includes/libs/rdbms/ChronologyProtector.php

   1 <?php
   2 /**
   3  * Generator of database load balancing objects.
   4  *
   5  * This program is free software; you can redistribute it and/or modify
   6  * it under the terms of the GNU General Public License as published by
   7  * the Free Software Foundation; either version 2 of the License, or
   8  * (at your option) any later version.
   9  *
  10  * This program is distributed in the hope that it will be useful,
  11  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  13  * GNU General Public License for more details.
  14  *
  15  * You should have received a copy of the GNU General Public License along
  16  * with this program; if not, write to the Free Software Foundation, Inc.,
  17  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  18  * http://www.gnu.org/copyleft/gpl.html
  19  *
  20  * @file
  21  * @ingroup Database
  22  */
  23
  24 namespace Wikimedia\Rdbms;
  25
  26 use Psr\Log\LoggerAwareInterface;
  27 use Psr\Log\LoggerInterface;
  28 use Psr\Log\NullLogger;
  29 use Wikimedia\WaitConditionLoop;
  30 use BagOStuff;
  31
  32 /**
  33  * Class for ensuring a consistent ordering of events as seen by the user, despite replication.
  34  * Kind of like Hawking's [[Chronology Protection Agency]].
  35  */
  36 class ChronologyProtector implements LoggerAwareInterface {
  37         /** @var BagOStuff */
  38         protected $store;
  39         /** @var LoggerInterface */
  40         protected $logger;
  41
  42         /** @var string Storage key name */
  43         protected $key;
  44         /** @var string Hash of client parameters */
  45         protected $clientId;
  46         /** @var int|null Expected minimum index of the last write to the position store */
  47         protected $waitForPosIndex;
  48         /** @var int Max seconds to wait on positions to appear */
  49         protected $waitForPosStoreTimeout = self::POS_STORE_WAIT_TIMEOUT;
  50         /** @var bool Whether to no-op all method calls */
  51         protected $enabled = true;
  52         /** @var bool Whether to check and wait on positions */
  53         protected $wait = true;
  54
  55         /** @var bool Whether the client data was loaded */
  56         protected $initialized = false;
  57         /** @var DBMasterPos[] Map of (DB master name => position) */
  58         protected $startupPositions = [];
  59         /** @var DBMasterPos[] Map of (DB master name => position) */
  60         protected $shutdownPositions = [];
  61         /** @var float[] Map of (DB master name => 1) */
  62         protected $shutdownTouchDBs = [];
  63
  64         /** @var int Seconds to store positions */
  65         const POSITION_TTL = 60;
  66         /** @var int Max time to wait for positions to appear */
  67         const POS_STORE_WAIT_TIMEOUT = 5;
  68
  69         /**
  70          * @param BagOStuff $store
  71          * @param array[] $client Map of (ip: <IP>, agent: <user-agent>)
  72          * @param int|null $posIndex Write counter index [optional]
  73          * @since 1.27
  74          */
  75         public function __construct( BagOStuff $store, array $client, $posIndex = null ) {
  76                 $this->store = $store;
  77                 $this->clientId = md5( $client['ip'] . "\n" . $client['agent'] );
  78                 $this->key = $store->makeGlobalKey( __CLASS__, $this->clientId, 'v1' );
  79                 $this->waitForPosIndex = $posIndex;
  80                 $this->logger = new NullLogger();
  81         }
  82
  83         public function setLogger( LoggerInterface $logger ) {
  84                 $this->logger = $logger;
  85         }
  86
  87         /**
  88          * @param bool $enabled Whether to no-op all method calls
  89          * @since 1.27
  90          */
  91         public function setEnabled( $enabled ) {
  92                 $this->enabled = $enabled;
  93         }
  94
  95         /**
  96          * @param bool $enabled Whether to check and wait on positions
  97          * @since 1.27
  98          */
  99         public function setWaitEnabled( $enabled ) {
 100                 $this->wait = $enabled;
 101         }
 102
 103         /**
 104          * Initialise a ILoadBalancer to give it appropriate chronology protection.
 105          *
 106          * If the stash has a previous master position recorded, this will try to
 107          * make sure that the next query to a replica DB of that master will see changes up
 108          * to that position by delaying execution. The delay may timeout and allow stale
 109          * data if no non-lagged replica DBs are available.
