3 * Database load monitoring
10 * An interface for database load monitoring
14 interface LoadMonitor
{
16 * Construct a new LoadMonitor with a given LoadBalancer parent
18 * @param LoadBalancer $parent
20 function __construct( $parent );
23 * Perform pre-connection load ratio adjustment.
25 * @param $group String: the selected query group
28 function scaleLoads( &$loads, $group = false, $wiki = false );
31 * Perform post-connection backoff.
33 * If the connection is in overload, this should return a backoff factor
34 * which will be used to control polling time. The number of threads
35 * connected is a good measure.
37 * If there is no overload, zero can be returned.
39 * A threshold thread count is given, the concrete class may compare this
40 * to the running thread count. The threshold may be false, which indicates
41 * that the sysadmin has not configured this feature.
43 * @param $conn DatabaseBase
44 * @param $threshold Float
46 function postConnectionBackoff( $conn, $threshold );
49 * Return an estimate of replication lag for each server
51 * @param $serverIndexes
56 function getLagTimes( $serverIndexes, $wiki );
59 class LoadMonitor_Null
implements LoadMonitor
{
60 function __construct( $parent ) {
63 function scaleLoads( &$loads, $group = false, $wiki = false ) {
66 function postConnectionBackoff( $conn, $threshold ) {
69 function getLagTimes( $serverIndexes, $wiki ) {
70 return array_fill_keys( $serverIndexes, 0 );
76 * Basic MySQL load monitor with no external dependencies
77 * Uses memcached to cache the replication lag for a short time
81 class LoadMonitor_MySQL
implements LoadMonitor
{
89 * @param LoadBalancer $parent
91 function __construct( $parent ) {
92 $this->parent
= $parent;
100 function scaleLoads( &$loads, $group = false, $wiki = false ) {
104 * @param $serverIndexes
108 function getLagTimes( $serverIndexes, $wiki ) {
109 if ( count( $serverIndexes ) == 1 && reset( $serverIndexes ) == 0 ) {
110 // Single server only, just return zero without caching
111 return array( 0 => 0 );
114 wfProfileIn( __METHOD__
);
119 if ( empty( $wgMemc ) )
120 $wgMemc = wfGetMainCache();
122 $masterName = $this->parent
->getServerName( 0 );
123 $memcKey = wfMemcKey( 'lag_times', $masterName );
124 $times = $wgMemc->get( $memcKey );
126 # Randomly recache with probability rising over $expiry
127 $elapsed = time() - $times['timestamp'];
128 $chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
129 if ( mt_rand( 0, $chance ) != 0 ) {
130 unset( $times['timestamp'] );
131 wfProfileOut( __METHOD__
);
134 wfIncrStats( 'lag_cache_miss_expired' );
136 wfIncrStats( 'lag_cache_miss_absent' );
139 # Cache key missing or expired
142 foreach ( $serverIndexes as $i ) {
143 if ($i == 0) { # Master
145 } elseif ( false !== ( $conn = $this->parent
->getAnyOpenConnection( $i ) ) ) {
146 $times[$i] = $conn->getLag();
147 } elseif ( false !== ( $conn = $this->parent
->openConnection( $i, $wiki ) ) ) {
148 $times[$i] = $conn->getLag();
152 # Add a timestamp key so we know when it was cached
153 $times['timestamp'] = time();
154 $wgMemc->set( $memcKey, $times, $expiry );
156 # But don't give the timestamp to the caller
157 unset($times['timestamp']);
160 wfProfileOut( __METHOD__
);
165 * @param $conn DatabaseBase
169 function postConnectionBackoff( $conn, $threshold ) {
173 $status = $conn->getMysqlStatus("Thread%");
174 if ( $status['Threads_running'] > $threshold ) {
175 $server = $conn->getProperty( 'mServer' );
176 wfLogDBError( "LB backoff from $server - Threads_running = {$status['Threads_running']}\n" );
177 return $status['Threads_connected'];