includes/libs/objectcache/WANObjectCache.php

   1 <?php
   2 /**
   3  * This program is free software; you can redistribute it and/or modify
   4  * it under the terms of the GNU General Public License as published by
   5  * the Free Software Foundation; either version 2 of the License, or
   6  * (at your option) any later version.
   7  *
   8  * This program is distributed in the hope that it will be useful,
   9  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  10  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  11  * GNU General Public License for more details.
  12  *
  13  * You should have received a copy of the GNU General Public License along
  14  * with this program; if not, write to the Free Software Foundation, Inc.,
  15  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  16  * http://www.gnu.org/copyleft/gpl.html
  17  *
  18  * @file
  19  * @ingroup Cache
  20  * @author Aaron Schulz
  21  */
  22
  23 /**
  24  * Multi-datacenter aware caching interface
  25  *
  26  * All operations go to the local cache, except the delete()
  27  * and touchCheckKey(), which broadcast to all clusters.
  28  * This class is intended for caching data from primary stores.
  29  * If the get() method does not return a value, then the caller
  30  * should query the new value and backfill the cache using set().
  31  * When the source data changes, the delete() method should be called.
  32  * Since delete() is expensive, it should be avoided. One can do so if:
  33  *   - a) The object cached is immutable; or
  34  *   - b) Validity is checked against the source after get(); or
  35  *   - c) Using a modest TTL is reasonably correct and performant
  36  * Consider using getWithSetCallback() instead of the get()/set() cycle.
  37  *
  38  * Instances of this class must be configured to point to a valid
  39  * PubSub endpoint, and there must be listeners on the cache servers
  40  * that subscribe to the endpoint and update the caches.
  41  *
  42  * Broadcasted operations like delete() and touchCheckKey() are done
  43  * synchronously in the local cluster, but are relayed asynchronously.
  44  * This means that callers in other datacenters will see older values
  45  * for a however many milliseconds the datacenters are apart. As with
  46  * any cache, this should not be relied on for cases where reads are
  47  * used to determine writes to source (e.g. non-cache) data stores.
  48  *
  49  * All values are wrapped in metadata arrays. Keys use a "WANCache:" prefix
  50  * to avoid collisions with keys that are not wrapped as metadata arrays. The
  51  * prefixes are as follows:
  52  *   - a) "WANCache:v" : used for regular value keys
  53  *   - b) "WANCache:s" : used for temporarily storing values of tombstoned keys
  54  *   - c) "WANCache:t" : used for storing timestamp "check" keys
  55  *
  56  * @ingroup Cache
  57  * @since 1.26
  58  */
  59 class WANObjectCache {
  60         /** @var BagOStuff The local cluster cache */
  61         protected $cache;
  62         /** @var string Cache pool name */
  63         protected $pool;
  64         /** @var EventRelayer */
  65         protected $relayer;
  66
  67         /** @var int */
  68         protected $lastRelayError = self::ERR_NONE;
  69
  70         /** Seconds to tombstone keys on delete() */
  71         const HOLDOFF_TTL = 10;
  72         /** Seconds to keep dependency purge keys around */
  73         const CHECK_KEY_TTL = 31536000; // 1 year
  74         /** Seconds to keep lock keys around */
  75         const LOCK_TTL = 5;
  76
  77         /** Cache format version number */
  78         const VERSION = 1;
  79
  80         /** Fields of value holder arrays */
  81         const FLD_VERSION = 0;
  82         const FLD_VALUE = 1;
  83         const FLD_TTL = 2;
  84         const FLD_TIME = 3;
  85
  86         /** Possible values for getLastError() */
  87         const ERR_NONE = 0; // no error
  88         const ERR_NO_RESPONSE = 1; // no response
  89         const ERR_UNREACHABLE = 2; // can't connect
  90         const ERR_UNEXPECTED = 3; // response gave some error
  91         const ERR_RELAY = 4; // relay broadcast failed
  92
  93         const VALUE_KEY_PREFIX = 'WANCache:v:';
  94         const STASH_KEY_PREFIX = 'WANCache:s:';
  95         const TIME_KEY_PREFIX = 'WANCache:t:';
  96
  97         const PURGE_VAL_PREFIX = 'PURGED:';
  98
  99         /**
 100          * @param array $params
 101          *   - cache   : BagOStuff object
 102          *   - pool    : pool name
 103          *   - relayer : EventRelayer object
 104          */
 105         public function __construct( array $params ) {
 106                 $this->cache = $params['cache'];
 107                 $this->pool = $params['pool'];
 108                 $this->relayer = $params['relayer'];
 109         }
 110
 111         /**
 112          * Fetch the value of a key from cache
 113          *
 114          * If passed in, $curTTL is set to the remaining TTL (current time left):
 115          *   - a) INF; if the key exists and has no TTL
 116          *   - b) float (>=0); if the key exists and has a TTL
 117          *   - c) float (<0); if the key is tombstoned or expired by $checkKeys
 118          *   - d) null; if the key does not exist and is not tombstoned
 119          *
 120          * If a key is tombstoned, $curTTL will reflect the time since delete().
