LoadBalancer.php

Go to the documentation of this file.

<?php
namespace Wikimedia\Rdbms;
 
use InvalidArgumentException;
use LogicException;
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;
use RuntimeException;
use Throwable;
use UnexpectedValueException;
use Wikimedia\ArrayUtils\ArrayUtils;
use Wikimedia\ObjectCache\BagOStuff;
use Wikimedia\ObjectCache\EmptyBagOStuff;
use Wikimedia\ObjectCache\WANObjectCache;
use Wikimedia\ScopedCallback;
use Wikimedia\Stats\StatsFactory;
 
class LoadBalancer implements ILoadBalancerForOwner {
    private $loadMonitor;
    private $srvCache;
    private $wanCache;
    private $databaseFactory;
 
    private $trxProfiler;
    private $statsFactory;
    private $logger;
    private $errorLogger;
    private $localDomain;
    private $cliMode;
 
    private $conns;
 
    private $clusterName;
    private $serverInfo;
    private $loads;
    private $defaultGroup;
 
    private $tableAliases = [];
    private $domainAliases = [];
    private $trxRecurringCallbacks = [];
    private $tempTablesOnlyMode = [];
 
    private $trxRoundFname = null;
    private $trxRoundStage = self::ROUND_CURSORY;
    private $readIndexByGroup = [];
    private $waitForPos;
    private $laggedReplicaMode = false;
    private $readOnlyReason = false;
    private $connectionCounter = 0;
    private $disabled = false;
    private ?ChronologyProtector $chronologyProtector = null;
    private $chronologyProtectorCalled = false;
 
    private $lastErrorConn;
 
    private $nonLocalDomainCache = [];
    private ?string $uniqueIdentifier = null;
 
    private $modcount = 0;
 
    private const INFO_SERVER_INDEX = 'serverIndex';
    private const INFO_CONN_CATEGORY = 'connCategory';
 
    private const CONN_HELD_WARN_THRESHOLD = 10;
 
    private const MAX_WAIT_DEFAULT = 10;
    private const TTL_CACHE_READONLY = 5;
 
    private const CATEGORY_ROUND = 'round';
    private const CATEGORY_AUTOCOMMIT = 'auto-commit';
    private const CATEGORY_GAUGE = 'gauge';
 
    private const ROUND_CURSORY = 'cursory';
    private const ROUND_FINALIZED = 'finalized';
    private const ROUND_APPROVED = 'approved';
    private const ROUND_COMMIT_CALLBACKS = 'commit-callbacks';
    private const ROUND_ROLLBACK_CALLBACKS = 'rollback-callbacks';
    private const ROUND_ERROR = 'error';
 
    private const READER_INDEX_NONE = -1;
 
    public function __construct( array $params ) {
        $this->configure( $params );
 
        $this->conns = self::newTrackedConnectionsArray();
    }
 
    protected function configure( array $params ): void {
        $this->localDomain = isset( $params['localDomain'] )
            ? DatabaseDomain::newFromId( $params['localDomain'] )
            : DatabaseDomain::newUnspecified();
 
        $this->serverInfo = new ServerInfo();
        $this->loads = [];
        foreach ( $this->serverInfo->normalizeServerMaps( $params['servers'] ?? [] ) as $i => $server ) {
            $this->serverInfo->addServer( $i, $server );
            $this->loads[$i] = $server['load'];
        }
        // If the cluster name is not specified, fallback to the current primary name
        $this->clusterName = $params['clusterName']
            ?? $this->serverInfo->getServerName( ServerInfo::WRITER_INDEX );
 
        if ( isset( $params['readOnlyReason'] ) && is_string( $params['readOnlyReason'] ) ) {
            $this->readOnlyReason = $params['readOnlyReason'];
        }
 
        $this->srvCache = $params['srvCache'] ?? new EmptyBagOStuff();
        $this->wanCache = $params['wanCache'] ?? WANObjectCache::newEmpty();
 
        // Note: this parameter is normally absent. It is injectable for testing purposes only.
        $this->databaseFactory = $params['databaseFactory'] ?? new DatabaseFactory( $params );
 
        $this->errorLogger = $params['errorLogger'] ?? static function ( Throwable $e ) {
            trigger_error( get_class( $e ) . ': ' . $e->getMessage(), E_USER_WARNING );
        };
        $this->logger = $params['logger'] ?? new NullLogger();
 
        $this->trxProfiler = $params['trxProfiler'] ?? new TransactionProfiler();
        $this->statsFactory = $params['statsFactory'] ?? StatsFactory::newNull();
 
        // Set up LoadMonitor
        $loadMonitorConfig = $params['loadMonitor'] ?? [ 'class' => LoadMonitorNull::class ];
        $compat = [
            'LoadMonitor' => LoadMonitor::class,
            'LoadMonitorNull' => LoadMonitorNull::class
        ];
        $class = $loadMonitorConfig['class'];
        // @phan-suppress-next-line PhanImpossibleCondition
        if ( isset( $compat[$class] ) ) {
            $class = $compat[$class];
        }
        $this->loadMonitor = new $class(
            $this,
            $this->srvCache,
            $this->wanCache,
            $this->logger,
            $this->statsFactory,
            $loadMonitorConfig
        );
 
        if ( isset( $params['chronologyProtector'] ) ) {
            $this->chronologyProtector = $params['chronologyProtector'];
        }
 
        if ( isset( $params['roundStage'] ) ) {
            if ( $params['roundStage'] === self::STAGE_POSTCOMMIT_CALLBACKS ) {
                $this->trxRoundStage = self::ROUND_COMMIT_CALLBACKS;
            } elseif ( $params['roundStage'] === self::STAGE_POSTROLLBACK_CALLBACKS ) {
                $this->trxRoundStage = self::ROUND_ROLLBACK_CALLBACKS;
            }
        }
 
        $this->cliMode = $params['cliMode'] ?? ( PHP_SAPI === 'cli' || PHP_SAPI === 'phpdbg' );
 
        $this->defaultGroup = $params['defaultGroup'] ?? self::GROUP_GENERIC;
        if ( empty( $params['shuffleSharding'] ) ) {
            $this->uniqueIdentifier = null;
        } else {
            $this->uniqueIdentifier = $params['uniqueIdentifier'] ?? null;
        }
    }
 
    private static function newTrackedConnectionsArray(): array {
        // Note that CATEGORY_GAUGE connections are untracked
        return [
            self::CATEGORY_ROUND => [],
            self::CATEGORY_AUTOCOMMIT => []
        ];
    }
 
    public function getClusterName(): string {
        return $this->clusterName;
    }
 
    public function getLocalDomainID(): string {
        return $this->localDomain->getId();
    }
 
    public function resolveDomainID( DatabaseDomain|string|false $domain ): string {
        return $this->resolveDomainInstance( $domain )->getId();
    }
 
    final protected function resolveDomainInstance( DatabaseDomain|string|false $domain ): DatabaseDomain {
        if ( $domain instanceof DatabaseDomain ) {
            return $domain; // already a domain instance
        } elseif ( $domain === false || $domain === $this->localDomain->getId() ) {
            return $this->localDomain;
        } elseif ( isset( $this->domainAliases[$domain] ) ) {
            $this->domainAliases[$domain] =
                DatabaseDomain::newFromId( $this->domainAliases[$domain] );
 
            return $this->domainAliases[$domain];
        }
 
        $cachedDomain = $this->nonLocalDomainCache[$domain] ?? null;
        if ( $cachedDomain === null ) {
            $cachedDomain = DatabaseDomain::newFromId( $domain );
            $this->nonLocalDomainCache = [ $domain => $cachedDomain ];
        }
 