 110          *
 111          * @param ILoadBalancer $lb
 112          * @return void
 113          */
 114         public function initLB( ILoadBalancer $lb ) {
 115                 if ( !$this->enabled || $lb->getServerCount() <= 1 ) {
 116                         return; // non-replicated setup or disabled
 117                 }
 118
 119                 $this->initPositions();
 120
 121                 $masterName = $lb->getServerName( $lb->getWriterIndex() );
 122                 if (
 123                         isset( $this->startupPositions[$masterName] ) &&
 124                         $this->startupPositions[$masterName] instanceof DBMasterPos
 125                 ) {
 126                         $pos = $this->startupPositions[$masterName];
 127                         $this->logger->info( __METHOD__ . ": LB for '$masterName' set to pos $pos\n" );
 128                         $lb->waitFor( $pos );
 129                 }
 130         }
 131
 132         /**
 133          * Notify the ChronologyProtector that the ILoadBalancer is about to shut
 134          * down. Saves replication positions.
 135          *
 136          * @param ILoadBalancer $lb
 137          * @return void
 138          */
 139         public function shutdownLB( ILoadBalancer $lb ) {
 140                 if ( !$this->enabled ) {
 141                         return; // not enabled
 142                 } elseif ( !$lb->hasOrMadeRecentMasterChanges( INF ) ) {
 143                         // Only save the position if writes have been done on the connection
 144                         return;
 145                 }
 146
 147                 $masterName = $lb->getServerName( $lb->getWriterIndex() );
 148                 if ( $lb->getServerCount() > 1 ) {
 149                         $pos = $lb->getMasterPos();
 150                         $this->logger->info( __METHOD__ . ": LB for '$masterName' has pos $pos\n" );
 151                         $this->shutdownPositions[$masterName] = $pos;
 152                 } else {
 153                         $this->logger->info( __METHOD__ . ": DB '$masterName' touched\n" );
 154                 }
 155                 $this->shutdownTouchDBs[$masterName] = 1;
 156         }
 157
 158         /**
 159          * Notify the ChronologyProtector that the LBFactory is done calling shutdownLB() for now.
 160          * May commit chronology data to persistent storage.
 161          *
 162          * @param callable|null $workCallback Work to do instead of waiting on syncing positions
 163          * @param string $mode One of (sync, async); whether to wait on remote datacenters
 164          * @param int|null &$cpIndex DB position key write counter; incremented on update
 165          * @return DBMasterPos[] Empty on success; returns the (db name => position) map on failure
 166          */
 167         public function shutdown( callable $workCallback = null, $mode = 'sync', &$cpIndex = null ) {
 168                 if ( !$this->enabled ) {
 169                         return [];
 170                 }
 171
 172                 $store = $this->store;
 173                 // Some callers might want to know if a user recently touched a DB.
 174                 // These writes do not need to block on all datacenters receiving them.
 175                 foreach ( $this->shutdownTouchDBs as $dbName => $unused ) {
 176                         $store->set(
 177                                 $this->getTouchedKey( $this->store, $dbName ),
 178                                 microtime( true ),
 179                                 $store::TTL_DAY
 180                         );
 181                 }
 182
 183                 if ( !count( $this->shutdownPositions ) ) {
 184                         return []; // nothing to save
 185                 }
 186
 187                 $this->logger->info( __METHOD__ . ": saving master pos for " .
 188                         implode( ', ', array_keys( $this->shutdownPositions ) ) . "\n"
 189                 );
 190
 191                 // CP-protected writes should overwhemingly go to the master datacenter, so get DC-local
 192                 // lock to merge the values. Use a DC-local get() and a synchronous all-DC set(). This
 193                 // makes it possible for the BagOStuff class to write in parallel to all DCs with one RTT.
 194                 if ( $store->lock( $this->key, 3 ) ) {
 195                         if ( $workCallback ) {
 196                                 // Let the store run the work before blocking on a replication sync barrier. By the
 197                                 // time it's done with the work, the barrier should be fast if replication caught up.
 198                                 $store->addBusyCallback( $workCallback );
 199                         }
 200                         $ok = $store->set(
 201                                 $this->key,
 202                                 $this->mergePositions(
 203                                         $store->get( $this->key ),
 204                                         $this->shutdownPositions,
 205                                         $cpIndex
 206                                 ),
 207                                 self::POSITION_TTL,
 208                                 ( $mode === 'sync' ) ? $store::WRITE_SYNC : 0
 209                         );
 210                         $store->unlock( $this->key );
 211                 } else {
 212                         $ok = false;
 213                         $cpIndex = null; // nothing saved
 214                 }
 215
 216                 if ( !$ok ) {
 217                         $bouncedPositions = $this->shutdownPositions;
 218                         // Raced out too many times or stash is down
 219                         $this->logger->warning( __METHOD__ . ": failed to save master pos for " .