 121          *
 122          * The timestamp of $key will be checked against the last-purge timestamp
 123          * of each of $checkKeys. Those $checkKeys not in cache will have the last-purge
 124          * initialized to the current timestamp. If any of $checkKeys have a timestamp
 125          * greater than that of $key, then $curTTL will reflect how long ago $key
 126          * became invalid. Callers can use $curTTL to know when the value is stale.
 127          * The $checkKeys parameter allow mass invalidations by updating a single key:
 128          *   - a) Each "check" key represents "last purged" of some source data
 129          *   - b) Callers pass in relevant "check" keys as $checkKeys in get()
 130          *   - c) When the source data that "check" keys represent changes,
 131          *        the touchCheckKey() method is called on them
 132          *
 133          * For keys that are hot/expensive, consider using getWithSetCallback() instead.
 134          *
 135          * @param string $key Cache key
 136          * @param mixed $curTTL Approximate TTL left on the key if present [returned]
 137          * @param array $checkKeys List of "check" keys
 138          * @return mixed Value of cache key or false on failure
 139          */
 140         final public function get( $key, &$curTTL = null, array $checkKeys = array() ) {
 141                 $curTTLs = array();
 142                 $values = $this->getMulti( array( $key ), $curTTLs, $checkKeys );
 143                 $curTTL = isset( $curTTLs[$key] ) ? $curTTLs[$key] : null;
 144
 145                 return isset( $values[$key] ) ? $values[$key] : false;
 146         }
 147
 148         /**
 149          * Fetch the value of several keys from cache
 150          *
 151          * @see WANObjectCache::get()
 152          *
 153          * @param array $keys List of cache keys
 154          * @param array $curTTLs Map of (key => approximate TTL left) for existing keys [returned]
 155          * @param array $checkKeys List of "check" keys
 156          * @return array Map of (key => value) for keys that exist
 157          */
 158         final public function getMulti(
 159                 array $keys, &$curTTLs = array(), array $checkKeys = array()
 160         ) {
 161                 $result = array();
 162                 $curTTLs = array();
 163
 164                 $vPrefixLen = strlen( self::VALUE_KEY_PREFIX );
 165                 $valueKeys = self::prefixCacheKeys( $keys, self::VALUE_KEY_PREFIX );
 166                 $checkKeys = self::prefixCacheKeys( $checkKeys, self::TIME_KEY_PREFIX );
 167
 168                 // Fetch all of the raw values
 169                 $wrappedValues = $this->cache->getMulti( array_merge( $valueKeys, $checkKeys ) );
 170                 $now = microtime( true );
 171
 172                 // Get/initialize the timestamp of all the "check" keys
 173                 $checkKeyTimes = array();
 174                 foreach ( $checkKeys as $checkKey ) {
 175                         $timestamp = isset( $wrappedValues[$checkKey] )
 176                                 ? self::parsePurgeValue( $wrappedValues[$checkKey] )
 177                                 : false;
 178                         if ( !is_float( $timestamp ) ) {
 179                                 // Key is not set or invalid; regenerate
 180                                 $this->cache->add( $checkKey,
 181                                         self::PURGE_VAL_PREFIX . $now, self::CHECK_KEY_TTL );
 182                                 $timestamp = $now;
 183                         }
 184
 185                         $checkKeyTimes[] = $timestamp;
 186                 }
 187
 188                 // Get the main cache value for each key and validate them
 189                 foreach ( $valueKeys as $vKey ) {
 190                         if ( !isset( $wrappedValues[$vKey] ) ) {
 191                                 continue; // not found
 192                         }
 193
 194                         $key = substr( $vKey, $vPrefixLen ); // unprefix
 195
 196                         list( $value, $curTTL ) = $this->unwrap( $wrappedValues[$vKey], $now );
 197                         if ( $value !== false ) {
 198                                 $result[$key] = $value;
 199                                 foreach ( $checkKeyTimes as $checkKeyTime ) {
 200                                         // Force dependant keys to be invalid for a while after purging
 201                                         // to reduce race conditions involving stale data getting cached
 202                                         $safeTimestamp = $checkKeyTime + self::HOLDOFF_TTL;
 203                                         if ( $safeTimestamp >= $wrappedValues[$vKey][self::FLD_TIME] ) {
 204                                                 $curTTL = min( $curTTL, $checkKeyTime - $now );
 205                                         }
 206                                 }
 207                         }
 208
 209                         $curTTLs[$key] = $curTTL;
 210                 }
 211
 212                 return $result;
 213         }
 214
 215         /**
 216          * Set the value of a key from cache
 217          *
 218          * Simply calling this method when source data changes is not valid because
 219          * the changes do not replicate to the other WAN sites. In that case, delete()
 220          * should be used instead. This method is intended for use on cache misses.
