includes/libs/HashRing.php

   1 <?php
   2 /**
   3  * Convenience class for weighted consistent hash rings.
   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  */
  22
  23 /**
  24  * Convenience class for weighted consistent hash rings
  25  *
  26  * This deterministically maps "keys" to a set of "locations" while avoiding clumping
  27  *
  28  * Each location is represented by a number of nodes on a ring proportionate to the ratio
  29  * of its weight compared to the total location weight. Note positions are deterministically
  30  * derived from the hash of the location name. Nodes are responsible for the portion of the
  31  * ring, counter-clockwise, up until the next node. Locations are responsible for all portions
  32  * of the ring that the location's nodes are responsible for.
  33  *
  34  * A location that is temporarily "ejected" is said to be absent from the "live" ring.
  35  * If no location ejections are active, then the base ring and live ring are identical.
  36  *
  37  * @since 1.22
  38  */
  39 class HashRing implements Serializable {
  40         /** @var string Hashing algorithm for hash() */
  41         protected $algo;
  42         /** @var int[] Non-empty (location => integer weight) */
  43         protected $weightByLocation;
  44         /** @var int[] Map of (location => UNIX timestamp) */
  45         protected $ejectExpiryByLocation;
  46
  47         /** @var array[] Non-empty position-ordered list of (position, location name) */
  48         protected $baseRing;
  49         /** @var array[] Non-empty position-ordered list of (position, location name) */
  50         protected $liveRing;
  51
  52         /** @var float Number of positions on the ring */
  53         const RING_SIZE = 4294967296.0; // 2^32
  54         /** @var integer Overall number of node groups per server */
  55         const HASHES_PER_LOCATION = 40;
  56         /** @var integer Number of nodes in a node group */
  57         const SECTORS_PER_HASH = 4;
  58
  59         const KEY_POS = 0;
  60         const KEY_LOCATION = 1;
  61
  62         /** @var int Consider all locations */
  63         const RING_ALL = 0;
  64         /** @var int Only consider "live" locations */
  65         const RING_LIVE = 1;
  66
  67         /**
  68          * Make a consistent hash ring given a set of locations and their weight values
  69          *
  70          * @param int[] $map Map of (location => weight)
  71          * @param string $algo Hashing algorithm listed in hash_algos() [optional]
  72          * @param int[] $ejections Map of (location => UNIX timestamp) for ejection expiries
  73          * @since 1.31
  74          */
  75         public function __construct( array $map, $algo = 'sha1', array $ejections = [] ) {
  76                 $this->init( $map, $algo, $ejections );
  77         }
  78
  79         /**
  80          * @param int[] $map Map of (location => integer)
  81          * @param string $algo Hashing algorithm
  82          * @param int[] $ejections Map of (location => UNIX timestamp) for ejection expires
  83          */
  84         protected function init( array $map, $algo, array $ejections ) {
  85                 if ( !in_array( $algo, hash_algos(), true ) ) {
  86                         throw new RuntimeException( __METHOD__ . ": unsupported '$algo' hash algorithm." );
  87                 }
  88
  89                 $weightByLocation = array_filter( $map );
  90                 if ( $weightByLocation === [] ) {
  91                         throw new UnexpectedValueException( "No locations with non-zero weight." );
  92                 } elseif ( min( $map ) < 0 ) {
  93                         throw new InvalidArgumentException( "Location weight cannot be negative." );
  94                 }
  95
  96                 $this->algo = $algo;
  97                 $this->weightByLocation = $weightByLocation;
  98                 $this->ejectExpiryByLocation = $ejections;
  99                 $this->baseRing = $this->buildLocationRing( $this->weightByLocation );
 100         }
 101
 102         /**
 103          * Get the location of an item on the ring
 104          *
 105          * @param string $item
 106          * @return string Location
 107          * @throws UnexpectedValueException
 108          */
 109         final public function getLocation( $item ) {
 110                 return $this->getLocations( $item, 1 )[0];
 111         }
 112
 113         /**
 114          * Get the location of an item on the ring followed by the next ring locations
 115          *
 116          * @param string $item
 117          * @param int $limit Maximum number of locations to return
 118          * @param int $from One of the RING_* class constants
 119          * @return string[] List of locations
 120          * @throws InvalidArgumentException
 121          * @throws UnexpectedValueException
 122          */
 123         public function getLocations( $item, $limit, $from = self::RING_ALL ) {
 124                 if ( $from === self::RING_ALL ) {
 125                         $ring = $this->baseRing;
 126                 } elseif ( $from === self::RING_LIVE ) {
 127                         $ring = $this->getLiveRing();