 220                                 implode( ', ', array_keys( $this->shutdownPositions ) ) . "\n"
 221                         );
 222                 } elseif ( $mode === 'sync' &&
 223                         $store->getQoS( $store::ATTR_SYNCWRITES ) < $store::QOS_SYNCWRITES_BE
 224                 ) {
 225                         // Positions may not be in all datacenters, force LBFactory to play it safe
 226                         $this->logger->info( __METHOD__ . ": store may not support synchronous writes." );
 227                         $bouncedPositions = $this->shutdownPositions;
 228                 } else {
 229                         $bouncedPositions = [];
 230                 }
 231
 232                 return $bouncedPositions;
 233         }
 234
 235         /**
 236          * @param string $dbName DB master name (e.g. "db1052")
 237          * @return float|bool UNIX timestamp when client last touched the DB; false if not on record
 238          * @since 1.28
 239          */
 240         public function getTouched( $dbName ) {
 241                 return $this->store->get( $this->getTouchedKey( $this->store, $dbName ) );
 242         }
 243
 244         /**
 245          * @param BagOStuff $store
 246          * @param string $dbName
 247          * @return string
 248          */
 249         private function getTouchedKey( BagOStuff $store, $dbName ) {
 250                 return $store->makeGlobalKey( __CLASS__, 'mtime', $this->clientId, $dbName );
 251         }
 252
 253         /**
 254          * Load in previous master positions for the client
 255          */
 256         protected function initPositions() {
 257                 if ( $this->initialized ) {
 258                         return;
 259                 }
 260
 261                 $this->initialized = true;
 262                 if ( $this->wait ) {
 263                         // If there is an expectation to see master positions from a certain write
 264                         // index or higher, then block until it appears, or until a timeout is reached.
 265                         // Since the write index restarts each time the key is created, it is possible that
 266                         // a lagged store has a matching key write index. However, in that case, it should
 267                         // already be expired and thus treated as non-existing, maintaining correctness.
 268                         if ( $this->waitForPosIndex > 0 ) {
 269                                 $data = null;
 270                                 $loop = new WaitConditionLoop(
 271                                         function () use ( &$data ) {
 272                                                 $data = $this->store->get( $this->key );
 273                                                 if ( !is_array( $data ) ) {
 274                                                         return WaitConditionLoop::CONDITION_CONTINUE; // not found yet
 275                                                 } elseif ( !isset( $data['writeIndex'] ) ) {
 276                                                         return WaitConditionLoop::CONDITION_REACHED; // b/c
 277                                                 }
 278
 279                                                 return ( $data['writeIndex'] >= $this->waitForPosIndex )
 280                                                         ? WaitConditionLoop::CONDITION_REACHED
 281                                                         : WaitConditionLoop::CONDITION_CONTINUE;
 282                                         },
 283                                         $this->waitForPosStoreTimeout
 284                                 );
 285                                 $result = $loop->invoke();
 286                                 $waitedMs = $loop->getLastWaitTime() * 1e3;
 287
 288                                 if ( $result == $loop::CONDITION_REACHED ) {
 289                                         $msg = "expected and found pos index {$this->waitForPosIndex} ({$waitedMs}ms)";
 290                                         $this->logger->debug( $msg );
 291                                 } else {
 292                                         $msg = "expected but missed pos index {$this->waitForPosIndex} ({$waitedMs}ms)";
 293                                         $this->logger->info( $msg );
 294                                 }
 295                         } else {
 296                                 $data = $this->store->get( $this->key );
 297                         }
 298
 299                         $this->startupPositions = $data ? $data['positions'] : [];
 300                         $this->logger->info( __METHOD__ . ": key is {$this->key} (read)\n" );
 301                 } else {
 302                         $this->startupPositions = [];
 303                         $this->logger->info( __METHOD__ . ": key is {$this->key} (unread)\n" );
 304                 }
 305         }
 306
 307         /**
 308          * @param array|bool $curValue
 309          * @param DBMasterPos[] $shutdownPositions
 310          * @param int|null &$cpIndex
 311          * @return array
 312          */
 313         protected function mergePositions( $curValue, array $shutdownPositions, &$cpIndex = null ) {
 314                 /** @var DBMasterPos[] $curPositions */
 315                 $curPositions = isset( $curValue['positions'] ) ? $curValue['positions'] : [];
 316                 // Use the newest positions for each DB master
 317                 foreach ( $shutdownPositions as $db => $pos ) {
 318                         if (
 319                                 !isset( $curPositions[$db] ) ||
 320                                 !( $curPositions[$db] instanceof DBMasterPos ) ||
 321                                 $pos->asOfTime() > $curPositions[$db]->asOfTime()
 322                         ) {
 323                                 $curPositions[$db] = $pos;
 324                         }
 325                 }
 326
 327                 $cpIndex = isset( $curValue['writeIndex'] ) ? $curValue['writeIndex'] : 0;
 328
 329                 return [
 330                         'positions' => $curPositions,
 331                         'writeIndex' => ++$cpIndex
 332                 ];
 333         }
 334 }