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<?php
/**
* An interface for database load monitoring
*/
interface LoadMonitor {
/**
* Construct a new LoadMonitor with a given LoadBalancer parent
*/
function __construct( $parent );
/**
* Perform pre-connection load ratio adjustment.
* @param array $loads
* @param string $group The selected query group
* @param string $wiki
*/
function scaleLoads( &$loads, $group = false, $wiki = false );
/**
* Perform post-connection backoff.
*
* If the connection is in overload, this should return a backoff factor
* which will be used to control polling time. The number of threads
* connected is a good measure.
*
* If there is no overload, zero can be returned.
*
* A threshold thread count is given, the concrete class may compare this
* to the running thread count. The threshold may be false, which indicates
* that the sysadmin has not configured this feature.
*
* @param Database $conn
* @param float $threshold
*/
function postConnectionBackoff( $conn, $threshold );
/**
* Return an estimate of replication lag for each server
*/
function getLagTimes( $serverIndexes, $wiki );
}
/**
* Basic MySQL load monitor with no external dependencies
* Uses memcached to cache the replication lag for a short time
*/
class LoadMonitor_MySQL implements LoadMonitor {
var $parent; // LoadBalancer
function __construct( $parent ) {
$this->parent = $parent;
}
function scaleLoads( &$loads, $group = false, $wiki = false ) {
}
function getLagTimes( $serverIndexes, $wiki ) {
wfProfileIn( __METHOD__ );
$expiry = 5;
$requestRate = 10;
global $wgMemc;
if ( empty( $wgMemc ) )
$wgMemc = wfGetMainCache();
$masterName = $this->parent->getServerName( 0 );
$memcKey = wfMemcKey( 'lag_times', $masterName );
$times = $wgMemc->get( $memcKey );
if ( $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'] );
wfProfileOut( __METHOD__ );
return $times;
}
wfIncrStats( 'lag_cache_miss_expired' );
} else {
wfIncrStats( 'lag_cache_miss_absent' );
}
# Cache key missing or expired
$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();
}
}
# Add a timestamp key so we know when it was cached
$times['timestamp'] = time();
$wgMemc->set( $memcKey, $times, $expiry );
# But don't give the timestamp to the caller
unset($times['timestamp']);
$lagTimes = $times;
wfProfileOut( __METHOD__ );
return $lagTimes;
}
function postConnectionBackoff( $conn, $threshold ) {
if ( !$threshold ) {
return 0;
}
$status = $conn->getStatus("Thread%");
if ( $status['Threads_running'] > $threshold ) {
return $status['Threads_connected'];
} else {
return 0;
}
}
}
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