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 Seconds to store position write index cookies (safely less than POSITION_TTL) */
  67         const POSITION_COOKIE_TTL = 60;
  68         /** @var int Max time to wait for positions to appear */
  69         const POS_STORE_WAIT_TIMEOUT = 5;
  70
  71         /**
  72          * @param BagOStuff $store
  73          * @param array[] $client Map of (ip: <IP>, agent: <user-agent>)
  74          * @param int|null $posIndex Write counter index [optional]
  75          * @since 1.27
  76          */
  77         public function __construct( BagOStuff $store, array $client, $posIndex = null ) {
  78                 $this->store = $store;
  79                 $this->clientId = md5( $client['ip'] . "\n" . $client['agent'] );
  80                 $this->key = $store->makeGlobalKey( __CLASS__, $this->clientId, 'v2' );
  81                 $this->waitForPosIndex = $posIndex;
  82                 $this->logger = new NullLogger();
  83         }
  84
  85         public function setLogger( LoggerInterface $logger ) {
  86                 $this->logger = $logger;
  87         }
  88
  89         /**
  90          * @param bool $enabled Whether to no-op all method calls
  91          * @since 1.27
  92          */
  93         public function setEnabled( $enabled ) {
  94                 $this->enabled = $enabled;
  95         }
  96
  97         /**
  98          * @param bool $enabled Whether to check and wait on positions
  99          * @since 1.27
 100          */
 101         public function setWaitEnabled( $enabled ) {
 102                 $this->wait = $enabled;
 103         }
 104
 105         /**
 106          * Initialise a ILoadBalancer to give it appropriate chronology protection.
 107          *
 108          * If the stash has a previous master position recorded, this will try to
 109          * make sure that the next query to a replica DB of that master will see changes up
 110          * to that position by delaying execution. The delay may timeout and allow stale
 111          * data if no non-lagged replica DBs are available.
 112          *
 113          * @param ILoadBalancer $lb
 114          * @return void
 115          */
 116         public function initLB( ILoadBalancer $lb ) {
 117                 if ( !$this->enabled || $lb->getServerCount() <= 1 ) {
 118                         return; // non-replicated setup or disabled
 119                 }
 120
 121                 $this->initPositions();
 122
 123                 $masterName = $lb->getServerName( $lb->getWriterIndex() );
 124                 if (
 125                         isset( $this->startupPositions[$masterName] ) &&
 126                         $this->startupPositions[$masterName] instanceof DBMasterPos
 127                 ) {
 128                         $pos = $this->startupPositions[$masterName];
 129                         $this->logger->debug( __METHOD__ . ": LB for '$masterName' set to pos $pos\n" );
 130                         $lb->waitFor( $pos );
 131                 }
 132         }
 133
 134         /**
 135          * Notify the ChronologyProtector that the ILoadBalancer is about to shut
 136          * down. Saves replication positions.
 137          *
 138          * @param ILoadBalancer $lb
 139          * @return void
 140          */
 141         public function shutdownLB( ILoadBalancer $lb ) {
 142                 if ( !$this->enabled ) {
 143                         return; // not enabled
 144                 } elseif ( !$lb->hasOrMadeRecentMasterChanges( INF ) ) {
 145                         // Only save the position if writes have been done on the connection
 146                         return;
 147                 }
 148
 149                 $masterName = $lb->getServerName( $lb->getWriterIndex() );
 150                 if ( $lb->getServerCount() > 1 ) {
 151                         $pos = $lb->getMasterPos();
 152                         if ( $pos ) {
 153                                 $this->logger->debug( __METHOD__ . ": LB for '$masterName' has pos $pos\n" );
 154                                 $this->shutdownPositions[$masterName] = $pos;
 155                         }
 156                 } else {
 157                         $this->logger->debug( __METHOD__ . ": DB '$masterName' touched\n" );
 158                 }
 159                 $this->shutdownTouchDBs[$masterName] = 1;
 160         }
 161
 162         /**
 163          * Notify the ChronologyProtector that the LBFactory is done calling shutdownLB() for now.
 164          * May commit chronology data to persistent storage.