 128                 } else {
 129                         throw new InvalidArgumentException( "Invalid ring source specified." );
 130                 }
 131
 132                 // Locate the node index for this item's position on the hash ring
 133                 $itemIndex = $this->findNodeIndexForPosition( $this->getItemPosition( $item ), $ring );
 134
 135                 $locations = [];
 136                 $currentIndex = null;
 137                 while ( count( $locations ) < $limit ) {
 138                         if ( $currentIndex === null ) {
 139                                 $currentIndex = $itemIndex;
 140                         } else {
 141                                 $currentIndex = $this->getNextClockwiseNodeIndex( $currentIndex, $ring );
 142                                 if ( $currentIndex === $itemIndex ) {
 143                                         break; // all nodes visited
 144                                 }
 145                         }
 146                         $nodeLocation = $ring[$currentIndex][self::KEY_LOCATION];
 147                         if ( !in_array( $nodeLocation, $locations, true ) ) {
 148                                 // Ignore other nodes for the same locations already added
 149                                 $locations[] = $nodeLocation;
 150                         }
 151                 }
 152
 153                 return $locations;
 154         }
 155
 156         /**
 157          * @param float $position
 158          * @param array[] $ring Either the base or live ring
 159          * @return int|null
 160          */
 161         private function findNodeIndexForPosition( $position, $ring ) {
 162                 $count = count( $ring );
 163                 if ( $count === 0 ) {
 164                         return null;
 165                 }
 166
 167                 $index = null;
 168                 $lowPos = 0;
 169                 $highPos = $count;
 170                 while ( true ) {
 171                         $midPos = (int)( ( $lowPos + $highPos ) / 2 );
 172                         if ( $midPos === $count ) {
 173                                 $index = 0;
 174                                 break;
 175                         }
 176
 177                         $midVal = $ring[$midPos][self::KEY_POS];
 178                         $midMinusOneVal = ( $midPos === 0 ) ? 0 : $ring[$midPos - 1][self::KEY_POS];
 179                         if ( $position <= $midVal && $position > $midMinusOneVal ) {
 180                                 $index = $midPos;
 181                                 break;
 182                         }
 183
 184                         if ( $midVal < $position ) {
 185                                 $lowPos = $midPos + 1;
 186                         } else {
 187                                 $highPos = $midPos - 1;
 188                         }
 189
 190                         if ( $lowPos > $highPos ) {
 191                                 $index = 0;
 192                                 break;
 193                         }
 194                 }
 195
 196                 return $index;
 197         }
 198
 199         /**
 200          * Get the map of locations to weight (does not include zero weight items)
 201          *
 202          * @return int[]
 203          */
 204         public function getLocationWeights() {
 205                 return $this->weightByLocation;
 206         }
 207
 208         /**
 209          * Remove a location from the "live" hash ring
 210          *
 211          * @param string $location
 212          * @param int $ttl Seconds
 213          * @return bool Whether some non-ejected locations are left
 214          * @throws UnexpectedValueException
 215          */
 216         public function ejectFromLiveRing( $location, $ttl ) {
 217                 if ( !isset( $this->weightByLocation[$location] ) ) {
 218                         throw new UnexpectedValueException( "No location '$location' in the ring." );
 219                 }
 220
 221                 $expiry = $this->getCurrentTime() + $ttl;
 222                 $this->ejectExpiryByLocation[$location] = $expiry;
 223
 224                 $this->liveRing = null; // invalidate ring cache
 225
 226                 return ( count( $this->ejectExpiryByLocation ) < count( $this->weightByLocation ) );
 227         }
 228
 229         /**
 230          * Get the location of an item on the "live" ring
 231          *
 232          * @param string $item
 233          * @return string Location
 234          * @throws UnexpectedValueException
 235          */
 236         final public function getLiveLocation( $item ) {
 237                 return $this->getLocations( $item, 1, self::RING_LIVE )[0];
 238         }
 239
 240         /**
 241          * Get the location of an item on the "live" ring, as well as the next locations
 242          *
 243          * @param string $item
 244          * @param int $limit Maximum number of locations to return
 245          * @return string[] List of locations
 246          * @throws UnexpectedValueException
 247          */
 248         final public function getLiveLocations( $item, $limit ) {