        return $cachedDomain;
    }
 
    protected function sanitizeConnectionFlags( $flags, $domain ) {
        if ( self::fieldHasBit( $flags, self::CONN_TRX_AUTOCOMMIT ) ) {
            // Callers use CONN_TRX_AUTOCOMMIT to bypass REPEATABLE-READ staleness without
            // resorting to row locks (e.g. FOR UPDATE) or to make small out-of-band commits
            // during larger transactions. This is useful for avoiding lock contention.
            // Assuming all servers are of the same type (or similar), which is overwhelmingly
            // the case, use the primary server information to get the attributes. The information
            // for $i cannot be used since it might be DB_REPLICA, which might require connection
            // attempts in order to be resolved into a real server index.
            $attributes = $this->getServerAttributes( ServerInfo::WRITER_INDEX );
            if ( $attributes[Database::ATTR_DB_LEVEL_LOCKING] ) {
                // The RDBMS does not support concurrent writes (e.g. SQLite), so attempts
                // to use separate connections would just cause self-deadlocks. Note that
                // REPEATABLE-READ staleness is not an issue since DB-level locking means
                // that transactions are Strict Serializable anyway.
                $flags &= ~self::CONN_TRX_AUTOCOMMIT;
                $type = $this->serverInfo->getServerType( ServerInfo::WRITER_INDEX );
                $this->logger->info( __METHOD__ . ": CONN_TRX_AUTOCOMMIT disallowed ($type)" );
            } elseif ( isset( $this->tempTablesOnlyMode[$domain] ) ) {
                // T202116: integration tests are active and queries should be all be using
                // temporary clone tables (via prefix). Such tables are not visible across
                // different connections nor can there be REPEATABLE-READ snapshot staleness,
                // so use the same connection for everything.
                $flags &= ~self::CONN_TRX_AUTOCOMMIT;
            }
        }
 
        return $flags;
    }
 
    private function enforceConnectionFlags( IDatabase $conn, $flags ) {
        if (
            self::fieldHasBit( $flags, self::CONN_TRX_AUTOCOMMIT ) ||
            // Handles with open transactions are avoided since they might be subject
            // to REPEATABLE-READ snapshots, which could affect the lag estimate query.
            self::fieldHasBit( $flags, self::CONN_UNTRACKED_GAUGE )
        ) {
            if ( $conn->trxLevel() ) {
                throw new DBUnexpectedError(
                    $conn,
                    'Handle requested with autocommit-mode yet it has a transaction'
                );
            }
 
            $conn->clearFlag( $conn::DBO_TRX ); // auto-commit mode
        }
    }
 
    private function getRandomNonLagged( array $loads, $sessionLagLimit = INF ) {
        $lags = $this->getLagTimes();
 
        $dbs = [];
        foreach ( $loads as $i => $load ) {
            $srvName = $this->serverInfo->getServerName( $i );
            $dbs[$srvName] = $i;
        }
        // Unset excessively lagged servers from the load group
        foreach ( $lags as $i => $lag ) {
            if ( $i !== ServerInfo::WRITER_INDEX ) {
                // How much lag normally counts as "excessive" for this server
                $maxServerLag = $this->serverInfo->getServerMaxLag( $i );
                // How much lag counts as "excessive" for this server given the session
                $maxServerLag = min( $maxServerLag, $sessionLagLimit );
 
                $srvName = $this->serverInfo->getServerName( $i );
                if ( $lag === false && !is_infinite( $maxServerLag ) ) {
                    $this->logger->debug(
                        __METHOD__ . ": server {db_server} is not replicating?",
                        [ 'db_server' => $srvName ]
                    );
                    unset( $loads[$i] );
                    unset( $dbs[$srvName] );
                } elseif ( $lag > $maxServerLag ) {
                    $this->logger->debug(
                        __METHOD__ .
                            ": server {db_server} has {lag} seconds of lag (>= {maxlag})",
                        [ 'db_server' => $srvName, 'lag' => $lag, 'maxlag' => $maxServerLag ]
                    );
                    unset( $loads[$i] );
                    unset( $dbs[$srvName] );
                }
            }
        }
 
        if ( array_sum( $loads ) == 0 ) {
            // All the replicas with non-zero weight are lagged and the primary has zero load.
            // Inform caller so that it can use switch to read-only mode and use a lagged replica.
            return false;
        }
 
        // Return a server index based on weighted random selection
        return $this->shuffleSharding( $dbs, $loads );
    }
 
    private function shuffleSharding( $dbs, $loads ) {
        // shuffle sharding doesn't make sense in small groups
        if ( count( $loads ) <= 3 || $this->uniqueIdentifier === null ) {
            return ArrayUtils::pickRandom( $loads );
        }
 
        ArrayUtils::consistentHashSort( $dbs, $this->uniqueIdentifier );
        $keys = array_values( $dbs );
        return ArrayUtils::pickRandom( [
            $keys[0] => $loads[$keys[0]],
            $keys[1] => $loads[$keys[1]],
            $keys[2] => $loads[$keys[2]],
        ] );
    }
 
    public function getReaderIndex( $group = false ) {
        if ( !$this->serverInfo->hasReplicaServers() ) {
            // There is only one possible server to use (the primary)
            return ServerInfo::WRITER_INDEX;
        }
 
        if (
            $group === 'dump' ||
            $group === 'vslow' ||
            $group === [ 'dump' ] ||
            $group === [ 'vslow' ] ||
            $this->defaultGroup === 'dump' ||
            $this->defaultGroup === 'vslow'
        ) {
            $group = 'vslow';
        } else {
            $group = self::GROUP_GENERIC;
        }
 
        $index = $this->getExistingReaderIndex( $group );
        if ( $index !== self::READER_INDEX_NONE ) {
            // A reader index was already selected for this query group. Keep using it,
            // since any session replication position was already waited on and any
            // active transaction will be reused (e.g. for point-in-time snapshots).
            return $index;
        }
 
        // Get the server weight array for this load group
        if ( $group === 'vslow' ) {
            // pick the replica with the lowest weight above zero as the vslow
            $lowestWeight = INF;
            $loads = $this->loads;
            foreach ( $this->loads as $index => $weight ) {
                if ( $weight > 0 && $weight < $lowestWeight ) {
                    $loads = [ $index => $weight ];
                    $lowestWeight = $weight;
                }
            }
        } else {
            $loads = $this->loads;
        }
 
        if ( !$loads ) {
            $this->logger->info( __METHOD__ . ": no loads for group $group" );
            return false;
        }
 
        // Load any session replication positions, before any connection attempts,
        // since reading them afterwards can only cause more delay due to possibly
        // seeing even higher replication positions (e.g. from concurrent requests).
        $this->loadSessionPrimaryPos();
 
        // Scale the configured load weights according to each server's load/state.
        // This can sometimes trigger server connections due to cache regeneration.
        $this->loadMonitor->scaleLoads( $loads );
 
        // Pick a server, accounting for load weights, lag, and session consistency
        $i = $this->pickReaderIndex( $loads );
        if ( $i === false ) {
            // Connection attempts failed
            return false;
        }
 
        // If data seen by queries is expected to reflect writes from a prior transaction,
        // then wait for the chosen server to apply those changes. This is used to improve
        // session consistency.
        if ( !$this->awaitSessionPrimaryPos( $i ) ) {
            // Data will be outdated compared to what was expected
            $this->setLaggedReplicaMode();
        }
 
        // Keep using this server for DB_REPLICA handles for this group
        if ( $i < 0 ) {
            throw new UnexpectedValueException( "Cannot set a negative read server index" );
        }
        $this->readIndexByGroup[$group] = $i;
 
        $serverName = $this->getServerName( $i );
        $this->logger->debug( __METHOD__ . ": using server $serverName for group '$group'" );
 
        return $i;
    }
 
    protected function getExistingReaderIndex( $group ) {
        return $this->readIndexByGroup[$group] ?? self::READER_INDEX_NONE;
    }
 
    private function pickReaderIndex( array $loads ) {
        if ( $loads === [] ) {
            throw new InvalidArgumentException( "Load group array is empty" );
        }
 
