MediaWiki  REL1_31
LoadMonitor.php
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1 <?php
22 namespace Wikimedia\Rdbms;
23 
24 use Psr\Log\LoggerInterface;
25 use Psr\Log\NullLogger;
26 use Wikimedia\ScopedCallback;
29 
36 class LoadMonitor implements ILoadMonitor {
38  protected $parent;
40  protected $srvCache;
42  protected $wanCache;
44  protected $replLogger;
45 
47  private $movingAveRatio;
50 
52  const VERSION = 1;
54  const LAG_WARN_THRESHOLD = 10;
55 
64  public function __construct(
66  ) {
67  $this->parent = $lb;
68  $this->srvCache = $srvCache;
69  $this->wanCache = $wCache;
70  $this->replLogger = new NullLogger();
71 
72  $this->movingAveRatio = isset( $options['movingAveRatio'] )
73  ? $options['movingAveRatio']
74  : 0.1;
75  $this->lagWarnThreshold = isset( $options['lagWarnThreshold'] )
76  ? $options['lagWarnThreshold']
77  : self::LAG_WARN_THRESHOLD;
78  }
79 
80  public function setLogger( LoggerInterface $logger ) {
81  $this->replLogger = $logger;
82  }
83 
84  final public function scaleLoads( array &$weightByServer, $domain ) {
85  $serverIndexes = array_keys( $weightByServer );
86  $states = $this->getServerStates( $serverIndexes, $domain );
87  $newScalesByServer = $states['weightScales'];
88  foreach ( $weightByServer as $i => $weight ) {
89  if ( isset( $newScalesByServer[$i] ) ) {
90  $weightByServer[$i] = $weight * $newScalesByServer[$i];
91  } else { // server recently added to config?
92  $host = $this->parent->getServerName( $i );
93  $this->replLogger->error( __METHOD__ . ": host $host not in cache" );
94  }
95  }
96  }
97 
98  final public function getLagTimes( array $serverIndexes, $domain ) {
99  return $this->getServerStates( $serverIndexes, $domain )['lagTimes'];
100  }
101 
102  protected function getServerStates( array $serverIndexes, $domain ) {
103  $writerIndex = $this->parent->getWriterIndex();
104  if ( count( $serverIndexes ) == 1 && reset( $serverIndexes ) == $writerIndex ) {
105  # Single server only, just return zero without caching
106  return [
107  'lagTimes' => [ $writerIndex => 0 ],
108  'weightScales' => [ $writerIndex => 1.0 ]
109  ];
110  }
111 
112  $key = $this->getCacheKey( $serverIndexes );
113  # Randomize TTLs to reduce stampedes (4.0 - 5.0 sec)
114  $ttl = mt_rand( 4e6, 5e6 ) / 1e6;
115  # Keep keys around longer as fallbacks
116  $staleTTL = 60;
117 
118  # (a) Check the local APC cache
119  $value = $this->srvCache->get( $key );
120  if ( $value && $value['timestamp'] > ( microtime( true ) - $ttl ) ) {
121  $this->replLogger->debug( __METHOD__ . ": got lag times ($key) from local cache" );
122  return $value; // cache hit
123  }
124  $staleValue = $value ?: false;
125 
126  # (b) Check the shared cache and backfill APC
127  $value = $this->wanCache->get( $key );
128  if ( $value && $value['timestamp'] > ( microtime( true ) - $ttl ) ) {
129  $this->srvCache->set( $key, $value, $staleTTL );
130  $this->replLogger->debug( __METHOD__ . ": got lag times ($key) from main cache" );
131 
132  return $value; // cache hit
133  }
134  $staleValue = $value ?: $staleValue;
135 
136  # (c) Cache key missing or expired; regenerate and backfill
137  if ( $this->srvCache->lock( $key, 0, 10 ) ) {
138  # Let only this process update the cache value on this server
139  $sCache = $this->srvCache;
141  $unlocker = new ScopedCallback( function () use ( $sCache, $key ) {
142  $sCache->unlock( $key );
143  } );
144  } elseif ( $staleValue ) {
145  # Could not acquire lock but an old cache exists, so use it
146  return $staleValue;
147  }
148 
149  $lagTimes = [];
150  $weightScales = [];
151  $movAveRatio = $this->movingAveRatio;
152  foreach ( $serverIndexes as $i ) {
153  if ( $i == $this->parent->getWriterIndex() ) {
154  $lagTimes[$i] = 0; // master always has no lag
155  $weightScales[$i] = 1.0; // nominal weight
156  continue;
157  }
158 
159  # Handles with open transactions are avoided since they might be subject
160  # to REPEATABLE-READ snapshots, which could affect the lag estimate query.