 165          *
 166          * @param callable|null $workCallback Work to do instead of waiting on syncing positions
 167          * @param string $mode One of (sync, async); whether to wait on remote datacenters
 168          * @param int|null &$cpIndex DB position key write counter; incremented on update
 169          * @return DBMasterPos[] Empty on success; returns the (db name => position) map on failure
 170          */
 171         public function shutdown( callable $workCallback = null, $mode = 'sync', &$cpIndex = null ) {
 172                 if ( !$this->enabled ) {
 173                         return [];
 174                 }
 175
 176                 $store = $this->store;
 177                 // Some callers might want to know if a user recently touched a DB.
 178                 // These writes do not need to block on all datacenters receiving them.
 179                 foreach ( $this->shutdownTouchDBs as $dbName => $unused ) {
 180                         $store->set(
 181                                 $this->getTouchedKey( $this->store, $dbName ),
 182                                 microtime( true ),
 183                                 $store::TTL_DAY
 184                         );
 185                 }
 186
 187                 if ( !count( $this->shutdownPositions ) ) {
 188                         return []; // nothing to save
 189                 }
 190
 191                 $this->logger->debug( __METHOD__ . ": saving master pos for " .
 192                         implode( ', ', array_keys( $this->shutdownPositions ) ) . "\n"
 193                 );
 194
 195                 // CP-protected writes should overwhemingly go to the master datacenter, so get DC-local
 196                 // lock to merge the values. Use a DC-local get() and a synchronous all-DC set(). This
 197                 // makes it possible for the BagOStuff class to write in parallel to all DCs with one RTT.
 198                 if ( $store->lock( $this->key, 3 ) ) {
 199                         if ( $workCallback ) {
 200                                 // Let the store run the work before blocking on a replication sync barrier. By the
 201                                 // time it's done with the work, the barrier should be fast if replication caught up.
 202                                 $store->addBusyCallback( $workCallback );
 203                         }
 204                         $ok = $store->set(
 205                                 $this->key,
 206                                 $this->mergePositions(
 207                                         $store->get( $this->key ),
 208                                         $this->shutdownPositions,
 209                                         $cpIndex
 210                                 ),
 211                                 self::POSITION_TTL,
 212                                 ( $mode === 'sync' ) ? $store::WRITE_SYNC : 0
 213                         );
 214                         $store->unlock( $this->key );
 215                 } else {
 216                         $ok = false;
 217                 }
 218
 219                 if ( !$ok ) {
 220                         $cpIndex = null; // nothing saved
 221                         $bouncedPositions = $this->shutdownPositions;
 222                         // Raced out too many times or stash is down
 223                         $this->logger->warning( __METHOD__ . ": failed to save master pos for " .
 224                                 implode( ', ', array_keys( $this->shutdownPositions ) ) . "\n"
 225                         );
 226                 } elseif ( $mode === 'sync' &&
 227                         $store->getQoS( $store::ATTR_SYNCWRITES ) < $store::QOS_SYNCWRITES_BE
 228                 ) {
 229                         // Positions may not be in all datacenters, force LBFactory to play it safe
 230                         $this->logger->info( __METHOD__ . ": store may not support synchronous writes." );
 231                         $bouncedPositions = $this->shutdownPositions;
 232                 } else {
 233                         $bouncedPositions = [];
 234                 }
 235
 236                 return $bouncedPositions;
 237         }
 238
 239         /**
 240          * @param string $dbName DB master name (e.g. "db1052")
 241          * @return float|bool UNIX timestamp when client last touched the DB; false if not on record
 242          * @since 1.28
 243          */
 244         public function getTouched( $dbName ) {
 245                 return $this->store->get( $this->getTouchedKey( $this->store, $dbName ) );
 246         }
 247
 248         /**
 249          * @param BagOStuff $store
 250          * @param string $dbName
 251          * @return string
 252          */
 253         private function getTouchedKey( BagOStuff $store, $dbName ) {
 254                 return $store->makeGlobalKey( __CLASS__, 'mtime', $this->clientId, $dbName );
 255         }
 256
 257         /**
 258          * Load in previous master positions for the client
 259          */
 260         protected function initPositions() {
 261                 if ( $this->initialized ) {
 262                         return;
 263                 }
 264
 265                 $this->initialized = true;
 266                 if ( $this->wait ) {
 267                         // If there is an expectation to see master positions from a certain write
 268                         // index or higher, then block until it appears, or until a timeout is reached.