        $i = false;
        // Quickly look through the available servers for a server that meets criteria...
        $currentLoads = $loads;
        while ( count( $currentLoads ) ) {
            if ( $this->waitForPos && $this->waitForPos->asOfTime() ) {
                $this->logger->debug( __METHOD__ . ": session has replication position" );
                // ChronologyProtector::getSessionPrimaryPos called in loadSessionPrimaryPos()
                // sets "waitForPos" for session consistency. This triggers doWait() after
                // connect, so it's especially good to avoid lagged servers so as to avoid
                // excessive delay in that method.
                $ago = microtime( true ) - $this->waitForPos->asOfTime();
                // Aim for <= 1 second of waiting (being too picky can backfire)
                $i = $this->getRandomNonLagged( $currentLoads, $ago + 1 );
            } else {
                // Any server with less lag than it's 'max lag' param is preferable
                $i = $this->getRandomNonLagged( $currentLoads );
            }
 
            if ( $i === false && count( $currentLoads ) ) {
                $this->setLaggedReplicaMode();
                // All replica DBs lagged, just pick anything.
                $i = ArrayUtils::pickRandom( $currentLoads );
            }
 
            if ( $i === false ) {
                // pickRandom() returned false.
                // This is permanent and means the configuration or LoadMonitor
                // wants us to return false.
                $this->logger->debug( __METHOD__ . ": no suitable server found" );
                return false;
            }
 
            $serverName = $this->getServerName( $i );
            $this->logger->debug( __METHOD__ . ": connecting to $serverName..." );
 
            // Get a connection to this server without triggering complementary connections
            // to other servers (due to things like lag or read-only checks). We want to avoid
            // the risk of overhead and recursion here.
            $conn = $this->getServerConnection( $i, self::DOMAIN_ANY, self::CONN_SILENCE_ERRORS );
            if ( !$conn ) {
                $this->logger->warning( __METHOD__ . ": failed connecting to $serverName" );
                unset( $currentLoads[$i] ); // avoid this server next iteration
                continue;
            }
 
            // Return this server
            break;
        }
 
        // If all servers were down, quit now
        if ( $currentLoads === [] ) {
            $this->logger->error( __METHOD__ . ": all servers down" );
        }
 
        return $i;
    }
 
    public function waitForAll( DBPrimaryPos $pos, $timeout = null ) {
        $timeout = $timeout ?: self::MAX_WAIT_DEFAULT;
 
        $oldPos = $this->waitForPos;
        try {
            $this->waitForPos = $pos;
 
            $failedReplicas = [];
            foreach ( $this->serverInfo->getStreamingReplicaIndexes() as $i ) {
                if ( isset( $this->loads[$i] ) && $this->loads[$i] > 0 ) {
                    $start = microtime( true );
                    $ok = $this->awaitSessionPrimaryPos( $i, $timeout );
                    if ( !$ok ) {
                        $failedReplicas[] = $this->getServerName( $i );
                    }
                    $timeout -= intval( microtime( true ) - $start );
                }
            }
 
            // Stop spamming logs when only one replica is lagging and we have 5+ replicas.
            // Mediawiki automatically stops sending queries to the lagged one.
            $failed = $failedReplicas && ( count( $failedReplicas ) > 1 || $this->getServerCount() < 5 );
            if ( $failed ) {
                $this->logger->error(
                    "Timed out waiting for replication to reach {raw_pos}",
                    [
                        'raw_pos' => $pos->__toString(),
                        'failed_hosts' => $failedReplicas,
                        'timeout' => $timeout,
                        'exception' => new RuntimeException()
                    ]
                );
            }
 
            return !$failed;
        } finally {
            // Restore the old position; this is used for throttling, not lag-protection
            $this->waitForPos = $oldPos;
        }
    }
 
    public function getAnyOpenConnection( $i, $flags = 0 ) {
        $i = ( $i === self::DB_PRIMARY ) ? ServerInfo::WRITER_INDEX : $i;
        $conn = false;
        foreach ( $this->conns as $type => $poolConnsByServer ) {
            if ( $i === self::DB_REPLICA ) {
                // Consider all existing connections to any server
                $applicableConnsByServer = $poolConnsByServer;
            } else {
                // Consider all existing connections to a specific server
                $applicableConnsByServer = isset( $poolConnsByServer[$i] )
                    ? [ $i => $poolConnsByServer[$i] ]
                    : [];
            }
 
            $conn = $this->pickAnyOpenConnection( $applicableConnsByServer );
            if ( $conn ) {
                $this->logger->debug( __METHOD__ . ": found '$type' connection to #$i." );
                break;
            }
        }
 
        if ( $conn ) {
            $this->enforceConnectionFlags( $conn, $flags );
        }
 
        return $conn;
    }
 
    private function pickAnyOpenConnection( array $connsByServer ) {
        foreach ( $connsByServer as $i => $conns ) {
            foreach ( $conns as $conn ) {
                if ( !$conn->isOpen() ) {
                    $this->logger->warning(
                        __METHOD__ .
                        ": pooled DB handle for {db_server} (#$i) has no open connection.",
                        $this->getConnLogContext( $conn )
                    );
                    continue; // some sort of error occurred?
                }
                return $conn;
            }
        }
 
        return false;
    }
 
    private function awaitSessionPrimaryPos( $index, $timeout = null ) {
        $timeout = max( 1, intval( $timeout ?: self::MAX_WAIT_DEFAULT ) );
 
        if ( !$this->waitForPos || $index === ServerInfo::WRITER_INDEX ) {
            return true;
        }
 
        $srvName = $this->getServerName( $index );
 
        // Check if we already know that the DB has reached this point
        $key = $this->srvCache->makeGlobalKey( __CLASS__, 'last-known-pos', $srvName, 'v2' );
 
        $position = $this->srvCache->get( $key );
        if ( !is_array( $position ) ) {
            $knownReachedPos = null;
        } else {
            $class = $position['_type_'];
            $knownReachedPos = $class::newFromArray( $position );
        }
        if (
            $knownReachedPos instanceof DBPrimaryPos &&
            $knownReachedPos->hasReached( $this->waitForPos )
        ) {
            $this->logger->debug(
                __METHOD__ .
                ": replica DB {db_server} known to be caught up (pos >= $knownReachedPos).",
                [ 'db_server' => $srvName ]
            );
 
            return true;
        }
 
        $close = false; // close the connection afterwards
        $flags = self::CONN_SILENCE_ERRORS;
        // Check if there is an existing connection that can be used
        $conn = $this->getAnyOpenConnection( $index, $flags );
        if ( !$conn ) {
            // Get a connection to this server without triggering complementary connections
            // to other servers (due to things like lag or read-only checks). We want to avoid
            // the risk of overhead and recursion here.
            $conn = $this->getServerConnection( $index, self::DOMAIN_ANY, $flags );
            if ( !$conn ) {
                $this->logger->warning(
                    __METHOD__ . ': failed to connect to {db_server}',
                    [ 'db_server' => $srvName ]
                );
 
                return false;
            }
            // Avoid connection spam in waitForAll() when connections
            // are made just for the sake of doing this lag check.
            $close = true;
        }
 
        $this->logger->info(
            __METHOD__ .
            ': waiting for replica DB {db_server} to catch up...',
            $this->getConnLogContext( $conn )
        );
 
        $result = $conn->primaryPosWait( $this->waitForPos, $timeout );
 
        $ok = ( $result !== null && $result != -1 );
        if ( $ok ) {
            // Remember that the DB reached this point
            $this->srvCache->set( $key, $this->waitForPos->toArray(), BagOStuff::TTL_DAY );
        }
 
        if ( $close ) {
            $this->closeConnection( $conn );
        }
 
        return $ok;
    }
 
    public function getConnection( $i, $groups = [], string|false $domain = false, $flags = 0 ) {
        if ( self::fieldHasBit( $flags, self::CONN_SILENCE_ERRORS ) ) {
            throw new UnexpectedValueException(
                __METHOD__ . ' got CONN_SILENCE_ERRORS; connection is already deferred'
            );
        }
 