161  $flags = ILoadBalancer::CONN_TRX_AUTOCOMMIT;
162  $conn = $this->parent->getAnyOpenConnection( $i, $flags );
163  if ( $conn ) {
164  $close = false; // already open
165  } else {
166  $conn = $this->parent->openConnection( $i, ILoadBalancer::DOMAIN_ANY, $flags );
167  $close = true; // new connection
168  }
169 
170  $lastWeight = isset( $staleValue['weightScales'][$i] )
171  ? $staleValue['weightScales'][$i]
172  : 1.0;
173  $coefficient = $this->getWeightScale( $i, $conn ?: null );
174  $newWeight = $movAveRatio * $coefficient + ( 1 - $movAveRatio ) * $lastWeight;
175 
176  // Scale from 10% to 100% of nominal weight
177  $weightScales[$i] = max( $newWeight, 0.10 );
178 
179  $host = $this->parent->getServerName( $i );
180 
181  if ( !$conn ) {
182  $lagTimes[$i] = false;
183  $this->replLogger->error(
184  __METHOD__ . ": host {db_server} is unreachable",
185  [ 'db_server' => $host ]
186  );
187  continue;
188  }
189 
190  if ( $conn->getLBInfo( 'is static' ) ) {
191  $lagTimes[$i] = 0;
192  } else {
193  $lagTimes[$i] = $conn->getLag();
194  if ( $lagTimes[$i] === false ) {
195  $this->replLogger->error(
196  __METHOD__ . ": host {db_server} is not replicating?",
197  [ 'db_server' => $host ]
198  );
199  } elseif ( $lagTimes[$i] > $this->lagWarnThreshold ) {
200  $this->replLogger->error(
201  "Server {host} has {lag} seconds of lag (>= {maxlag})",
202  [
203  'host' => $host,
204  'lag' => $lagTimes[$i],
205  'maxlag' => $this->lagWarnThreshold
206  ]
207  );
208  }
209  }
210 
211  if ( $close ) {
212  # Close the connection to avoid sleeper connections piling up.
213  # Note that the caller will pick one of these DBs and reconnect,
214  # which is slightly inefficient, but this only matters for the lag
215  # time cache miss cache, which is far less common that cache hits.
216  $this->parent->closeConnection( $conn );
217  }
218  }
219 
220  # Add a timestamp key so we know when it was cached
221  $value = [
222  'lagTimes' => $lagTimes,
223  'weightScales' => $weightScales,
224  'timestamp' => microtime( true )
225  ];
226  $this->wanCache->set( $key, $value, $staleTTL );
227  $this->srvCache->set( $key, $value, $staleTTL );
228  $this->replLogger->info( __METHOD__ . ": re-calculated lag times ($key)" );
229 
230  return $value;
231  }
232 
238  protected function getWeightScale( $index, IDatabase $conn = null ) {
239  return $conn ? 1.0 : 0.0;
240  }
241 
242  private function getCacheKey( array $serverIndexes ) {
243  sort( $serverIndexes );
244  // Lag is per-server, not per-DB, so key on the master DB name
245  return $this->srvCache->makeGlobalKey(
246  'lag-times',
247  self::VERSION,
248  $this->parent->getServerName( $this->parent->getWriterIndex() ),
249  implode( '-', $serverIndexes )
250  );
251  }
252 }
Wikimedia\Rdbms\LoadMonitor\$replLogger
LoggerInterface $replLogger
Definition: LoadMonitor.php:44
Wikimedia\Rdbms\LoadMonitor
Basic DB load monitor with no external dependencies Uses memcached to cache the replication lag for a...
Definition: LoadMonitor.php:36
use
Apache License January AND DISTRIBUTION Definitions License shall mean the terms and conditions for use
Definition: APACHE-LICENSE-2.0.txt:10
Wikimedia\Rdbms\LoadMonitor\$srvCache
BagOStuff $srvCache
Definition: LoadMonitor.php:40
array
the array() calling protocol came about after MediaWiki 1.4rc1.
