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, 'v2' );
  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                         $this->logger->debug( __METHOD__ . ": LB for '$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                         if ( $pos ) {
 151                                 $this->logger->debug( __METHOD__ . ": LB for '$masterName' has pos $pos\n" );
 152                                 $this->shutdownPositions[$masterName] = $pos;
 153                         }
 154                 } else {
 155                         $this->logger->debug( __METHOD__ . ": DB '$masterName' touched\n" );
 156                 }
 157                 $this->shutdownTouchDBs[$masterName] = 1;
 158         }
 159
 160         /**
 161          * Notify the ChronologyProtector that the LBFactory is done calling shutdownLB() for now.
 162          * May commit chronology data to persistent storage.
 163          *
 164          * @param callable|null $workCallback Work to do instead of waiting on syncing positions
 165          * @param string $mode One of (sync, async); whether to wait on remote datacenters
 166          * @param int|null &$cpIndex DB position key write counter; incremented on update
 167          * @return DBMasterPos[] Empty on success; returns the (db name => position) map on failure
 168          */
 169         public function shutdown( callable $workCallback = null, $mode = 'sync', &$cpIndex = null ) {
 170                 if ( !$this->enabled ) {
 171                         return [];
 172                 }
 173
 174                 $store = $this->store;
 175                 // Some callers might want to know if a user recently touched a DB.
 176                 // These writes do not need to block on all datacenters receiving them.
 177                 foreach ( $this->shutdownTouchDBs as $dbName => $unused ) {
 178                         $store->set(
 179                                 $this->getTouchedKey( $this->store, $dbName ),
 180                                 microtime( true ),
 181                                 $store::TTL_DAY
 182                         );
 183                 }
 184
 185                 if ( !count( $this->shutdownPositions ) ) {
 186                         return []; // nothing to save
 187                 }
 188
 189                 $this->logger->debug( __METHOD__ . ": saving master pos for " .
 190                         implode( ', ', array_keys( $this->shutdownPositions ) ) . "\n"
 191                 );
 192
 193                 // CP-protected writes should overwhelmingly go to the master datacenter, so use a
 194                 // DC-local lock to merge the values. Use a DC-local get() and a synchronous all-DC
 195                 // set(). This makes it possible for the BagOStuff class to write in parallel to all
 196                 // DCs with one RTT. The use of WRITE_SYNC avoids needing READ_LATEST for the get().
 197                 if ( $store->lock( $this->key, 3 ) ) {
 198                         if ( $workCallback ) {
 199                                 // Let the store run the work before blocking on a replication sync barrier.
 200                                 // If replication caught up while the work finished, the barrier will be fast.
 201                                 $store->addBusyCallback( $workCallback );
 202                         }
 203                         $ok = $store->set(
 204                                 $this->key,
 205                                 $this->mergePositions(
 206                                         $store->get( $this->key ),
 207                                         $this->shutdownPositions,
 208                                         $cpIndex
 209                                 ),
 210                                 self::POSITION_TTL,
 211                                 ( $mode === 'sync' ) ? $store::WRITE_SYNC : 0
 212                         );
 213                         $store->unlock( $this->key );
 214                 } else {
 215                         $ok = false;
 216                 }
 217
 218                 if ( !$ok ) {
 219                         $cpIndex = null; // nothing saved
 220                         $bouncedPositions = $this->shutdownPositions;
 221                         // Raced out too many times or stash is down
 222                         $this->logger->warning( __METHOD__ . ": failed to save master pos for " .
 223                                 implode( ', ', array_keys( $this->shutdownPositions ) ) . "\n"
 224                         );
 225                 } elseif ( $mode === 'sync' &&
 226                         $store->getQoS( $store::ATTR_SYNCWRITES ) < $store::QOS_SYNCWRITES_BE
 227                 ) {
 228                         // Positions may not be in all datacenters, force LBFactory to play it safe
 229                         $this->logger->info( __METHOD__ . ": store may not support synchronous writes." );
 230                         $bouncedPositions = $this->shutdownPositions;
 231                 } else {
 232                         $bouncedPositions = [];
 233                 }
 234
 235                 return $bouncedPositions;
 236         }
 237
 238         /**
 239          * @param string $dbName DB master name (e.g. "db1052")
 240          * @return float|bool UNIX timestamp when client last touched the DB; false if not on record
 241          * @since 1.28
 242          */
 243         public function getTouched( $dbName ) {
 244                 return $this->store->get( $this->getTouchedKey( $this->store, $dbName ) );
 245         }
 246
 247         /**
 248          * @param BagOStuff $store
 249          * @param string $dbName
 250          * @return string
 251          */
 252         private function getTouchedKey( BagOStuff $store, $dbName ) {
 253                 return $store->makeGlobalKey( __CLASS__, 'mtime', $this->clientId, $dbName );
 254         }
 255
 256         /**
 257          * Load in previous master positions for the client
 258          */
 259         protected function initPositions() {
 260                 if ( $this->initialized ) {
 261                         return;
 262                 }
 263
 264                 $this->initialized = true;
 265                 if ( $this->wait ) {
 266                         // If there is an expectation to see master positions from a certain write
 267                         // index or higher, then block until it appears, or until a timeout is reached.