 269                         // Since the write index restarts each time the key is created, it is possible that
 270                         // a lagged store has a matching key write index. However, in that case, it should
 271                         // already be expired and thus treated as non-existing, maintaining correctness.
 272                         if ( $this->waitForPosIndex > 0 ) {
 273                                 $data = null;
 274                                 $indexReached = null; // highest index reached in the position store
 275                                 $loop = new WaitConditionLoop(
 276                                         function () use ( &$data, &$indexReached ) {
 277                                                 $data = $this->store->get( $this->key );
 278                                                 if ( !is_array( $data ) ) {
 279                                                         return WaitConditionLoop::CONDITION_CONTINUE; // not found yet
 280                                                 } elseif ( !isset( $data['writeIndex'] ) ) {
 281                                                         return WaitConditionLoop::CONDITION_REACHED; // b/c
 282                                                 }
 283                                                 $indexReached = max( $data['writeIndex'], $indexReached );
 284
 285                                                 return ( $data['writeIndex'] >= $this->waitForPosIndex )
 286                                                         ? WaitConditionLoop::CONDITION_REACHED
 287                                                         : WaitConditionLoop::CONDITION_CONTINUE;
 288                                         },
 289                                         $this->waitForPosStoreTimeout
 290                                 );
 291                                 $result = $loop->invoke();
 292                                 $waitedMs = $loop->getLastWaitTime() * 1e3;
 293
 294                                 if ( $result == $loop::CONDITION_REACHED ) {
 295                                         $this->logger->debug(
 296                                                 __METHOD__ . ": expected and found position index.",
 297                                                 [
 298                                                         'cpPosIndex' => $this->waitForPosIndex,
 299                                                         'waitTimeMs' => $waitedMs
 300                                                 ]
 301                                         );
 302                                 } else {
 303                                         $this->logger->warning(
 304                                                 __METHOD__ . ": expected but failed to find position index.",
 305                                                 [
 306                                                         'cpPosIndex' => $this->waitForPosIndex,
 307                                                         'indexReached' => $indexReached,
 308                                                         'waitTimeMs' => $waitedMs
 309                                                 ]
 310                                         );
 311                                 }
 312                         } else {
 313                                 $data = $this->store->get( $this->key );
 314                         }
 315
 316                         $this->startupPositions = $data ? $data['positions'] : [];
 317                         $this->logger->debug( __METHOD__ . ": key is {$this->key} (read)\n" );
 318                 } else {
 319                         $this->startupPositions = [];
 320                         $this->logger->debug( __METHOD__ . ": key is {$this->key} (unread)\n" );
 321                 }
 322         }
 323
 324         /**
 325          * @param array|bool $curValue
 326          * @param DBMasterPos[] $shutdownPositions
 327          * @param int|null &$cpIndex
 328          * @return array
 329          */
 330         protected function mergePositions( $curValue, array $shutdownPositions, &$cpIndex = null ) {
 331                 /** @var DBMasterPos[] $curPositions */
 332                 $curPositions = isset( $curValue['positions'] ) ? $curValue['positions'] : [];
 333                 // Use the newest positions for each DB master
 334                 foreach ( $shutdownPositions as $db => $pos ) {
 335                         if (
 336                                 !isset( $curPositions[$db] ) ||
 337                                 !( $curPositions[$db] instanceof DBMasterPos ) ||
 338                                 $pos->asOfTime() > $curPositions[$db]->asOfTime()
 339                         ) {
 340                                 $curPositions[$db] = $pos;
 341                         }
 342                 }
 343
 344                 $cpIndex = isset( $curValue['writeIndex'] ) ? $curValue['writeIndex'] : 0;
 345
 346                 return [
 347                         'positions' => $curPositions,
 348                         'writeIndex' => ++$cpIndex
 349                 ];
 350         }
 351 }