1 files changed, 121 insertions, 0 deletions
diff --git a/includes/db/LoadMonitor.php b/includes/db/LoadMonitor.php
new file mode 100644
index 00000000..8e16f1a1
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+++ b/includes/db/LoadMonitor.php
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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;
+		$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;
+		}
+	}
+}
+