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         /** Idiom for set()/getWithSetCallback() TTL */
  78         const TTL_NONE = 0;
  79
  80         /** Cache format version number */
  81         const VERSION = 1;
  82
  83         /** Fields of value holder arrays */
  84         const FLD_VERSION = 0;
  85         const FLD_VALUE = 1;
  86         const FLD_TTL = 2;
  87         const FLD_TIME = 3;
  88
  89         /** Possible values for getLastError() */
  90         const ERR_NONE = 0; // no error
  91         const ERR_NO_RESPONSE = 1; // no response
  92         const ERR_UNREACHABLE = 2; // can't connect
  93         const ERR_UNEXPECTED = 3; // response gave some error
  94         const ERR_RELAY = 4; // relay broadcast failed
  95
  96         const VALUE_KEY_PREFIX = 'WANCache:v:';
  97         const STASH_KEY_PREFIX = 'WANCache:s:';
  98         const TIME_KEY_PREFIX = 'WANCache:t:';
  99
 100         const PURGE_VAL_PREFIX = 'PURGED:';
 101
 102         /**
 103          * @param array $params
 104          *   - cache   : BagOStuff object
 105          *   - pool    : pool name
 106          *   - relayer : EventRelayer object
 107          */
 108         public function __construct( array $params ) {
 109                 $this->cache = $params['cache'];
 110                 $this->pool = $params['pool'];
 111                 $this->relayer = $params['relayer'];
 112         }
 113
 114         /**
 115          * Fetch the value of a key from cache
 116          *
 117          * If passed in, $curTTL is set to the remaining TTL (current time left):
 118          *   - a) INF; if the key exists and has no TTL
 119          *   - b) float (>=0); if the key exists and has a TTL
 120          *   - c) float (<0); if the key is tombstoned or expired by $checkKeys
 121          *   - d) null; if the key does not exist and is not tombstoned
 122          *
 123          * If a key is tombstoned, $curTTL will reflect the time since delete().
 124          *
 125          * The timestamp of $key will be checked against the last-purge timestamp
 126          * of each of $checkKeys. Those $checkKeys not in cache will have the last-purge
 127          * initialized to the current timestamp. If any of $checkKeys have a timestamp
 128          * greater than that of $key, then $curTTL will reflect how long ago $key
 129          * became invalid. Callers can use $curTTL to know when the value is stale.
 130          * The $checkKeys parameter allow mass invalidations by updating a single key:
 131          *   - a) Each "check" key represents "last purged" of some source data
 132          *   - b) Callers pass in relevant "check" keys as $checkKeys in get()
 133          *   - c) When the source data that "check" keys represent changes,
 134          *        the touchCheckKey() method is called on them
 135          *
 136          * For keys that are hot/expensive, consider using getWithSetCallback() instead.