        $domain = $this->resolveDomainID( $domain );
        $role = ( $i === self::DB_PRIMARY || $i === ServerInfo::WRITER_INDEX )
            ? self::DB_PRIMARY
            : self::DB_REPLICA;
 
        return new DBConnRef( $this, [ $i, $groups, $domain, $flags ], $role, $this->modcount );
    }
 
    public function getConnectionInternal( $i, $groups = [], $domain = false, $flags = 0 ): IDatabase {
        $domain = $this->resolveDomainID( $domain );
        $flags = $this->sanitizeConnectionFlags( $flags, $domain );
        // If given DB_PRIMARY/DB_REPLICA, resolve it to a specific server index. Resolving
        // DB_REPLICA might trigger getServerConnection() calls due to the getReaderIndex()
        // connectivity checks or LoadMonitor::scaleLoads() server state cache regeneration.
        // The use of getServerConnection() instead of getConnection() avoids infinite loops.
        $serverIndex = $i;
        if ( $i === self::DB_PRIMARY ) {
            $serverIndex = ServerInfo::WRITER_INDEX;
        } elseif ( $i === self::DB_REPLICA ) {
            $groupIndex = $this->getReaderIndex( $groups );
            if ( $groupIndex !== false ) {
                // Group connection succeeded
                $serverIndex = $groupIndex;
            }
            if ( $serverIndex < 0 ) {
                $this->reportConnectionError( 'could not connect to any replica DB server' );
            }
        } elseif ( !$this->serverInfo->hasServerIndex( $i ) ) {
            throw new UnexpectedValueException( "Invalid server index index #$i" );
        }
        // Get an open connection to that server (might trigger a new connection)
        return $this->getServerConnection( $serverIndex, $domain, $flags );
    }
 
    public function getServerConnection( $i, $domain, $flags = 0 ) {
        $domainInstance = DatabaseDomain::newFromId( $domain );
        // Number of connections made before getting the server index and handle
        $priorConnectionsMade = $this->connectionCounter;
        // Get an open connection to this server (might trigger a new connection)
        $conn = $this->reuseOrOpenConnectionForNewRef( $i, $domainInstance, $flags );
        // Throw an error or otherwise bail out if the connection attempt failed
        if ( !( $conn instanceof IDatabaseForOwner ) ) {
            if ( !self::fieldHasBit( $flags, self::CONN_SILENCE_ERRORS ) ) {
                $this->reportConnectionError();
            }
 
            return false;
        }
 
        // Profile any new connections caused by this method
        if ( $this->connectionCounter > $priorConnectionsMade ) {
            $this->trxProfiler->recordConnection(
                $conn->getServerName(),
                $conn->getDBname(),
                ( $i === ServerInfo::WRITER_INDEX && $this->hasStreamingReplicaServers() )
            );
        }
 
        if ( !$conn->isOpen() ) {
            $this->lastErrorConn = $conn;
            // Connection was made but later unrecoverably lost for some reason.
            // Do not return a handle that will just throw exceptions on use, but
            // let the calling code, e.g. getReaderIndex(), try another server.
            if ( !self::fieldHasBit( $flags, self::CONN_SILENCE_ERRORS ) ) {
                $this->reportConnectionError();
            }
            return false;
        }
 
        return $conn;
    }
 
    public function getMaintenanceConnectionRef(
        $i,
        $groups = [],
        $domain = false,
        $flags = 0
    ): DBConnRef {
        if ( self::fieldHasBit( $flags, self::CONN_SILENCE_ERRORS ) ) {
            throw new UnexpectedValueException(
                __METHOD__ . ' CONN_SILENCE_ERRORS is not supported'
            );
        }
 
        $domain = $this->resolveDomainID( $domain );
        $role = ( $i === self::DB_PRIMARY || $i === ServerInfo::WRITER_INDEX )
            ? self::DB_PRIMARY
            : self::DB_REPLICA;
 
        return new DBConnRef( $this, [ $i, $groups, $domain, $flags ], $role, $this->modcount );
    }
 
    private function reuseOrOpenConnectionForNewRef( $i, DatabaseDomain $domain, $flags = 0 ) {
        // Figure out which connection pool to use based on the flags
        if ( $this->fieldHasBit( $flags, self::CONN_UNTRACKED_GAUGE ) ) {
            // Use low timeouts, use autocommit mode, ignore transaction rounds
            $category = self::CATEGORY_GAUGE;
        } elseif ( self::fieldHasBit( $flags, self::CONN_TRX_AUTOCOMMIT ) ) {
            // Use autocommit mode, ignore transaction rounds
            $category = self::CATEGORY_AUTOCOMMIT;
        } else {
            // Respect DBO_DEFAULT, respect transaction rounds
            $category = self::CATEGORY_ROUND;
        }
 
        $conn = null;
        // Reuse a free connection in the pool from any domain if possible. There should only
        // be one connection in this pool unless either:
        //  - a) IDatabase::databasesAreIndependent() returns true (e.g. postgres) and two
        //       or more database domains have been used during the load balancer's lifetime
        //  - b) Two or more nested function calls used getConnection() on different domains.
        foreach ( ( $this->conns[$category][$i] ?? [] ) as $poolConn ) {
            // Check if any required DB domain changes for the new reference are possible
            // Calling selectDomain() would trigger a reconnect, which will break if a
            // transaction is active or if there is any other meaningful session state.
            $isShareable = !(
                $poolConn->databasesAreIndependent() &&
                $domain->getDatabase() !== null &&
                $domain->getDatabase() !== $poolConn->getDBname()
            );
            if ( $isShareable ) {
                $conn = $poolConn;
                // Make any required DB domain changes for the new reference
                if ( !$domain->isUnspecified() ) {
                    $conn->selectDomain( $domain );
                }
                $this->logger->debug( __METHOD__ . ": reusing connection for $i/$domain" );
                break;
            }
        }
 
        // If necessary, try to open a new connection and add it to the pool
        if ( !$conn ) {
            $conn = $this->reallyOpenConnection(
                $i,
                $domain,
                [ self::INFO_CONN_CATEGORY => $category ]
            );
            if ( $conn->isOpen() ) {
                // Make Database::isReadOnly() respect server-side and configuration-based
                // read-only mode. Note that replica handles are always seen as read-only
                // in Database::isReadOnly() and Database::assertIsWritablePrimary().
                if ( $i === ServerInfo::WRITER_INDEX ) {
                    if ( $this->readOnlyReason !== false ) {
                        $readOnlyReason = $this->readOnlyReason;
                    } elseif ( $this->isPrimaryRunningReadOnly( $conn ) ) {
                        $readOnlyReason = 'The primary database server is running in read-only mode.';
                    } else {
                        $readOnlyReason = false;
                    }
                    $conn->setLBInfo( $conn::LB_READ_ONLY_REASON, $readOnlyReason );
                }
                // Connection obtained; check if it belongs to a tracked connection category
                if ( isset( $this->conns[$category] ) ) {
                    // Track this connection for future reuse
                    $this->conns[$category][$i][] = $conn;
                }
            } else {
                $this->logger->warning( __METHOD__ . ": connection error for $i/$domain" );
                $this->lastErrorConn = $conn;
                $conn = null;
            }
        }
 
        if ( $conn instanceof IDatabaseForOwner ) {
            // Check to make sure that the right domain is selected
            $this->assertConnectionDomain( $conn, $domain );
            // Check to make sure that the CONN_* flags are respected
            $this->enforceConnectionFlags( $conn, $flags );
        }
 
        return $conn;
    }
 
    private function assertConnectionDomain( Database $conn, DatabaseDomain $domain ) {
        if ( !$domain->isCompatible( $conn->getDomainID() ) ) {
            throw new UnexpectedValueException(
                "Got connection to '{$conn->getDomainID()}', but expected one for '{$domain}'"
            );
        }
    }
 
    public function getServerAttributes( $i ) {
        return $this->databaseFactory->attributesFromType(
            $this->getServerType( $i ),
            $this->serverInfo->getServerDriver( $i )
        );
    }
 
    protected function reallyOpenConnection( $i, DatabaseDomain $domain, array $lbInfo ) {
        if ( $this->disabled ) {
            throw new DBAccessError();
        }
 
        $server = $this->serverInfo->getServerInfoStrict( $i );
        if ( $lbInfo[self::INFO_CONN_CATEGORY] === self::CATEGORY_GAUGE ) {
            // Use low connection/read timeouts for connection used for gauging server health.
            // Gauge information should be cached and used to avoid outages. Indefinite hanging
            // while gauging servers would do the opposite.
            $server['connectTimeout'] = min( 1, $server['connectTimeout'] ?? INF );
            $server['receiveTimeout'] = min( 1, $server['receiveTimeout'] ?? INF );
            // Avoid implicit transactions and avoid any SET query for session variables during
            // Database::open(). If a server becomes slow, every extra query can cause significant
            // delays, even with low connect/receive timeouts.
            $server['flags'] ??= 0;
            $server['flags'] &= ~IDatabase::DBO_DEFAULT;
            $server['flags'] |= IDatabase::DBO_GAUGE;
        } else {
            // Use implicit transactions unless explicitly configured otherwise
            $server['flags'] ??= IDatabase::DBO_DEFAULT;
        }
 
        if ( !empty( $server['is static'] ) ) {
            $topologyRole = IDatabase::ROLE_STATIC_CLONE;
        } else {
            $topologyRole = ( $i === ServerInfo::WRITER_INDEX )
                ? IDatabase::ROLE_STREAMING_MASTER
                : IDatabase::ROLE_STREAMING_REPLICA;
        }
 
