wiki.techinc.nl/includes/libs/objectcache/WANObjectCache.php

<?php
/**
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup Cache
 */

use Psr\Log\LoggerAwareInterface;
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;

/**
 * Multi-datacenter aware caching interface
 *
 * All operations go to the local datacenter cache, except for delete(),
 * touchCheckKey(), and resetCheckKey(), which broadcast to all datacenters.
 *
 * This class is intended for caching data from primary stores.
 * If the get() method does not return a value, then the caller
 * should query the new value and backfill the cache using set().
 * The preferred way to do this logic is through getWithSetCallback().
 * When querying the store on cache miss, the closest DB replica
 * should be used. Try to avoid heavyweight DB master or quorum reads.
 * When the source data changes, a purge method should be called.
 * Since purges are expensive, they should be avoided. One can do so if:
 *   - a) The object cached is immutable; or
 *   - b) Validity is checked against the source after get(); or
 *   - c) Using a modest TTL is reasonably correct and performant
 *
 * The simplest purge method is delete().
 *
 * There are three supported ways to handle broadcasted operations:
 *   - a) Configure the 'purge' EventRelayer to point to a valid PubSub endpoint
 *         that has subscribed listeners on the cache servers applying the cache updates.
 *   - b) Ignore the 'purge' EventRelayer configuration (default is NullEventRelayer)
 *         and set up mcrouter as the underlying cache backend, using one of the memcached
 *         BagOStuff classes as 'cache'. Use OperationSelectorRoute in the mcrouter settings
 *         to configure 'set' and 'delete' operations to go to all DCs via AllAsyncRoute and
 *         configure other operations to go to the local DC via PoolRoute (for reference,
 *         see https://github.com/facebook/mcrouter/wiki/List-of-Route-Handles).
 *   - c) Ignore the 'purge' EventRelayer configuration (default is NullEventRelayer)
 *         and set up dynomite as cache middleware between the web servers and either
 *         memcached or redis. This will also broadcast all key setting operations, not just purges,
 *         which can be useful for cache warming. Writes are eventually consistent via the
 *         Dynamo replication model (see https://github.com/Netflix/dynomite).
 *
 * Broadcasted operations like delete() and touchCheckKey() are done asynchronously
 * in all datacenters this way, though the local one should likely be near immediate.
 *
 * This means that callers in all datacenters may see older values for however many
 * milliseconds that the purge took to reach that datacenter. As with any cache, this
 * should not be relied on for cases where reads are used to determine writes to source
 * (e.g. non-cache) data stores, except when reading immutable data.
 *
 * All values are wrapped in metadata arrays. Keys use a "WANCache:" prefix
 * to avoid collisions with keys that are not wrapped as metadata arrays. The
 * prefixes are as follows:
 *   - a) "WANCache:v" : used for regular value keys
 *   - b) "WANCache:i" : used for temporarily storing values of tombstoned keys
 *   - c) "WANCache:t" : used for storing timestamp "check" keys
 *   - d) "WANCache:m" : used for temporary mutex keys to avoid cache stampedes
 *
 * @ingroup Cache
 * @since 1.26
 */
class WANObjectCache implements IExpiringStore, LoggerAwareInterface {
	/** @var BagOStuff The local datacenter cache */
	protected $cache;
	/** @var HashBagOStuff[] Map of group PHP instance caches */
	protected $processCaches = [];
	/** @var string Purge channel name */
	protected $purgeChannel;
	/** @var EventRelayer Bus that handles purge broadcasts */
	protected $purgeRelayer;
	/** @var LoggerInterface */
	protected $logger;

	/** @var int ERR_* constant for the "last error" registry */
	protected $lastRelayError = self::ERR_NONE;

	/** @var int Callback stack depth for getWithSetCallback() */
	private $callbackDepth = 0;
	/** @var mixed[] Temporary warm-up cache */
	private $warmupCache = [];
	/** @var int Key fetched */
	private $warmupKeyMisses = 0;

	/** Max time expected to pass between delete() and DB commit finishing */
	const MAX_COMMIT_DELAY = 3;
	/** Max replication+snapshot lag before applying TTL_LAGGED or disallowing set() */
	const MAX_READ_LAG = 7;
	/** Seconds to tombstone keys on delete() */
	const HOLDOFF_TTL = 11; // MAX_COMMIT_DELAY + MAX_READ_LAG + 1

	/** Seconds to keep dependency purge keys around */
	const CHECK_KEY_TTL = self::TTL_YEAR;
	/** Seconds to keep lock keys around */
	const LOCK_TTL = 10;
	/** Default remaining TTL at which to consider pre-emptive regeneration */
	const LOW_TTL = 30;
	/** Default time-since-expiry on a miss that makes a key "hot" */
	const LOCK_TSE = 1;

	/** Never consider performing "popularity" refreshes until a key reaches this age */
	const AGE_NEW = 60;
	/** The time length of the "popularity" refresh window for hot keys */
	const HOT_TTR = 900;
	/** Hits/second for a refresh to be expected within the "popularity" window */
	const HIT_RATE_HIGH = 1;
	/** Seconds to ramp up to the "popularity" refresh chance after a key is no longer new */
	const RAMPUP_TTL = 30;