 137          *
 138          * @param string $key Cache key
 139          * @param mixed $curTTL Approximate TTL left on the key if present [returned]
 140          * @param array $checkKeys List of "check" keys
 141          * @return mixed Value of cache key or false on failure
 142          */
 143         final public function get( $key, &$curTTL = null, array $checkKeys = array() ) {
 144                 $curTTLs = array();
 145                 $values = $this->getMulti( array( $key ), $curTTLs, $checkKeys );
 146                 $curTTL = isset( $curTTLs[$key] ) ? $curTTLs[$key] : null;
 147
 148                 return isset( $values[$key] ) ? $values[$key] : false;
 149         }
 150
 151         /**
 152          * Fetch the value of several keys from cache
 153          *
 154          * @see WANObjectCache::get()
 155          *
 156          * @param array $keys List of cache keys
 157          * @param array $curTTLs Map of (key => approximate TTL left) for existing keys [returned]
 158          * @param array $checkKeys List of "check" keys
 159          * @return array Map of (key => value) for keys that exist
 160          */
 161         final public function getMulti(
 162                 array $keys, &$curTTLs = array(), array $checkKeys = array()
 163         ) {
 164                 $result = array();
 165                 $curTTLs = array();
 166
 167                 $vPrefixLen = strlen( self::VALUE_KEY_PREFIX );
 168                 $valueKeys = self::prefixCacheKeys( $keys, self::VALUE_KEY_PREFIX );
 169                 $checkKeys = self::prefixCacheKeys( $checkKeys, self::TIME_KEY_PREFIX );
 170
 171                 // Fetch all of the raw values
 172                 $wrappedValues = $this->cache->getMulti( array_merge( $valueKeys, $checkKeys ) );
 173                 $now = microtime( true );
 174
 175                 // Get/initialize the timestamp of all the "check" keys
 176                 $checkKeyTimes = array();
 177                 foreach ( $checkKeys as $checkKey ) {
 178                         $timestamp = isset( $wrappedValues[$checkKey] )
 179                                 ? self::parsePurgeValue( $wrappedValues[$checkKey] )
 180                                 : false;
 181                         if ( !is_float( $timestamp ) ) {
 182                                 // Key is not set or invalid; regenerate
 183                                 $this->cache->add( $checkKey,
 184                                         self::PURGE_VAL_PREFIX . $now, self::CHECK_KEY_TTL );
 185                                 $timestamp = $now;
 186                         }
 187
 188                         $checkKeyTimes[] = $timestamp;
 189                 }
 190
 191                 // Get the main cache value for each key and validate them
 192                 foreach ( $valueKeys as $vKey ) {
 193                         if ( !isset( $wrappedValues[$vKey] ) ) {
 194                                 continue; // not found
 195                         }
 196
 197                         $key = substr( $vKey, $vPrefixLen ); // unprefix
 198
 199                         list( $value, $curTTL ) = $this->unwrap( $wrappedValues[$vKey], $now );
 200                         if ( $value !== false ) {
 201                                 $result[$key] = $value;
 202                                 foreach ( $checkKeyTimes as $checkKeyTime ) {
 203                                         // Force dependant keys to be invalid for a while after purging
 204                                         // to reduce race conditions involving stale data getting cached
 205                                         $safeTimestamp = $checkKeyTime + self::HOLDOFF_TTL;
 206                                         if ( $safeTimestamp >= $wrappedValues[$vKey][self::FLD_TIME] ) {
 207                                                 $curTTL = min( $curTTL, $checkKeyTime - $now );
 208                                         }
 209                                 }
 210                         }
 211
 212                         $curTTLs[$key] = $curTTL;
 213                 }
 214
 215                 return $result;
 216         }
 217
 218         /**
 219          * Set the value of a key from cache
 220          *
 221          * Simply calling this method when source data changes is not valid because
 222          * the changes do not replicate to the other WAN sites. In that case, delete()
 223          * should be used instead. This method is intended for use on cache misses.
 224          *
 225          * @param string $key Cache key
 226          * @param mixed $value
 227          * @param integer $ttl Seconds to live [0=forever]
 228          * @return bool Success
 229          */
 230         final public function set( $key, $value, $ttl = 0 ) {
 231                 $key = self::VALUE_KEY_PREFIX . $key;
 232                 $wrapped = $this->wrap( $value, $ttl );
 233
 234                 $func = function ( $cache, $key, $cWrapped ) use ( $wrapped ) {
 235                         return ( is_string( $cWrapped ) )
 236                                 ? false // key is tombstoned; do nothing
 237                                 : $wrapped;
 238                 };
 239
 240                 return $this->cache->merge( $key, $func, $ttl, 1 );
 241         }
 242
 243         /**
 244          * Purge a key from all clusters
 245          *
 246          * This instantiates a hold-off period where the key cannot be
 247          * written to avoid race conditions where dependent keys get updated
 248          * with a stale value (e.g. from a DB slave).
 249          *
 250          * This should only be called when the underlying data (being cached)
 251          * changes in a significant way. If called twice on the same key, then
 252          * the last TTL takes precedence.