 221          *
 222          * @param string $key Cache key
 223          * @param mixed $value
 224          * @param integer $ttl Seconds to live [0=forever]
 225          * @return bool Success
 226          */
 227         final public function set( $key, $value, $ttl = 0 ) {
 228                 $key = self::VALUE_KEY_PREFIX . $key;
 229                 $wrapped = $this->wrap( $value, $ttl );
 230
 231                 $func = function ( $cache, $key, $cWrapped ) use ( $wrapped ) {
 232                         return ( is_string( $cWrapped ) )
 233                                 ? false // key is tombstoned; do nothing
 234                                 : $wrapped;
 235                 };
 236
 237                 return $this->cache->merge( $key, $func, $ttl, 1 );
 238         }
 239
 240         /**
 241          * Purge a key from all clusters
 242          *
 243          * This instantiates a hold-off period where the key cannot be
 244          * written to avoid race conditions where dependent keys get updated
 245          * with a stale value (e.g. from a DB slave).
 246          *
 247          * This should only be called when the underlying data (being cached)
 248          * changes in a significant way. If called twice on the same key, then
 249          * the last TTL takes precedence.
 250          *
 251          * @param string $key Cache key
 252          * @param integer $ttl How long to block writes to the key [seconds]
 253          * @return bool True if the item was purged or not found, false on failure
 254          */
 255         final public function delete( $key, $ttl = self::HOLDOFF_TTL ) {
 256                 $key = self::VALUE_KEY_PREFIX . $key;
 257                 // Update the local cluster immediately
 258                 $ok = $this->cache->set( $key, self::PURGE_VAL_PREFIX . microtime( true ), $ttl );
 259                 // Publish the purge to all clusters
 260                 return $this->relayPurge( $key, $ttl ) && $ok;
 261         }
 262
 263         /**
 264          * Fetch the value of a timestamp "check" key
 265          *
 266          * @param string $key
 267          * @return float|bool TS_UNIX timestamp of the key; false if not present
 268          */
 269         final public function getCheckKeyTime( $key ) {
 270                 return self::parsePurgeValue( $this->cache->get( self::TIME_KEY_PREFIX . $key ) );
 271         }
 272
 273         /**
 274          * Purge a "check" key from all clusters, invalidating keys that use it
 275          *
 276          * This should only be called when the underlying data (being cached)
 277          * changes in a significant way, and it is impractical to call delete()
 278          * on all keys that should be changed. When get() is called on those
 279          * keys, the relevant "check" keys must be supplied for this to work.
 280          *
 281          * The "check" key essentially represents a last-modified field.
 282          * It is set in the future a few seconds when this is called, to
 283          * avoid race conditions where dependent keys get updated with a
 284          * stale value (e.g. from a DB slave).
 285          *
 286          * @see WANObjectCache::get()
 287          *
 288          * @param string $key Cache key
 289          * @return bool True if the item was purged or not found, false on failure
 290          */
 291         final public function touchCheckKey( $key ) {
 292                 $key = self::TIME_KEY_PREFIX . $key;
 293                 // Update the local cluster immediately
 294                 $ok = $this->cache->set( $key,
 295                         self::PURGE_VAL_PREFIX . microtime( true ), self::CHECK_KEY_TTL );
 296                 // Publish the purge to all clusters
 297                 return $this->relayPurge( $key, self::CHECK_KEY_TTL ) && $ok;
 298         }
 299
 300         /**
 301          * Method to fetch/regenerate cache keys
 302          *
 303          * On cache miss, the key will be set to the callback result,
 304          * unless the callback returns false. The arguments supplied are:
 305          *     (current value or false, &$ttl)
 306          * The callback function returns the new value given the current
 307          * value (false if not present). Preemptive re-caching and $checkKeys
 308          * can result in a non-false current value. The TTL of the new value
 309          * can be set dynamically by altering $ttl in the callback (by reference).