        $conn = $this->databaseFactory->create(
            $server['type'],
            array_merge( $server, [
                // Basic replication role information
                'topologyRole' => $topologyRole,
                // Use the database specified in $domain (null means "none or entrypoint DB");
                // fallback to the $server default if the RDBMs is an embedded library using a
                // file on disk since there would be nothing to access to without a DB/file name.
                'dbname' => $this->getServerAttributes( $i )[Database::ATTR_DB_IS_FILE]
                    ? ( $domain->getDatabase() ?? $server['dbname'] ?? null )
                    : $domain->getDatabase(),
                // Override the $server default schema with that of $domain if specified
                'schema' => $domain->getSchema(),
                // Use the table prefix specified in $domain
                'tablePrefix' => $domain->getTablePrefix(),
                'srvCache' => $this->srvCache,
                'logger' => $this->logger,
                'errorLogger' => $this->errorLogger,
                'trxProfiler' => $this->trxProfiler,
                'lbInfo' => [ self::INFO_SERVER_INDEX => $i ] + $lbInfo
            ] ),
            Database::NEW_UNCONNECTED
        );
        // Set alternative table names before any queries can be issued
        $conn->setTableAliases( $this->tableAliases );
        // Account for any active transaction round and listeners
        $this->syncConnectionRoundState( $conn );
        if ( $i === ServerInfo::WRITER_INDEX ) {
            foreach ( $this->trxRecurringCallbacks as $name => $callback ) {
                $conn->setTransactionListener( $name, $callback );
            }
        }
 
        // Make the connection handle live
        try {
            $conn->initConnection();
            ++$this->connectionCounter;
        } catch ( DBConnectionError ) {
            $this->lastErrorConn = $conn;
            // ignore; let the DB handle the logging
        }
 
        if ( $conn->isOpen() ) {
            $this->logger->debug( __METHOD__ . ": opened new connection for $i/$domain" );
        } else {
            $this->logger->warning(
                __METHOD__ . ": connection error for $i/{db_domain}",
                [ 'db_domain' => $domain->getId() ]
            );
        }
 
        // Log when many connection are made during a single request/script
        $count = 0;
        foreach ( $this->conns as $poolConnsByServer ) {
            foreach ( $poolConnsByServer as $serverConns ) {
                $count += count( $serverConns );
            }
        }
        if ( $count >= self::CONN_HELD_WARN_THRESHOLD ) {
            $this->logger->warning(
                __METHOD__ . ": {connections}+ connections made (primary={primarydb})",
                $this->getConnLogContext(
                    $conn,
                    [
                        'connections' => $count,
                        'primarydb' => $this->serverInfo->getPrimaryServerName(),
                        'db_domain' => $domain->getId()
                    ]
                )
            );
        }
 
        $this->assertConnectionDomain( $conn, $domain );
 
        return $conn;
    }
 
    private function loadSessionPrimaryPos() {
        if ( !$this->chronologyProtectorCalled && $this->chronologyProtector ) {
            $this->chronologyProtectorCalled = true;
            $pos = $this->chronologyProtector->getSessionPrimaryPos( $this );
            $this->logger->debug( __METHOD__ . ': executed chronology callback.' );
            if ( $pos ) {
                if ( !$this->waitForPos || $pos->hasReached( $this->waitForPos ) ) {
                    $this->waitForPos = $pos;
                }
            }
        }
    }
 
    private function reportConnectionError( $extraLbError = '' ): never {
        if ( $this->lastErrorConn instanceof IDatabaseForOwner ) {
            $srvName = $this->lastErrorConn->getServerName();
            $lastDbError = $this->lastErrorConn->lastError() ?: 'unknown error';
 
            $exception = new DBConnectionError(
                $this->lastErrorConn,
                $extraLbError
                    ? "{$extraLbError}; {$lastDbError} ({$srvName})"
                    : "{$lastDbError} ({$srvName})"
            );
 
            if ( $extraLbError ) {
                $this->logger->warning(
                    __METHOD__ . ": $extraLbError; {last_error} ({db_server})",
                    $this->getConnLogContext(
                        $this->lastErrorConn,
                        [
                            'method' => __METHOD__,
                            'last_error' => $lastDbError
                        ]
                    )
                );
            }
        } else {
            $exception = new DBConnectionError(
                null,
                $extraLbError ?: 'could not connect to the DB server'
            );
 
            if ( $extraLbError ) {
                $this->logger->error(
                    __METHOD__ . ": $extraLbError",
                    [
                        'method' => __METHOD__,
                        'last_error' => '(last connection error missing)'
                    ]
                );
            }
        }
 
        throw $exception;
    }
 
    public function getServerCount() {
        return $this->serverInfo->getServerCount();
    }
 
    public function hasReplicaServers() {
        return $this->serverInfo->hasReplicaServers();
    }
 
    public function hasStreamingReplicaServers() {
        return $this->serverInfo->hasStreamingReplicaServers();
    }
 
    public function getServerName( $i ): string {
        return $this->serverInfo->getServerName( $i );
    }
 
    public function getServerInfo( $i ) {
        return $this->serverInfo->getServerInfo( $i );
    }
 
    public function getServerType( $i ) {
        return $this->serverInfo->getServerType( $i );
    }
 
    public function getPrimaryPos() {
        $conn = $this->getAnyOpenConnection( ServerInfo::WRITER_INDEX );
        if ( $conn ) {
            return $conn->getPrimaryPos();
        }
 
        $conn = $this->getConnectionInternal( ServerInfo::WRITER_INDEX, [], false, self::CONN_SILENCE_ERRORS );
        // @phan-suppress-next-line PhanRedundantCondition
        if ( !$conn ) {
            $this->reportConnectionError();
        }
 
        // ::getConnectionInternal() should return IDatabaseForOwner but changing signature
        // is not straightforward (being implemented in Wikibase)
        '@phan-var IDatabaseForOwner $conn';
        try {
            return $conn->getPrimaryPos();
        } finally {
            $this->closeConnection( $conn );
        }
    }
 
    public function reconfigure( array $params ) {
        $anyServerDepooled = false;
 
        $paramServers = $params['servers'];
        $newIndexByServerIndex = $this->serverInfo->reconfigureServers( $paramServers );
        foreach ( $newIndexByServerIndex as $i => $ni ) {
            if ( $ni === null ) {
                // Server no longer exists in the new config
                $anyServerDepooled = true;
                // Note that if the primary server is depooled and a replica server promoted
                // to new primary, then DB_PRIMARY handles will fail with server index errors
                unset( $this->loads[$i] );
            }
        }
 
        if ( $anyServerDepooled ) {
            // NOTE: We could close all connection here, but some may be in the middle of
            //       a transaction. So instead, we leave it to DBConnRef to close the
            //       connection when it detects that the modcount has changed and no
            //       transaction is open.
            $this->logger->info( 'Reconfiguring dbs!' );
            // Unpin DB_REPLICA connection groups from server indexes
            $this->readIndexByGroup = [];
            // We could close all connection here, but some may be in the middle of a
            // transaction. So instead, we leave it to DBConnRef to close the connection
            // when it detects that the modcount has changed and no transaction is open.
            $this->conns = self::newTrackedConnectionsArray();
            // Bump modification counter to invalidate the connections held by DBConnRef
            // instances. This will cause the next call to a method on the DBConnRef
            // to get a new connection from getConnectionInternal()
            $this->modcount++;
        }
    }
 
    public function disable( $fname = __METHOD__ ) {
        $this->closeAll( $fname );
        $this->disabled = true;
    }
 
    public function closeAll( $fname = __METHOD__ ) {
        $scope = ScopedCallback::newScopedIgnoreUserAbort();
        foreach ( $this->getOpenConnections() as $conn ) {
            $conn->close( $fname );
        }
 