 253          *
 254          * @param string $key Cache key
 255          * @param integer $ttl How long to block writes to the key [seconds]
 256          * @return bool True if the item was purged or not found, false on failure
 257          */
 258         final public function delete( $key, $ttl = self::HOLDOFF_TTL ) {
 259                 $key = self::VALUE_KEY_PREFIX . $key;
 260                 // Update the local cluster immediately
 261                 $ok = $this->cache->set( $key, self::PURGE_VAL_PREFIX . microtime( true ), $ttl );
 262                 // Publish the purge to all clusters
 263                 return $this->relayPurge( $key, $ttl ) && $ok;
 264         }
 265
 266         /**
 267          * Fetch the value of a timestamp "check" key
 268          *
 269          * Note that "check" keys won't collide with other regular keys
 270          *
 271          * @param string $key
 272          * @return float|bool TS_UNIX timestamp of the key; false if not present
 273          */
 274         final public function getCheckKeyTime( $key ) {
 275                 return self::parsePurgeValue( $this->cache->get( self::TIME_KEY_PREFIX . $key ) );
 276         }
 277
 278         /**
 279          * Purge a "check" key from all clusters, invalidating keys that use it
 280          *
 281          * This should only be called when the underlying data (being cached)
 282          * changes in a significant way, and it is impractical to call delete()
 283          * on all keys that should be changed. When get() is called on those
 284          * keys, the relevant "check" keys must be supplied for this to work.
 285          *
 286          * The "check" key essentially represents a last-modified field.
 287          * It is set in the future a few seconds when this is called, to
 288          * avoid race conditions where dependent keys get updated with a
 289          * stale value (e.g. from a DB slave).
 290          *
 291          * Note that "check" keys won't collide with other regular keys
 292          *
 293          * @see WANObjectCache::get()
 294          *
 295          * @param string $key Cache key
 296          * @return bool True if the item was purged or not found, false on failure
 297          */
 298         final public function touchCheckKey( $key ) {
 299                 $key = self::TIME_KEY_PREFIX . $key;
 300                 // Update the local cluster immediately
 301                 $ok = $this->cache->set( $key,
 302                         self::PURGE_VAL_PREFIX . microtime( true ), self::CHECK_KEY_TTL );
 303                 // Publish the purge to all clusters
 304                 return $this->relayPurge( $key, self::CHECK_KEY_TTL ) && $ok;
 305         }
 306
 307         /**
 308          * Method to fetch/regenerate cache keys
 309          *
 310          * On cache miss, the key will be set to the callback result,
 311          * unless the callback returns false. The arguments supplied are:
 312          *     (current value or false, &$ttl)
 313          * The callback function returns the new value given the current
 314          * value (false if not present). Preemptive re-caching and $checkKeys
 315          * can result in a non-false current value. The TTL of the new value
 316          * can be set dynamically by altering $ttl in the callback (by reference).
 317          *
 318          * Usually, callbacks ignore the current value, but it can be used
 319          * to maintain "most recent X" values that come from time or sequence
 320          * based source data, provided that the "as of" id/time is tracked.
 321          *
 322          * Usage of $checkKeys is similar to get()/getMulti(). However,
 323          * rather than the caller having to inspect a "current time left"
 324          * variable (e.g. $curTTL, $curTTLs), a cache regeneration will be
 325          * triggered using the callback.
 326          *
 327          * The simplest way to avoid stampedes for hot keys is to use
 328          * the 'lockTSE' option in $opts. If cache purges are needed, also:
 329          *   a) Pass $key into $checkKeys
 330          *   b) Use touchCheckKey( $key ) instead of delete( $key )
 331          * Following this pattern lets the old cache be used until a
 332          * single thread updates it as needed. Also consider tweaking
 333          * the 'lowTTL' parameter.