 310          *
 311          * Usually, callbacks ignore the current value, but it can be used
 312          * to maintain "most recent X" values that come from time or sequence
 313          * based source data, provided that the "as of" id/time is tracked.
 314          *
 315          * Usage of $checkKeys is similar to get()/getMulti(). However,
 316          * rather than the caller having to inspect a "current time left"
 317          * variable (e.g. $curTTL, $curTTLs), a cache regeneration will be
 318          * triggered using the callback.
 319          *
 320          * The simplest way to avoid stampedes for hot keys is to use
 321          * the 'lockTSE' option in $opts. If cache purges are needed, also:
 322          *   a) Pass $key into $checkKeys
 323          *   b) Use touchCheckKey( $key ) instead of delete( $key )
 324          * Following this pattern lets the old cache be used until a
 325          * single thread updates it as needed. Also consider tweaking
 326          * the 'lowTTL' parameter.
 327          *
 328          * Example usage:
 329          * @code
 330          *     $key = wfMemcKey( 'cat-recent-actions', $catId );
 331          *     // Function that derives the new key value given the old value
 332          *     $callback = function( $cValue, &$ttl ) { ... };
 333          *     // Get the key value from cache or from source on cache miss;
 334          *     // try to only let one cluster thread manage doing cache updates
 335          *     $opts = array( 'lockTSE' => 5, 'lowTTL' => 10 );
 336          *     $value = $cache->getWithSetCallback( $key, $callback, 60, array(), $opts );
 337          * @endcode
 338          *
 339          * Example usage:
 340          * @code
 341          *     $key = wfMemcKey( 'cat-state', $catId );
 342          *     // The "check" keys that represent things the value depends on;
 343          *     // Calling touchCheckKey() on them invalidates "cat-state"
 344          *     $checkKeys = array(
 345          *         wfMemcKey( 'water-bowls', $houseId ),
 346          *         wfMemcKey( 'food-bowls', $houseId ),
 347          *         wfMemcKey( 'people-present', $houseId )
 348          *     );
 349          *     // Function that derives the new key value
 350          *     $callback = function() { ... };
 351          *     // Get the key value from cache or from source on cache miss;
 352          *     // try to only let one cluster thread manage doing cache updates
 353          *     $opts = array( 'lockTSE' => 5, 'lowTTL' => 10 );
 354          *     $value = $cache->getWithSetCallback( $key, $callback, 60, $checkKeys, $opts );
 355          * @endcode
 356          *
 357          * @see WANObjectCache::get()
 358          *
 359          * @param string $key Cache key
 360          * @param callable $callback Value generation function
 361          * @param integer $ttl Seconds to live when the key is updated [0=forever]
 362          * @param array $checkKeys List of "check" keys
 363          * @param array $opts Options map:
 364          *   - lowTTL  : consider pre-emptive updates when the current TTL (sec)
 365          *               of the key is less than this. It becomes more likely
 366          *               over time, becoming a certainty once the key is expired.
 367          *   - lockTSE : if the key is tombstoned or expired (by $checkKeys) less
 368          *               than this many seconds ago, then try to have a single
 369          *               thread handle cache regeneration at any given time.
 370          *               Other threads will try to use stale values if possible.
 371          *               If, on miss, the time since expiration is low, the assumption
 372          *               is that the key is hot and that a stampede is worth avoiding.
 373          *   - tempTTL : when 'lockTSE' is set, this determines the TTL of the temp
 374          *               key used to cache values while a key is tombstoned.
 375          *               This avoids excessive regeneration of hot keys on delete() but
 376          *               may result in stale values.
