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<?php
/**
* Database load monitoring.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License along
* with this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
* http://www.gnu.org/copyleft/gpl.html
*
* @file
* @ingroup Database
*/
/**
* An interface for database load monitoring
*
* @ingroup Database
*/
interface LoadMonitor {
/**
* Construct a new LoadMonitor with a given LoadBalancer parent
*
* @param LoadBalancer $parent
*/
public function __construct( $parent );
/**
* Perform pre-connection load ratio adjustment.
* @param array $loads
* @param string|bool $group The selected query group. Default: false
* @param string|bool $wiki Default: false
*/
public function scaleLoads( &$loads, $group = false, $wiki = false );
/**
* Return an estimate of replication lag for each server
*
* @param array $serverIndexes
* @param string $wiki
*
* @return array
*/
public function getLagTimes( $serverIndexes, $wiki );
}
class LoadMonitorNull implements LoadMonitor {
public function __construct( $parent ) {
}
public function scaleLoads( &$loads, $group = false, $wiki = false ) {
}
public function getLagTimes( $serverIndexes, $wiki ) {
return array_fill_keys( $serverIndexes, 0 );
}
}
/**
* Basic MySQL load monitor with no external dependencies
* Uses memcached to cache the replication lag for a short time
*
* @ingroup Database
*/
class LoadMonitorMySQL implements LoadMonitor {
/** @var LoadBalancer */
public $parent;
/** @var BagOStuff */
protected $cache;
public function __construct( $parent ) {
global $wgMemc;
$this->parent = $parent;
$this->cache = $wgMemc ?: wfGetMainCache();
}
public function scaleLoads( &$loads, $group = false, $wiki = false ) {
}
public function getLagTimes( $serverIndexes, $wiki ) {
if ( count( $serverIndexes ) == 1 && reset( $serverIndexes ) == 0 ) {
// Single server only, just return zero without caching
return array( 0 => 0 );
}
$section = new ProfileSection( __METHOD__ );
$expiry = 5;
$requestRate = 10;
$cache = $this->cache;
$masterName = $this->parent->getServerName( 0 );
$memcKey = wfMemcKey( 'lag_times', $masterName );
$times = $cache->get( $memcKey );
if ( is_array( $times ) ) {
# Randomly recache with probability rising over $expiry
$elapsed = time() - $times['timestamp'];
$chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
if ( mt_rand( 0, $chance ) != 0 ) {
unset( $times['timestamp'] ); // hide from caller
return $times;
}
wfIncrStats( 'lag_cache_miss_expired' );
} else {
wfIncrStats( 'lag_cache_miss_absent' );
}
# Cache key missing or expired
if ( $cache->add( "$memcKey:lock", 1, 10 ) ) {
# Let this process alone update the cache value
$unlocker = new ScopedCallback( function () use ( $cache, $memcKey ) {
$cache->delete( $memcKey );
} );
} elseif ( is_array( $times ) ) {
# Could not acquire lock but an old cache exists, so use it
unset( $times['timestamp'] ); // hide from caller
return $times;
}
$times = array();
foreach ( $serverIndexes as $i ) {
if ( $i == 0 ) { # Master
$times[$i] = 0;
} elseif ( false !== ( $conn = $this->parent->getAnyOpenConnection( $i ) ) ) {
$times[$i] = $conn->getLag();
} elseif ( false !== ( $conn = $this->parent->openConnection( $i, $wiki ) ) ) {
$times[$i] = $conn->getLag();
// Close the connection to avoid sleeper connections piling up.
// Note that the caller will pick one of these DBs and reconnect,
// which is slightly inefficient, but this only matters for the lag
// time cache miss cache, which is far less common that cache hits.
$this->parent->closeConnection( $conn );
}
}
# Add a timestamp key so we know when it was cached
$times['timestamp'] = time();
$cache->set( $memcKey, $times, $expiry + 10 );
unset( $times['timestamp'] ); // hide from caller
return $times;
}
}
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