 249                 return $this->getLocations( $item, $limit, self::RING_LIVE );
 250         }
 251
 252         /**
 253          * Get the map of "live" locations to weight (does not include zero weight items)
 254          *
 255          * @return int[]
 256          * @throws UnexpectedValueException
 257          */
 258         public function getLiveLocationWeights() {
 259                 $now = $this->getCurrentTime();
 260
 261                 return array_diff_key(
 262                         $this->weightByLocation,
 263                         array_filter(
 264                                 $this->ejectExpiryByLocation,
 265                                 function ( $expiry ) use ( $now ) {
 266                                         return ( $expiry > $now );
 267                                 }
 268                         )
 269                 );
 270         }
 271
 272         /**
 273          * @param int[] $weightByLocation
 274          * @return array[]
 275          */
 276         private function buildLocationRing( array $weightByLocation ) {
 277                 $locationCount = count( $weightByLocation );
 278                 $totalWeight = array_sum( $weightByLocation );
 279
 280                 $ring = [];
 281                 // Assign nodes to all locations based on location weight
 282                 $claimed = []; // (position as string => (node, index))
 283                 foreach ( $weightByLocation as $location => $weight ) {
 284                         $ratio = $weight / $totalWeight;
 285                         // There $locationCount * (HASHES_PER_LOCATION * 4) nodes available;
 286                         // assign a few groups of nodes to this location based on its weight.
 287                         $nodesQuartets = intval( $ratio * self::HASHES_PER_LOCATION * $locationCount );
 288                         for ( $qi = 0; $qi < $nodesQuartets; ++$qi ) {
 289                                 // For efficiency, get 4 points per hash call and 4X node count.
 290                                 // If $algo is MD5, then this matches that of with libketama.
 291                                 // See https://github.com/RJ/ketama/blob/master/libketama/ketama.c
 292                                 $positions = $this->getNodePositionQuartet( "{$location}-{$qi}" );
 293                                 foreach ( $positions as $gi => $position ) {
 294                                         $node = ( $qi * self::SECTORS_PER_HASH + $gi ) . "@$location";
 295                                         $posKey = (string)$position; // large integer
 296                                         if ( isset( $claimed[$posKey] ) ) {
 297                                                 // Disallow duplicates for sanity (name decides precedence)
 298                                                 if ( $claimed[$posKey]['node'] > $node ) {
 299                                                         continue;
 300                                                 } else {
 301                                                         unset( $ring[$claimed[$posKey]['index']] );
 302                                                 }
 303                                         }
 304                                         $ring[] = [
 305                                                 self::KEY_POS => $position,
 306                                                 self::KEY_LOCATION => $location
 307                                         ];
 308                                         $claimed[$posKey] = [ 'node' => $node, 'index' => count( $ring ) - 1 ];
 309                                 }
 310                         }
 311                 }
 312                 // Sort the locations into clockwise order based on the hash ring position
 313                 usort( $ring, function ( $a, $b ) {
 314                         if ( $a[self::KEY_POS] === $b[self::KEY_POS] ) {
 315                                 throw new UnexpectedValueException( 'Duplicate node positions.' );
 316                         }
 317
 318                         return ( $a[self::KEY_POS] < $b[self::KEY_POS] ? -1 : 1 );
 319                 } );
 320
 321                 return $ring;
 322         }
 323
 324         /**
 325          * @param string $item Key
 326          * @return float Ring position; integral number in [0, self::RING_SIZE - 1]
 327          */
 328         private function getItemPosition( $item ) {
 329                 // If $algo is MD5, then this matches that of with libketama.
 330                 // See https://github.com/RJ/ketama/blob/master/libketama/ketama.c
 331                 $octets = substr( hash( $this->algo, (string)$item, true ), 0, 4 );
 332                 if ( strlen( $octets ) != 4 ) {
 333                         throw new UnexpectedValueException( __METHOD__ . ": {$this->algo} is < 32 bits." );
 334                 }
 335
 336                 $pos = unpack( 'V', $octets )[1];
 337                 if ( $pos < 0 ) {
 338                         // Most-significant-bit is set, causing unpack() to return a negative integer due
 339                         // to the fact that it returns a signed int. Cast it to an unsigned integer string.