 377          * @return mixed Value to use for the key
 378          */
 379         final public function getWithSetCallback(
 380                 $key, $callback, $ttl, array $checkKeys = array(), array $opts = array()
 381         ) {
 382                 $lowTTL = isset( $opts['lowTTL'] ) ? $opts['lowTTL'] : min( 10, $ttl );
 383                 $lockTSE = isset( $opts['lockTSE'] ) ? $opts['lockTSE'] : -1;
 384                 $tempTTL = isset( $opts['tempTTL'] ) ? $opts['tempTTL'] : 5;
 385
 386                 // Get the current key value
 387                 $curTTL = null;
 388                 $cValue = $this->get( $key, $curTTL, $checkKeys ); // current value
 389                 $value = $cValue; // return value
 390
 391                 // Determine if a regeneration is desired
 392                 if ( $value !== false && $curTTL > 0 && !$this->worthRefresh( $curTTL, $lowTTL ) ) {
 393                         return $value;
 394                 }
 395
 396                 if ( !is_callable( $callback ) ) {
 397                         throw new InvalidArgumentException( "Invalid cache miss callback provided." );
 398                 }
 399
 400                 // Assume a key is hot if requested soon after invalidation
 401                 $isHot = ( $curTTL !== null && $curTTL <= 0 && abs( $curTTL ) <= $lockTSE );
 402                 $isTombstone = ( $curTTL !== null && $value === false );
 403
 404                 $locked = false;
 405                 if ( $isHot || $isTombstone ) {
 406                         // Acquire a cluster-local non-blocking lock
 407                         if ( $this->cache->lock( $key, 0, self::LOCK_TTL ) ) {
 408                                 // Lock acquired; this thread should update the key
 409                                 $locked = true;
 410                         } elseif ( $value !== false ) {
 411                                 // If it cannot be acquired; then the stale value can be used
 412                                 return $value;
 413                         } else {
 414                                 // Either another thread has the lock or the lock failed.
 415                                 // Use the stash value, which is likely from the prior thread.
 416                                 $value = $this->cache->get( self::STASH_KEY_PREFIX . $key );
 417                                 // Regenerate on timeout or if the other thread failed
 418                                 if ( $value !== false ) {
 419                                         return $value;
 420                                 }
 421                         }
 422                 }
 423
 424                 // Generate the new value from the callback...
 425                 $value = call_user_func_array( $callback, array( $cValue, &$ttl ) );
 426                 // When delete() is called, writes are write-holed by the tombstone,
 427                 // so use a special stash key to pass the new value around threads.
 428                 if ( $value !== false && ( $isHot || $isTombstone ) ) {
 429                         $this->cache->set( self::STASH_KEY_PREFIX . $key, $value, $tempTTL );
 430                 }
 431
 432                 if ( $locked ) {
 433                         $this->cache->unlock( $key );
 434                 }
 435
 436                 if ( $value !== false ) {
 437                         // Update the cache; this will fail if the key is tombstoned
 438                         $this->set( $key, $value, $ttl );
 439                 }
 440
 441                 return $value;
 442         }
 443
 444         /**
 445          * Get the "last error" registered; clearLastError() should be called manually
 446          * @return int ERR_* constant for the "last error" registry
 447          */
 448         final public function getLastError() {
 449                 if ( $this->lastRelayError ) {
 450                         // If the cache and the relayer failed, focus on the later.
 451                         // An update not making it to the relayer means it won't show up
 452                         // in other DCs (nor will consistent re-hashing see up-to-date values).
 453                         // On the other hand, if just the cache update failed, then it should
 454                         // eventually be applied by the relayer.
 455                         return $this->lastRelayError;
 456                 }
 457
 458                 $code = $this->cache->getLastError();
 459                 switch ( $code ) {
 460                         case BagOStuff::ERR_NONE:
 461                                 return self::ERR_NONE;
 462                         case BagOStuff::ERR_NO_RESPONSE:
 463                                 return self::ERR_NO_RESPONSE;
 464                         case BagOStuff::ERR_UNREACHABLE:
 465                                 return self::ERR_UNREACHABLE;
 466                         default:
 467                                 return self::ERR_UNEXPECTED;
 468                 }
 469         }
 470
 471         /**
 472          * Clear the "last error" registry
 473          */
 474         final public function clearLastError() {
 475                 $this->cache->clearLastError();
 476                 $this->lastRelayError = self::ERR_NONE;
 477         }
 478
 479         /**
 480          * Do the actual async bus purge of a key
 481          *
 482          * This must set the key to "PURGED:<UNIX timestamp>"
 483          *
 484          * @param string $key Cache key
 485          * @param integer $ttl How long to keep the tombstone [seconds]
 486          * @return bool Success
 487          */
 488         protected function relayPurge( $key, $ttl ) {
 489                 $event = $this->cache->modifySimpleRelayEvent( array(
 490                         'cmd' => 'set',
 491                         'key' => $key,
 492                         'val' => 'PURGED:$UNIXTIME$',
 493                         'ttl' => max( $ttl, 1 ),
 494                         'sbt' => true, // substitute $UNIXTIME$ with actual microtime
 495                 ) );
 496
 497                 $ok = $this->relayer->notify( "{$this->pool}:purge", $event );
 498                 if ( !$ok ) {
 499                         $this->lastRelayError = self::ERR_RELAY;
 500                 }
 501
 502                 return $ok;
 503         }
 504
 505         /**
 506          * Check if a key should be regenerated (using random probability)
 507          *
 508          * This returns false if $curTTL >= $lowTTL. Otherwise, the chance
 509          * of returning true increases steadily from 0% to 100% as the $curTTL
 510          * moves from $lowTTL to 0 seconds. This handles widely varying
 511          * levels of cache access traffic.