 334          *
 335          * Example usage:
 336          * @code
 337          *     $key = wfMemcKey( 'cat-recent-actions', $catId );
 338          *     // Function that derives the new key value given the old value
 339          *     $callback = function( $cValue, &$ttl ) { ... };
 340          *     // Get the key value from cache or from source on cache miss;
 341          *     // try to only let one cluster thread manage doing cache updates
 342          *     $opts = array( 'lockTSE' => 5, 'lowTTL' => 10 );
 343          *     $value = $cache->getWithSetCallback( $key, $callback, 60, array(), $opts );
 344          * @endcode
 345          *
 346          * Example usage:
 347          * @code
 348          *     $key = wfMemcKey( 'cat-state', $catId );
 349          *     // The "check" keys that represent things the value depends on;
 350          *     // Calling touchCheckKey() on them invalidates "cat-state"
 351          *     $checkKeys = array(
 352          *         wfMemcKey( 'water-bowls', $houseId ),
 353          *         wfMemcKey( 'food-bowls', $houseId ),
 354          *         wfMemcKey( 'people-present', $houseId )
 355          *     );
 356          *     // Function that derives the new key value
 357          *     $callback = function() { ... };
 358          *     // Get the key value from cache or from source on cache miss;
 359          *     // try to only let one cluster thread manage doing cache updates
 360          *     $opts = array( 'lockTSE' => 5, 'lowTTL' => 10 );
 361          *     $value = $cache->getWithSetCallback( $key, $callback, 60, $checkKeys, $opts );
 362          * @endcode
 363          *
 364          * @see WANObjectCache::get()
 365          *
 366          * @param string $key Cache key
 367          * @param callable $callback Value generation function
 368          * @param integer $ttl Seconds to live when the key is updated [0=forever]
 369          * @param array $checkKeys List of "check" keys
 370          * @param array $opts Options map:
 371          *   - lowTTL  : consider pre-emptive updates when the current TTL (sec)
 372          *               of the key is less than this. It becomes more likely
 373          *               over time, becoming a certainty once the key is expired.
 374          *   - lockTSE : if the key is tombstoned or expired (by $checkKeys) less
 375          *               than this many seconds ago, then try to have a single
 376          *               thread handle cache regeneration at any given time.
 377          *               Other threads will try to use stale values if possible.
 378          *               If, on miss, the time since expiration is low, the assumption
 379          *               is that the key is hot and that a stampede is worth avoiding.
 380          *   - tempTTL : when 'lockTSE' is set, this determines the TTL of the temp
 381          *               key used to cache values while a key is tombstoned.
 382          *               This avoids excessive regeneration of hot keys on delete() but
 383          *               may result in stale values.
 384          * @return mixed Value to use for the key
 385          */
 386         final public function getWithSetCallback(
 387                 $key, $callback, $ttl, array $checkKeys = array(), array $opts = array()
 388         ) {
 389                 $lowTTL = isset( $opts['lowTTL'] ) ? $opts['lowTTL'] : min( 10, $ttl );
 390                 $lockTSE = isset( $opts['lockTSE'] ) ? $opts['lockTSE'] : -1;
 391                 $tempTTL = isset( $opts['tempTTL'] ) ? $opts['tempTTL'] : 5;
 392
 393                 // Get the current key value
 394                 $curTTL = null;
 395                 $cValue = $this->get( $key, $curTTL, $checkKeys ); // current value
 396                 $value = $cValue; // return value
 397
 398                 // Determine if a regeneration is desired
 399                 if ( $value !== false && $curTTL > 0 && !$this->worthRefresh( $curTTL, $lowTTL ) ) {
 400                         return $value;
 401                 }
 402
 403                 if ( !is_callable( $callback ) ) {
 404                         throw new InvalidArgumentException( "Invalid cache miss callback provided." );
 405                 }
 406
 407                 // Assume a key is hot if requested soon after invalidation
 408                 $isHot = ( $curTTL !== null && $curTTL <= 0 && abs( $curTTL ) <= $lockTSE );
 409                 $isTombstone = ( $curTTL !== null && $value === false );
 410
 411                 $locked = false;
 412                 if ( $isHot || $isTombstone ) {
 413                         // Acquire a cluster-local non-blocking lock
 414                         if ( $this->cache->lock( $key, 0, self::LOCK_TTL ) ) {
 415                                 // Lock acquired; this thread should update the key
 416                                 $locked = true;
 417                         } elseif ( $value !== false ) {
 418                                 // If it cannot be acquired; then the stale value can be used
 419                                 return $value;
 420                         } else {
 421                                 // Either another thread has the lock or the lock failed.