        $this->conns = self::newTrackedConnectionsArray();
    }
 
    private function closeConnection( IDatabaseForOwner $conn ) {
        if ( $conn instanceof DBConnRef ) {
            // Avoid calling close() but still leaving the handle in the pool
            throw new RuntimeException( 'Cannot close DBConnRef instance; it must be shareable' );
        }
 
        $domain = $conn->getDomainID();
        $serverIndex = $conn->getLBInfo( self::INFO_SERVER_INDEX );
        if ( $serverIndex === null ) {
            throw new UnexpectedValueException( "Handle on '$domain' missing server index" );
        }
 
        $srvName = $this->serverInfo->getServerName( $serverIndex );
 
        $found = false;
        foreach ( $this->conns as $type => $poolConnsByServer ) {
            $key = array_search( $conn, $poolConnsByServer[$serverIndex] ?? [], true );
            if ( $key !== false ) {
                $found = true;
                unset( $this->conns[$type][$serverIndex][$key] );
            }
        }
 
        if ( !$found ) {
            $this->logger->warning(
                __METHOD__ .
                ": orphaned connection to database {$this->stringifyConn( $conn )} at '$srvName'."
            );
        }
 
        $this->logger->debug(
            __METHOD__ .
            ": closing connection to database {$this->stringifyConn( $conn )} at '$srvName'."
        );
 
        $conn->close( __METHOD__ );
    }
 
    public function finalizePrimaryChanges( $fname = __METHOD__ ) {
        $this->assertTransactionRoundStage( [ self::ROUND_CURSORY, self::ROUND_FINALIZED ] );
        $scope = ScopedCallback::newScopedIgnoreUserAbort();
 
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
        // Loop until callbacks stop adding callbacks on other connections
        $total = 0;
        do {
            $count = 0; // callbacks execution attempts
            foreach ( $this->getOpenPrimaryConnections() as $conn ) {
                // Run any pre-commit callbacks while leaving the post-commit ones suppressed.
                // Any error should cause all (peer) transactions to be rolled back together.
                $count += $conn->runOnTransactionPreCommitCallbacks();
            }
            $total += $count;
        } while ( $count > 0 );
        // Defer post-commit callbacks until after COMMIT/ROLLBACK happens on all handles
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            $conn->setTrxEndCallbackSuppression( true );
        }
        $this->trxRoundStage = self::ROUND_FINALIZED;
 
        return $total;
    }
 
    public function approvePrimaryChanges( int $maxWriteDuration, $fname = __METHOD__ ) {
        $this->assertTransactionRoundStage( self::ROUND_FINALIZED );
        $scope = ScopedCallback::newScopedIgnoreUserAbort();
 
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            // Any atomic sections should have been closed by now and there definitely should
            // not be any open transactions started by begin() from callers outside Database.
            if ( $conn->explicitTrxActive() ) {
                throw new DBTransactionError(
                    $conn,
                    "Explicit transaction still active; a caller might have failed to call " .
                    "endAtomic() or cancelAtomic()."
                );
            }
            // Assert that the time to replicate the transaction will be reasonable.
            // If this fails, then all DB transactions will be rollback back together.
            $time = $conn->pendingWriteQueryDuration( $conn::ESTIMATE_DB_APPLY );
            if ( $maxWriteDuration > 0 ) {
                if ( $time > $maxWriteDuration ) {
                    $humanTimeSec = round( $time, 3 );
                    throw new DBTransactionSizeError(
                        $conn,
                        "Transaction spent {time}s in writes, exceeding the {$maxWriteDuration}s limit",
                        // Message parameters for: transaction-duration-limit-exceeded
                        [ $time, $maxWriteDuration ],
                        null,
                        [ 'time' => $humanTimeSec ]
                    );
                } elseif ( $time > 0 ) {
                    $timeMs = $time * 1000;
                    $humanTimeMs = round( $timeMs, $timeMs > 1 ? 0 : 3 );
                    $this->logger->debug(
                        "Transaction spent {time_ms}ms in writes, under the {$maxWriteDuration}s limit",
                        [ 'time_ms' => $humanTimeMs ]
                    );
                }
            }
            // If a connection sits idle for too long it might be dropped, causing transaction
            // writes and session locks to be lost. Ping all the server connections before making
            // any attempt to commit the transactions belonging to the active transaction round.
            if ( $conn->writesOrCallbacksPending() || $conn->sessionLocksPending() ) {
                if ( !$conn->ping() ) {
                    throw new DBTransactionError(
                        $conn,
                        "Pre-commit ping failed on server {$conn->getServerName()}"
                    );
                }
            }
        }
        $this->trxRoundStage = self::ROUND_APPROVED;
    }
 
    public function beginPrimaryChanges( $fname = __METHOD__ ) {
        if ( $this->trxRoundFname !== null ) {
            throw new DBTransactionError(
                null,
                "Transaction round '{$this->trxRoundFname}' already started"
            );
        }
        $this->assertTransactionRoundStage( self::ROUND_CURSORY );
        $scope = ScopedCallback::newScopedIgnoreUserAbort();
 
        // Clear any empty transactions (no writes/callbacks) from the implicit round
        $this->flushPrimarySnapshots( $fname );
 
        $this->trxRoundFname = $fname;
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
        // Mark applicable handles as participating in this explicit transaction round.
        // For each of these handles, any writes and callbacks will be tied to a single
        // transaction. The (peer) handles will reject begin()/commit() calls unless they
        // are part of an en masse commit or an en masse rollback.
        foreach ( $this->getOpenConnections() as $conn ) {
            $this->syncConnectionRoundState( $conn );
        }
        $this->trxRoundStage = self::ROUND_CURSORY;
    }
 
    public function commitPrimaryChanges( $fname = __METHOD__ ) {
        $this->assertTransactionRoundStage( self::ROUND_APPROVED );
        $scope = ScopedCallback::newScopedIgnoreUserAbort();
 
        $failures = [];
 
        $this->trxRoundFname = null;
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
        // Commit any writes and clear any snapshots as well (callbacks require AUTOCOMMIT).
        // Note that callbacks should already be suppressed due to finalizePrimaryChanges().
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            try {
                $conn->commit( $fname, $conn::FLUSHING_ALL_PEERS );
            } catch ( DBError $e ) {
                ( $this->errorLogger )( $e );
                $failures[] = "{$conn->getServerName()}: {$e->getMessage()}";
            }
        }
        if ( $failures ) {
            throw new DBTransactionError(
                null,
                "Commit failed on server(s) " . implode( "\n", array_unique( $failures ) )
            );
        }
        // Unmark handles as participating in this explicit transaction round
        foreach ( $this->getOpenConnections() as $conn ) {
            $this->syncConnectionRoundState( $conn );
        }
        $this->trxRoundStage = self::ROUND_COMMIT_CALLBACKS;
    }
 
    public function runPrimaryTransactionIdleCallbacks( $fname = __METHOD__ ) {
        if ( $this->trxRoundStage === self::ROUND_COMMIT_CALLBACKS ) {
            $type = IDatabase::TRIGGER_COMMIT;
        } elseif ( $this->trxRoundStage === self::ROUND_ROLLBACK_CALLBACKS ) {
            $type = IDatabase::TRIGGER_ROLLBACK;
        } else {
            throw new DBTransactionError(
                null,
                "Transaction should be in the callback stage (not '{$this->trxRoundStage}')"
            );
        }
        $scope = ScopedCallback::newScopedIgnoreUserAbort();
 
        $oldStage = $this->trxRoundStage;
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
 
        // Now that the COMMIT/ROLLBACK step is over, enable post-commit callback runs
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            $conn->setTrxEndCallbackSuppression( false );
        }
 