Wikimedia\Rdbms\ILoadMonitor
An interface for database load monitoring.
Definition: ILoadMonitor.php:35
Wikimedia\Rdbms\LoadMonitor\$parent
ILoadBalancer $parent
Definition: LoadMonitor.php:38
Wikimedia\Rdbms
Definition: ChronologyProtector.php:24
Wikimedia\Rdbms\LoadMonitor\getCacheKey
getCacheKey(array $serverIndexes)
Definition: LoadMonitor.php:242
BagOStuff
interface is intended to be more or less compatible with the PHP memcached client.
Definition: BagOStuff.php:47
Wikimedia\Rdbms\LoadMonitor\getWeightScale
getWeightScale( $index, IDatabase $conn=null)
Definition: LoadMonitor.php:238
php
injection txt This is an overview of how MediaWiki makes use of dependency injection The design described here grew from the discussion of RFC T384 The term dependency this means that anything an object needs to operate should be injected from the the object itself should only know narrow no concrete implementation of the logic it relies on The requirement to inject everything typically results in an architecture that based on two main types of and essentially stateless service objects that use other service objects to operate on the value objects As of the beginning MediaWiki is only starting to use the DI approach Much of the code still relies on global state or direct resulting in a highly cyclical dependency which acts as the top level factory for services in MediaWiki which can be used to gain access to default instances of various services MediaWikiServices however also allows new services to be defined and default services to be redefined Services are defined or redefined by providing a callback the instantiator that will return a new instance of the service When it will create an instance of MediaWikiServices and populate it with the services defined in the files listed by thereby bootstrapping the DI framework Per $wgServiceWiringFiles lists includes ServiceWiring php
Definition: injection.txt:37
Wikimedia\Rdbms\IDatabase
Basic database interface for live and lazy-loaded relation database handles.
Definition: IDatabase.php:38
Wikimedia\Rdbms\LoadMonitor\$wanCache
WANObjectCache $wanCache
Definition: LoadMonitor.php:42
Wikimedia\Rdbms\LoadMonitor\$lagWarnThreshold
int $lagWarnThreshold
Amount of replication lag in seconds before warnings are logged.
Definition: LoadMonitor.php:49
Wikimedia\Rdbms\LoadMonitor\$movingAveRatio
float $movingAveRatio
Moving average ratio (e.g.
Definition: LoadMonitor.php:47
Wikimedia\Rdbms\LoadMonitor\scaleLoads
scaleLoads(array &$weightByServer, $domain)
Perform load ratio adjustment before deciding which server to use.
Definition: LoadMonitor.php:84
Wikimedia\Rdbms\LoadMonitor\getServerStates
getServerStates(array $serverIndexes, $domain)
Definition: LoadMonitor.php:102
$options
null means default in associative array with keys and values unescaped Should be merged with default with a value of false meaning to suppress the attribute in associative array with keys and values unescaped & $options
Definition: hooks.txt:2001
$value
$value
Definition: styleTest.css.php:45
Wikimedia\Rdbms\LoadMonitor\setLogger
setLogger(LoggerInterface $logger)
Definition: LoadMonitor.php:80
WANObjectCache
Multi-datacenter aware caching interface.
Definition: WANObjectCache.php:87
Wikimedia\Rdbms\LoadMonitor\getLagTimes
getLagTimes(array $serverIndexes, $domain)
Get an estimate of replication lag (in seconds) for each server.
Definition: LoadMonitor.php:98
as
This document is intended to provide useful advice for parties seeking to redistribute MediaWiki to end users It s targeted particularly at maintainers for Linux since it s been observed that distribution packages of MediaWiki often break We ve consistently had to recommend that users seeking support use official tarballs instead of their distribution s and this often solves whatever problem the user is having It would be nice if this could such as
Definition: distributors.txt:22
Wikimedia\Rdbms\LoadMonitor\__construct
__construct(ILoadBalancer $lb, BagOStuff $srvCache, WANObjectCache $wCache, array $options=[])
Definition: LoadMonitor.php:64
Wikimedia\Rdbms\ILoadBalancer
Database cluster connection, tracking, load balancing, and transaction manager interface.
Definition: ILoadBalancer.php:78