 422                                 // Use the stash value, which is likely from the prior thread.
 423                                 $value = $this->cache->get( self::STASH_KEY_PREFIX . $key );
 424                                 // Regenerate on timeout or if the other thread failed
 425                                 if ( $value !== false ) {
 426                                         return $value;
 427                                 }
 428                         }
 429                 }
 430
 431                 // Generate the new value from the callback...
 432                 $value = call_user_func_array( $callback, array( $cValue, &$ttl ) );
 433                 // When delete() is called, writes are write-holed by the tombstone,
 434                 // so use a special stash key to pass the new value around threads.
 435                 if ( $value !== false && ( $isHot || $isTombstone ) ) {
 436                         $this->cache->set( self::STASH_KEY_PREFIX . $key, $value, $tempTTL );
 437                 }
 438
 439                 if ( $locked ) {
 440                         $this->cache->unlock( $key );
 441                 }
 442
 443                 if ( $value !== false ) {
 444                         // Update the cache; this will fail if the key is tombstoned
 445                         $this->set( $key, $value, $ttl );
 446                 }
 447
 448                 return $value;
 449         }
 450
 451         /**
 452          * Get the "last error" registered; clearLastError() should be called manually
 453          * @return int ERR_* constant for the "last error" registry
 454          */
 455         final public function getLastError() {
 456                 if ( $this->lastRelayError ) {
 457                         // If the cache and the relayer failed, focus on the later.
 458                         // An update not making it to the relayer means it won't show up
 459                         // in other DCs (nor will consistent re-hashing see up-to-date values).
 460                         // On the other hand, if just the cache update failed, then it should
 461                         // eventually be applied by the relayer.
 462                         return $this->lastRelayError;
 463                 }
 464
 465                 $code = $this->cache->getLastError();
 466                 switch ( $code ) {
 467                         case BagOStuff::ERR_NONE:
 468                                 return self::ERR_NONE;
 469                         case BagOStuff::ERR_NO_RESPONSE:
 470                                 return self::ERR_NO_RESPONSE;
 471                         case BagOStuff::ERR_UNREACHABLE:
 472                                 return self::ERR_UNREACHABLE;
 473                         default:
 474                                 return self::ERR_UNEXPECTED;
 475                 }
 476         }
 477
 478         /**
 479          * Clear the "last error" registry
 480          */
 481         final public function clearLastError() {
 482                 $this->cache->clearLastError();
 483                 $this->lastRelayError = self::ERR_NONE;
 484         }
 485
 486         /**
 487          * Do the actual async bus purge of a key
 488          *
 489          * This must set the key to "PURGED:<UNIX timestamp>"
 490          *
 491          * @param string $key Cache key
 492          * @param integer $ttl How long to keep the tombstone [seconds]
 493          * @return bool Success
 494          */
 495         protected function relayPurge( $key, $ttl ) {
 496                 $event = $this->cache->modifySimpleRelayEvent( array(
 497                         'cmd' => 'set',
 498                         'key' => $key,
 499                         'val' => 'PURGED:$UNIXTIME$',
 500                         'ttl' => max( $ttl, 1 ),
 501                         'sbt' => true, // substitute $UNIXTIME$ with actual microtime
 502                 ) );
 503
 504                 $ok = $this->relayer->notify( "{$this->pool}:purge", $event );
 505                 if ( !$ok ) {
 506                         $this->lastRelayError = self::ERR_RELAY;
 507                 }
 508
 509                 return $ok;
 510         }
 511
 512         /**
 513          * Check if a key should be regenerated (using random probability)
 514          *
 515          * This returns false if $curTTL >= $lowTTL. Otherwise, the chance
 516          * of returning true increases steadily from 0% to 100% as the $curTTL
 517          * moves from $lowTTL to 0 seconds. This handles widely varying
 518          * levels of cache access traffic.