        $errors = [];
        $fname = __METHOD__;
        // Loop until callbacks stop adding callbacks on other connections
        do {
            // Run any pending callbacks for each connection...
            $count = 0; // callback execution attempts
            foreach ( $this->getOpenPrimaryConnections() as $conn ) {
                if ( $conn->trxLevel() ) {
                    continue; // retry in the next iteration, after commit() is called
                }
                $count += $conn->runOnTransactionIdleCallbacks( $type, $errors );
            }
            // Clear out any active transactions left over from callbacks...
            foreach ( $this->getOpenPrimaryConnections() as $conn ) {
                if ( $conn->writesPending() ) {
                    // A callback from another handle wrote to this one and DBO_TRX is set
                    $fnames = implode( ', ', $conn->pendingWriteCallers() );
                    $this->logger->info(
                        "$fname: found writes pending ($fnames).",
                        $this->getConnLogContext(
                            $conn,
                            [ 'exception' => new RuntimeException() ]
                        )
                    );
                    $this->statsFactory->getCounter( 'rdbms_callback_writes_found_total' )
                        ->setLabel( 'db_cluster', $this->getClusterName() )
                        ->setLabel( 'db_server', $conn->getServerName() )
                        ->increment();
                } elseif ( $conn->trxLevel() ) {
                    // A callback from another handle read from this one and DBO_TRX is set,
                    // which can easily happen if there is only one DB (no replicas)
                    $this->logger->debug( "$fname: found empty transaction." );
                }
                try {
                    $conn->commit( $fname, $conn::FLUSHING_ALL_PEERS );
                } catch ( DBError $ex ) {
                    $errors[] = $ex;
                }
            }
        } while ( $count > 0 );
 
        $this->trxRoundStage = $oldStage;
 
        return $errors[0] ?? null;
    }
 
    public function runPrimaryTransactionListenerCallbacks( $fname = __METHOD__ ) {
        if ( $this->trxRoundStage === self::ROUND_COMMIT_CALLBACKS ) {
            $type = IDatabase::TRIGGER_COMMIT;
        } elseif ( $this->trxRoundStage === self::ROUND_ROLLBACK_CALLBACKS ) {
            $type = IDatabase::TRIGGER_ROLLBACK;
        } else {
            throw new DBTransactionError(
                null,
                "Transaction should be in the callback stage (not '{$this->trxRoundStage}')"
            );
        }
        $scope = ScopedCallback::newScopedIgnoreUserAbort();
 
        $errors = [];
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            $conn->runTransactionListenerCallbacks( $type, $errors );
        }
        $this->trxRoundStage = self::ROUND_CURSORY;
 
        return $errors[0] ?? null;
    }
 
    public function rollbackPrimaryChanges( $fname = __METHOD__ ) {
        $scope = ScopedCallback::newScopedIgnoreUserAbort();
 
        $this->trxRoundFname = null;
        $this->trxRoundStage = self::ROUND_ERROR; // "failed" until proven otherwise
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            $conn->rollback( $fname, $conn::FLUSHING_ALL_PEERS );
        }
        // Unmark handles as participating in this explicit transaction round
        foreach ( $this->getOpenConnections() as $conn ) {
            $this->syncConnectionRoundState( $conn );
        }
        $this->trxRoundStage = self::ROUND_ROLLBACK_CALLBACKS;
    }
 
    public function flushPrimarySessions( $fname = __METHOD__ ) {
        $this->assertTransactionRoundStage( [ self::ROUND_CURSORY ] );
        if ( $this->hasPrimaryChanges() ) {
            // Any transaction should have been rolled back beforehand
            throw new DBTransactionError( null, "Cannot reset session while writes are pending" );
        }
 
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            $conn->flushSession( $fname, $conn::FLUSHING_ALL_PEERS );
        }
    }
 
    private function assertTransactionRoundStage( $stage ) {
        $stages = (array)$stage;
 
        if ( !in_array( $this->trxRoundStage, $stages, true ) ) {
            $stageList = implode(
                '/',
                array_map( static fn ( $v ) => "'$v'", $stages )
            );
            $transactionName = $this->trxRoundFname ?? '<unknown>';
            throw new DBTransactionError(
                null,
                "Transaction '$transactionName' round stage must be '$stageList' (not '{$this->trxRoundStage}')"
            );
        }
    }
 
    private function syncConnectionRoundState( Database $conn ) {
        if ( $conn->getLBInfo( self::INFO_CONN_CATEGORY ) !== self::CATEGORY_ROUND ) {
            return; // transaction rounds do not apply to these connections
        }
 
        if ( $this->trxRoundFname !== null ) {
            // Explicit transaction round
            $trxRoundLevel = 1;
        } else {
            // Implicit auto-commit round (cli mode) or implicit transaction round (web mode)
            $trxRoundLevel = ( $this->cliMode ? 0 : 1 );
        }
 
        // CATEGORY_ROUND DB handles with DBO_DEFAULT are considered "round aware" and the
        // load balancer will take over the logic of when DBO_TRX is set for such DB handles.
        // DBO_TRX will be set only when a transaction round is active (explicit or implicit).
        if ( $conn->getFlag( $conn::DBO_DEFAULT ) ) {
            if ( $trxRoundLevel ) {
                // Wrap queries in a transaction joined to the active transaction round
                $conn->setFlag( $conn::DBO_TRX );
            } else {
                // Do not wrap queries in a transaction (no active transaction round)
                $conn->clearFlag( $conn::DBO_TRX );
            }
        }
 
        // Note that DBO_TRX is normally only set above or by DatabaseFactory applying
        // DBO_DEFAULT. However, integration tests might directly force DBO_TRX in the
        // server configuration arrays. Such handles will still be flushed during calls
        // to {@link LoadBalancer::commitPrimaryChanges()}.
        if ( $conn->getFlag( $conn::DBO_TRX ) ) {
            // DB handle is participating in the active transaction round
            $conn->setLBInfo( $conn::LB_TRX_ROUND_LEVEL, $trxRoundLevel );
            $conn->setLBInfo( $conn::LB_TRX_ROUND_FNAME, $this->trxRoundFname );
        } else {
            // DB handle is not participating in any transaction rounds
            $conn->setLBInfo( $conn::LB_TRX_ROUND_LEVEL, 0 );
            $conn->setLBInfo( $conn::LB_TRX_ROUND_FNAME, null );
        }
    }
 
    public function flushReplicaSnapshots( $fname = __METHOD__ ) {
        foreach ( $this->conns as $poolConnsByServer ) {
            foreach ( $poolConnsByServer as $serverIndex => $serverConns ) {
                if ( $serverIndex === ServerInfo::WRITER_INDEX ) {
                    continue; // skip primary
                }
                foreach ( $serverConns as $conn ) {
                    $conn->flushSnapshot( $fname );
                }
            }
        }
    }
 
    public function flushPrimarySnapshots( $fname = __METHOD__ ) {
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            $conn->flushSnapshot( $fname );
        }
    }
 
    public function hasPrimaryConnection() {
        return (bool)$this->getAnyOpenConnection( ServerInfo::WRITER_INDEX );
    }
 
    public function hasPrimaryChanges() {
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            if ( $conn->writesOrCallbacksPending() ) {
                return true;
            }
        }
 
        return false;
    }
 
    public function lastPrimaryChangeTimestamp() {
        $lastTime = null;
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            $lastTime = max( $lastTime, $conn->lastDoneWrites() );
        }
 
        return $lastTime;
    }
 
    public function hasOrMadeRecentPrimaryChanges( $age = null ) {
        $age ??= self::MAX_WAIT_DEFAULT;
 
        return ( $this->hasPrimaryChanges()
            || $this->lastPrimaryChangeTimestamp() > microtime( true ) - $age );
    }
 
    public function pendingPrimaryChangeCallers() {
        $fnames = [];
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            $fnames = array_merge( $fnames, $conn->pendingWriteCallers() );
        }
 
        return $fnames;
    }
 
    public function explicitTrxActive() {
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            if ( $conn->explicitTrxActive() ) {
                return true;
            }
        }
        return false;
    }
 
    private function setLaggedReplicaMode(): void {
        $this->laggedReplicaMode = true;
        $this->logger->warning( __METHOD__ . ": setting lagged replica mode" );
    }
 
    public function laggedReplicaUsed() {
        return $this->laggedReplicaMode;
    }
 
    public function getReadOnlyReason() {
        if ( $this->readOnlyReason !== false ) {
            return $this->readOnlyReason;
        } elseif ( $this->isPrimaryRunningReadOnly() ) {
            return 'The primary database server is running in read-only mode.';
        }
 