 340                         $pos = sprintf( '%u', $pos );
 341                 }
 342
 343                 return (float)$pos;
 344         }
 345
 346         /**
 347          * @param string $nodeGroupName
 348          * @return float[] Four ring positions on [0, self::RING_SIZE - 1]
 349          */
 350         private function getNodePositionQuartet( $nodeGroupName ) {
 351                 $octets = substr( hash( $this->algo, (string)$nodeGroupName, true ), 0, 16 );
 352                 if ( strlen( $octets ) != 16 ) {
 353                         throw new UnexpectedValueException( __METHOD__ . ": {$this->algo} is < 128 bits." );
 354                 }
 355
 356                 $positions = [];
 357                 foreach ( unpack( 'V4', $octets ) as $signed ) {
 358                         $positions[] = (float)sprintf( '%u', $signed );
 359                 }
 360
 361                 return $positions;
 362         }
 363
 364         /**
 365          * @param int $i Valid index for a node in the ring
 366          * @param array[] $ring Either the base or live ring
 367          * @return int Valid index for a node in the ring
 368          */
 369         private function getNextClockwiseNodeIndex( $i, $ring ) {
 370                 if ( !isset( $ring[$i] ) ) {
 371                         throw new UnexpectedValueException( __METHOD__ . ": reference index is invalid." );
 372                 }
 373
 374                 $next = $i + 1;
 375
 376                 return ( $next < count( $ring ) ) ? $next : 0;
 377         }
 378
 379         /**
 380          * Get the "live" hash ring (which does not include ejected locations)
 381          *
 382          * @return array[]
 383          * @throws UnexpectedValueException
 384          */
 385         protected function getLiveRing() {
 386                 if ( !$this->ejectExpiryByLocation ) {
 387                         return $this->baseRing; // nothing ejected
 388                 }
 389
 390                 $now = $this->getCurrentTime();
 391
 392                 if ( $this->liveRing === null || min( $this->ejectExpiryByLocation ) <= $now ) {
 393                         // Live ring needs to be regerenated...
 394                         $this->ejectExpiryByLocation = array_filter(
 395                                 $this->ejectExpiryByLocation,
 396                                 function ( $expiry ) use ( $now ) {
 397                                         return ( $expiry > $now );
 398                                 }
 399                         );
 400
 401                         if ( count( $this->ejectExpiryByLocation ) ) {
 402                                 // Some locations are still ejected from the ring
 403                                 $liveRing = [];
 404                                 foreach ( $this->baseRing as $i => $nodeInfo ) {
 405                                         $location = $nodeInfo[self::KEY_LOCATION];
 406                                         if ( !isset( $this->ejectExpiryByLocation[$location] ) ) {
 407                                                 $liveRing[] = $nodeInfo;
 408                                         }
 409                                 }
 410                         } else {
 411                                 $liveRing = $this->baseRing;
 412                         }
 413
 414                         $this->liveRing = $liveRing;
 415                 }
 416
 417                 if ( !$this->liveRing ) {
 418                         throw new UnexpectedValueException( "The live ring is currently empty." );
 419                 }
 420
 421                 return $this->liveRing;
 422         }
 423
 424         /**
 425          * @return int UNIX timestamp
 426          */
 427         protected function getCurrentTime() {
 428                 return time();
 429         }
 430
 431         public function serialize() {
 432                 return serialize( [
 433                         'algorithm' => $this->algo,
 434                         'locations' => $this->weightByLocation,
 435                         'ejections' => $this->ejectExpiryByLocation
 436                 ] );
 437         }
 438
 439         public function unserialize( $serialized ) {
 440                 $data = unserialize( $serialized );
 441                 if ( is_array( $data ) ) {
 442                         $this->init( $data['locations'], $data['algorithm'], $data['ejections'] );
 443                 } else {
 444                         throw new UnexpectedValueException( __METHOD__ . ": unable to decode JSON." );
 445                 }
 446         }
 447 }