 519          *
 520          * @param float|INF $curTTL Approximate TTL left on the key if present
 521          * @param float $lowTTL Consider a refresh when $curTTL is less than this
 522          * @return bool
 523          */
 524         protected function worthRefresh( $curTTL, $lowTTL ) {
 525                 if ( $curTTL >= $lowTTL ) {
 526                         return false;
 527                 } elseif ( $curTTL <= 0 ) {
 528                         return true;
 529                 }
 530
 531                 $chance = ( 1 - $curTTL / $lowTTL );
 532
 533                 return mt_rand( 1, 1e9 ) <= 1e9 * $chance;
 534         }
 535
 536         /**
 537          * Do not use this method outside WANObjectCache
 538          *
 539          * @param mixed $value
 540          * @param integer $ttl [0=forever]
 541          * @return string
 542          */
 543         protected function wrap( $value, $ttl ) {
 544                 return array(
 545                         self::FLD_VERSION => self::VERSION,
 546                         self::FLD_VALUE => $value,
 547                         self::FLD_TTL => $ttl,
 548                         self::FLD_TIME => microtime( true )
 549                 );
 550         }
 551
 552         /**
 553          * Do not use this method outside WANObjectCache
 554          *
 555          * @param array|string|bool $wrapped
 556          * @param float $now Unix Current timestamp (preferrable pre-query)
 557          * @return array (mixed; false if absent/invalid, current time left)
 558          */
 559         protected function unwrap( $wrapped, $now ) {
 560                 // Check if the value is a tombstone
 561                 $purgeTimestamp = self::parsePurgeValue( $wrapped );
 562                 if ( is_float( $purgeTimestamp ) ) {
 563                         // Purged values should always have a negative current $ttl
 564                         $curTTL = min( -0.000001, $purgeTimestamp - $now );
 565                         return array( false, $curTTL );
 566                 }
 567
 568                 if ( !is_array( $wrapped ) // not found
 569                         || !isset( $wrapped[self::FLD_VERSION] ) // wrong format
 570                         || $wrapped[self::FLD_VERSION] !== self::VERSION // wrong version
 571                 ) {
 572                         return array( false, null );
 573                 }
 574
 575                 if ( $wrapped[self::FLD_TTL] > 0 ) {
 576                         // Get the approximate time left on the key
 577                         $age = $now - $wrapped[self::FLD_TIME];
 578                         $curTTL = max( $wrapped[self::FLD_TTL] - $age, 0.0 );
 579                 } else {
 580                         // Key had no TTL, so the time left is unbounded
 581                         $curTTL = INF;
 582                 }
 583
 584                 return array( $wrapped[self::FLD_VALUE], $curTTL );
 585         }
 586
 587         /**
 588          * @param array $keys
 589          * @param string $prefix
 590          * @return string[]
 591          */
 592         protected static function prefixCacheKeys( array $keys, $prefix ) {
 593                 $res = array();
 594                 foreach ( $keys as $key ) {
 595                         $res[] = $prefix . $key;
 596                 }
 597
 598                 return $res;
 599         }
 600
 601         /**
 602          * @param string $value String like "PURGED:<timestamp>"
 603          * @return float|bool UNIX timestamp or false on failure
 604          */
 605         protected static function parsePurgeValue( $value ) {
 606                 $m = array();
 607                 if ( is_string( $value ) &&
 608                         preg_match( '/^' . self::PURGE_VAL_PREFIX . '([^:]+)$/', $value, $m )
 609                 ) {
 610                         return (float)$m[1];
 611                 } else {
 612                         return false;
 613                 }
 614         }
 615 }