        return false;
    }
 
    private function isPrimaryRunningReadOnly( ?IDatabaseForOwner $conn = null ) {
        // Context will often be HTTP GET/HEAD; heavily cache the results
        return (bool)$this->wanCache->getWithSetCallback(
            // Note that table prefixes are not related to server-side read-only mode
            $this->wanCache->makeGlobalKey(
                'rdbms-server-readonly',
                $this->serverInfo->getPrimaryServerName()
            ),
            self::TTL_CACHE_READONLY,
            function ( $oldValue ) use ( $conn ) {
                $scope = $this->trxProfiler->silenceForScope();
                $conn ??= $this->getServerConnection(
                    ServerInfo::WRITER_INDEX,
                    self::DOMAIN_ANY,
                    self::CONN_SILENCE_ERRORS
                );
                if ( $conn ) {
                    try {
                        $value = (int)$conn->serverIsReadOnly();
                    } catch ( DBError ) {
                        $value = is_int( $oldValue ) ? $oldValue : 0;
                    }
                } else {
                    $value = 0;
                }
                ScopedCallback::consume( $scope );
 
                return $value;
            },
            [
                'busyValue' => 0,
                'pcTTL' => WANObjectCache::TTL_PROC_LONG
            ]
        );
    }
 
    public function pingAll() {
        $success = true;
        foreach ( $this->getOpenConnections() as $conn ) {
            if ( !$conn->ping() ) {
                $success = false;
            }
        }
 
        return $success;
    }
 
    private function getOpenConnections() {
        foreach ( $this->conns as $poolConnsByServer ) {
            foreach ( $poolConnsByServer as $serverConns ) {
                foreach ( $serverConns as $conn ) {
                    yield $conn;
                }
            }
        }
    }
 
    private function getOpenPrimaryConnections() {
        foreach ( $this->conns as $poolConnsByServer ) {
            foreach ( ( $poolConnsByServer[ServerInfo::WRITER_INDEX] ?? [] ) as $conn ) {
                yield $conn;
            }
        }
    }
 
    public function getMaxLag() {
        $host = '';
        $maxLag = -1;
        $maxIndex = 0;
 
        if ( $this->serverInfo->hasReplicaServers() ) {
            $lagTimes = $this->getLagTimes();
            foreach ( $lagTimes as $i => $lag ) {
                // Allowing the value to be unset due to stale cache (T361824)
                $load = $this->loads[$i] ?? 0;
                if ( $load > 0 && $lag > $maxLag ) {
                    $maxLag = $lag;
                    $host = $this->serverInfo->getServerInfoStrict( $i, 'host' );
                    $maxIndex = $i;
                }
            }
        }
 
        return [ $host, $maxLag, $maxIndex ];
    }
 
    public function getLagTimes() {
        if ( !$this->hasReplicaServers() ) {
            return [ ServerInfo::WRITER_INDEX => 0 ]; // no replication = no lag
        }
        $fname = __METHOD__;
        return $this->wanCache->getWithSetCallback(
            $this->wanCache->makeGlobalKey( 'rdbms-lags', $this->getClusterName() ),
            // Add jitter to avoid stampede
            10 + mt_rand( 1, 10 ),
            function () use ( $fname ) {
                $lags = [];
                foreach ( $this->serverInfo->getStreamingReplicaIndexes() as $i ) {
                    $conn = $this->getServerConnection(
                        $i,
                        self::DOMAIN_ANY,
                        self::CONN_SILENCE_ERRORS | self::CONN_UNTRACKED_GAUGE
                    );
                    if ( $conn ) {
                        $lags[$i] = $conn->getLag();
                        $conn->close( $fname );
                    } else {
                        $lags[$i] = false;
                    }
                }
                return $lags;
            },
            [ 'lockTSE' => 30 ]
        );
    }
 
    public function waitForPrimaryPos( IDatabase $conn ) {
        if ( $conn->getLBInfo( self::INFO_SERVER_INDEX ) === ServerInfo::WRITER_INDEX ) {
            return true; // not a replica DB server
        }
 
        // Get the current primary DB position, opening a connection only if needed
        $flags = self::CONN_SILENCE_ERRORS;
        $primaryConn = $this->getAnyOpenConnection( ServerInfo::WRITER_INDEX, $flags );
        if ( $primaryConn ) {
            $pos = $primaryConn->getPrimaryPos();
        } else {
            $primaryConn = $this->getServerConnection( ServerInfo::WRITER_INDEX, self::DOMAIN_ANY, $flags );
            if ( !$primaryConn ) {
                throw new DBReplicationWaitError(
                    null,
                    "Could not obtain a primary database connection to get the position"
                );
            }
            $pos = $primaryConn->getPrimaryPos();
            $this->closeConnection( $primaryConn );
        }
 
        if ( $pos instanceof DBPrimaryPos && $conn instanceof IDatabaseForOwner ) {
            $this->logger->debug( __METHOD__ . ': waiting' );
            $result = $conn->primaryPosWait( $pos, self::MAX_WAIT_DEFAULT );
            $ok = ( $result !== null && $result != -1 );
            if ( $ok ) {
                $this->logger->debug( __METHOD__ . ': done waiting (success)' );
            } else {
                $this->logger->debug( __METHOD__ . ': done waiting (failure)' );
            }
        } else {
            $ok = false; // something is misconfigured
            $this->logger->error(
                __METHOD__ . ': could not get primary pos for {db_server}',
                $this->getConnLogContext( $conn, [ 'exception' => new RuntimeException() ] )
            );
        }
 
        return $ok;
    }
 
    public function setTransactionListener( $name, ?callable $callback = null ) {
        if ( $callback ) {
            $this->trxRecurringCallbacks[$name] = $callback;
        } else {
            unset( $this->trxRecurringCallbacks[$name] );
        }
        foreach ( $this->getOpenPrimaryConnections() as $conn ) {
            $conn->setTransactionListener( $name, $callback );
        }
    }
 
    public function setTableAliases( array $aliases ) {
        $this->tableAliases = $aliases;
    }
 
    public function setDomainAliases( array $aliases ) {
        $this->domainAliases = $aliases;
    }
 
    public function setLocalDomainPrefix( $prefix ) {
        $oldLocalDomain = $this->localDomain;
        $this->localDomain = new DatabaseDomain(
            $this->localDomain->getDatabase(),
            $this->localDomain->getSchema(),
            $prefix
        );
 
        // Update the prefix for existing connections.
        // Existing DBConnRef handles will not be affected.
        foreach ( $this->getOpenConnections() as $conn ) {
            if ( $oldLocalDomain->equals( $conn->getDomainID() ) ) {
                $conn->tablePrefix( $prefix );
            }
        }
    }
 
    public function redefineLocalDomain( $domain ) {
        $this->closeAll( __METHOD__ );
        $this->localDomain = DatabaseDomain::newFromId( $domain );
    }
 
    public function setTempTablesOnlyMode( $value, $domain ) {
        $old = $this->tempTablesOnlyMode[$domain] ?? false;
        if ( $value ) {
            $this->tempTablesOnlyMode[$domain] = true;
        } else {
            unset( $this->tempTablesOnlyMode[$domain] );
        }
 
        return $old;
    }
 
    private function stringifyConn( IDatabase $conn ) {
        return $conn->getLBInfo( self::INFO_SERVER_INDEX ) . '/' . $conn->getDomainID();
    }
 
    private function fieldHasBit( int $flags, int $bit ) {
        return ( ( $flags & $bit ) === $bit );
    }
 
    protected function getConnLogContext( IDatabase $conn, array $extras = [] ) {
        return $extras + [
            'db_server' => $conn->getServerName(),
            'db_domain' => $conn->getDomainID(),
        ];
    }
 
    public function setMockTime( &$time ) {
        if ( !$this->loadMonitor instanceof LoadMonitor ) {
            throw new LogicException( 'Cannot set mock time on ' . $this::class . ' (consider adding ' .
                'setMockTime to ILoadMonitor).' );
        }
        $this->loadMonitor->setMockTime( $time );
    }
 
    public function setDefaultGroupName( string $defaultGroup ): void {
        $this->defaultGroup = $defaultGroup;
    }
}