */ public function getMaxLag(); /** * 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 * * @return float[]|int[]|false[] Map of (server index => lag) in order of server index */ public function getLagTimes(); /** * Wait for a replica DB to reach a specified primary position * * If $conn is not a replica server connection, then this will return true. * Otherwise, if $pos is not provided, this will connect to the primary server * to get an accurate position. * * @param IDatabase $conn Replica DB * @return bool Success * @since 1.37 */ public function waitForPrimaryPos( IDatabase $conn ); /** * Set a callback via IDatabase::setTransactionListener() on * all current and future primary connections of this load balancer * * @param string $name Callback name * @param callable|null $callback */ public function setTransactionListener( $name, ?callable $callback = null ); /** * 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 ); /** * Convert certain database domains to alternative ones. * * This can be used for backwards compatibility logic. * * @param DatabaseDomain[]|string[] $aliases Map of (domain alias => domain) * @since 1.35 */ public function setDomainAliases( array $aliases ); }
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