 268                         // Since the write index restarts each time the key is created, it is possible that
 269                         // a lagged store has a matching key write index. However, in that case, it should
 270                         // already be expired and thus treated as non-existing, maintaining correctness.
 271                         if ( $this->waitForPosIndex > 0 ) {
 272                                 $data = null;
 273                                 $indexReached = null; // highest index reached in the position store
 274                                 $loop = new WaitConditionLoop(
 275                                         function () use ( &$data, &$indexReached ) {
 276                                                 $data = $this->store->get( $this->key );
 277                                                 if ( !is_array( $data ) ) {
 278                                                         return WaitConditionLoop::CONDITION_CONTINUE; // not found yet
 279                                                 } elseif ( !isset( $data['writeIndex'] ) ) {
 280                                                         return WaitConditionLoop::CONDITION_REACHED; // b/c
 281                                                 }
 282                                                 $indexReached = max( $data['writeIndex'], $indexReached );
 283
 284                                                 return ( $data['writeIndex'] >= $this->waitForPosIndex )
 285                                                         ? WaitConditionLoop::CONDITION_REACHED
 286                                                         : WaitConditionLoop::CONDITION_CONTINUE;
 287                                         },
 288                                         $this->waitForPosStoreTimeout
 289                                 );
 290                                 $result = $loop->invoke();
 291                                 $waitedMs = $loop->getLastWaitTime() * 1e3;
 292
 293                                 if ( $result == $loop::CONDITION_REACHED ) {
 294                                         $this->logger->debug(
 295                                                 __METHOD__ . ": expected and found position index.",
 296                                                 [
 297                                                         'cpPosIndex' => $this->waitForPosIndex,
 298                                                         'waitTimeMs' => $waitedMs
 299                                                 ]
 300                                         );
 301                                 } else {
 302                                         $this->logger->warning(
 303                                                 __METHOD__ . ": expected but failed to find position index.",
 304                                                 [
 305                                                         'cpPosIndex' => $this->waitForPosIndex,
 306                                                         'indexReached' => $indexReached,
 307                                                         'waitTimeMs' => $waitedMs
 308                                                 ]
 309                                         );
 310                                 }
 311                         } else {
 312                                 $data = $this->store->get( $this->key );
 313                         }
 314
 315                         $this->startupPositions = $data ? $data['positions'] : [];
 316                         $this->logger->debug( __METHOD__ . ": key is {$this->key} (read)\n" );
 317                 } else {
 318                         $this->startupPositions = [];
 319                         $this->logger->debug( __METHOD__ . ": key is {$this->key} (unread)\n" );
 320                 }
 321         }
 322
 323         /**
 324          * @param array|bool $curValue
 325          * @param DBMasterPos[] $shutdownPositions
 326          * @param int|null &$cpIndex
 327          * @return array
 328          */
 329         protected function mergePositions( $curValue, array $shutdownPositions, &$cpIndex = null ) {
 330                 /** @var DBMasterPos[] $curPositions */
 331                 $curPositions = isset( $curValue['positions'] ) ? $curValue['positions'] : [];
 332                 // Use the newest positions for each DB master
 333                 foreach ( $shutdownPositions as $db => $pos ) {
 334                         if (
 335                                 !isset( $curPositions[$db] ) ||
 336                                 !( $curPositions[$db] instanceof DBMasterPos ) ||
 337                                 $pos->asOfTime() > $curPositions[$db]->asOfTime()
 338                         ) {
 339                                 $curPositions[$db] = $pos;
 340                         }
 341                 }
 342
 343                 $cpIndex = isset( $curValue['writeIndex'] ) ? $curValue['writeIndex'] : 0;
 344
 345                 return [
 346                         'positions' => $curPositions,
 347                         'writeIndex' => ++$cpIndex
 348                 ];
 349         }
 350 }