 512          *
 513          * @param float|INF $curTTL Approximate TTL left on the key if present
 514          * @param float $lowTTL Consider a refresh when $curTTL is less than this
 515          * @return bool
 516          */
 517         protected function worthRefresh( $curTTL, $lowTTL ) {
 518                 if ( $curTTL >= $lowTTL ) {
 519                         return false;
 520                 } elseif ( $curTTL <= 0 ) {
 521                         return true;
 522                 }
 523
 524                 $chance = ( 1 - $curTTL / $lowTTL );
 525
 526                 return mt_rand( 1, 1e9 ) <= 1e9 * $chance;
 527         }
 528
 529         /**
 530          * Do not use this method outside WANObjectCache
 531          *
 532          * @param mixed $value
 533          * @param integer $ttl [0=forever]
 534          * @return string
 535          */
 536         protected function wrap( $value, $ttl ) {
 537                 return array(
 538                         self::FLD_VERSION => self::VERSION,
 539                         self::FLD_VALUE => $value,
 540                         self::FLD_TTL => $ttl,
 541                         self::FLD_TIME => microtime( true )
 542                 );
 543         }
 544
 545         /**
 546          * Do not use this method outside WANObjectCache
 547          *
 548          * @param array|string|bool $wrapped
 549          * @param float $now Unix Current timestamp (preferrable pre-query)
 550          * @return array (mixed; false if absent/invalid, current time left)
 551          */
 552         protected function unwrap( $wrapped, $now ) {
 553                 // Check if the value is a tombstone
 554                 $purgeTimestamp = self::parsePurgeValue( $wrapped );
 555                 if ( is_float( $purgeTimestamp ) ) {
 556                         // Purged values should always have a negative current $ttl
 557                         $curTTL = min( -0.000001, $purgeTimestamp - $now );
 558                         return array( false, $curTTL );
 559                 }
 560
 561                 if ( !is_array( $wrapped ) // not found
 562                         || !isset( $wrapped[self::FLD_VERSION] ) // wrong format
 563                         || $wrapped[self::FLD_VERSION] !== self::VERSION // wrong version
 564                 ) {
 565                         return array( false, null );
 566                 }
 567
 568                 if ( $wrapped[self::FLD_TTL] > 0 ) {
 569                         // Get the approximate time left on the key
 570                         $age = $now - $wrapped[self::FLD_TIME];
 571                         $curTTL = max( $wrapped[self::FLD_TTL] - $age, 0.0 );
 572                 } else {
 573                         // Key had no TTL, so the time left is unbounded
 574                         $curTTL = INF;
 575                 }
 576
 577                 return array( $wrapped[self::FLD_VALUE], $curTTL );
 578         }
 579
 580         /**
 581          * @param array $keys
 582          * @param string $prefix
 583          * @return string[]
 584          */
 585         protected static function prefixCacheKeys( array $keys, $prefix ) {
 586                 $res = array();
 587                 foreach ( $keys as $key ) {
 588                         $res[] = $prefix . $key;
 589                 }
 590
 591                 return $res;
 592         }
 593
 594         /**
 595          * @param string $value String like "PURGED:<timestamp>"
 596          * @return float|bool UNIX timestamp or false on failure
 597          */
 598         protected static function parsePurgeValue( $value ) {
 599                 $m = array();
 600                 if ( is_string( $value ) &&
 601                         preg_match( '/^' . self::PURGE_VAL_PREFIX . '([^:]+)$/', $value, $m )
 602                 ) {
 603                         return (float)$m[1];
 604                 } else {
 605                         return false;
 606                 }
 607         }
 608 }