	/** Idiom for getWithSetCallback() callbacks to avoid calling set() */
	const TTL_UNCACHEABLE = -1;
	/** Idiom for getWithSetCallback() callbacks to 'lockTSE' logic */
	const TSE_NONE = -1;
	/** Max TTL to store keys when a data sourced is lagged */
	const TTL_LAGGED = 30;
	/** Idiom for delete() for "no hold-off" */
	const HOLDOFF_NONE = 0;
	/** Idiom for getWithSetCallback() for "no minimum required as-of timestamp" */
	const MIN_TIMESTAMP_NONE = 0.0;

	/** Tiny negative float to use when CTL comes up >= 0 due to clock skew */
	const TINY_NEGATIVE = -0.000001;

	/** Cache format version number */
	const VERSION = 1;

	const FLD_VERSION = 0; // key to cache version number
	const FLD_VALUE = 1; // key to the cached value
	const FLD_TTL = 2; // key to the original TTL
	const FLD_TIME = 3; // key to the cache time
	const FLD_FLAGS = 4; // key to the flags bitfield
	const FLD_HOLDOFF = 5; // key to any hold-off TTL

	/** @var int Treat this value as expired-on-arrival */
	const FLG_STALE = 1;

	const ERR_NONE = 0; // no error
	const ERR_NO_RESPONSE = 1; // no response
	const ERR_UNREACHABLE = 2; // can't connect
	const ERR_UNEXPECTED = 3; // response gave some error
	const ERR_RELAY = 4; // relay broadcast failed

	const VALUE_KEY_PREFIX = 'WANCache:v:';
	const INTERIM_KEY_PREFIX = 'WANCache:i:';
	const TIME_KEY_PREFIX = 'WANCache:t:';
	const MUTEX_KEY_PREFIX = 'WANCache:m:';

	const PURGE_VAL_PREFIX = 'PURGED:';

	const VFLD_DATA = 'WOC:d'; // key to the value of versioned data
	const VFLD_VERSION = 'WOC:v'; // key to the version of the value present

	const PC_PRIMARY = 'primary:1000'; // process cache name and max key count

	const DEFAULT_PURGE_CHANNEL = 'wancache-purge';

	/**
	 * @param array $params
	 *   - cache    : BagOStuff object for a persistent cache
	 *   - channels : Map of (action => channel string). Actions include "purge".
	 *   - relayers : Map of (action => EventRelayer object). Actions include "purge".
	 *   - logger   : LoggerInterface object
	 */
	public function __construct( array $params ) {
		$this->cache = $params['cache'];
		$this->purgeChannel = isset( $params['channels']['purge'] )
			? $params['channels']['purge']
			: self::DEFAULT_PURGE_CHANNEL;
		$this->purgeRelayer = isset( $params['relayers']['purge'] )
			? $params['relayers']['purge']
			: new EventRelayerNull( [] );
		$this->setLogger( isset( $params['logger'] ) ? $params['logger'] : new NullLogger() );
	}

	public function setLogger( LoggerInterface $logger ) {
		$this->logger = $logger;
	}

	/**
	 * Get an instance that wraps EmptyBagOStuff
	 *
	 * @return WANObjectCache
	 */
	public static function newEmpty() {
		return new self( [
			'cache'   => new EmptyBagOStuff(),
			'pool'    => 'empty'
		] );
	}

	/**
	 * Fetch the value of a key from cache
	 *
	 * If supplied, $curTTL is set to the remaining TTL (current time left):
	 *   - a) INF; if $key exists, has no TTL, and is not expired by $checkKeys
	 *   - b) float (>=0); if $key exists, has a TTL, and is not expired by $checkKeys
	 *   - c) float (<0); if $key is tombstoned, stale, or existing but expired by $checkKeys
	 *   - d) null; if $key does not exist and is not tombstoned
	 *
	 * If a key is tombstoned, $curTTL will reflect the time since delete().
	 *
	 * The timestamp of $key will be checked against the last-purge timestamp
	 * of each of $checkKeys. Those $checkKeys not in cache will have the last-purge
	 * initialized to the current timestamp. If any of $checkKeys have a timestamp
	 * greater than that of $key, then $curTTL will reflect how long ago $key
	 * became invalid. Callers can use $curTTL to know when the value is stale.
	 * The $checkKeys parameter allow mass invalidations by updating a single key:
	 *   - a) Each "check" key represents "last purged" of some source data
	 *   - b) Callers pass in relevant "check" keys as $checkKeys in get()
	 *   - c) When the source data that "check" keys represent changes,
	 *        the touchCheckKey() method is called on them
	 *
	 * Source data entities might exists in a DB that uses snapshot isolation
	 * (e.g. the default REPEATABLE-READ in innoDB). Even for mutable data, that
	 * isolation can largely be maintained by doing the following:
	 *   - a) Calling delete() on entity change *and* creation, before DB commit
	 *   - b) Keeping transaction duration shorter than delete() hold-off TTL
	 *
	 * However, pre-snapshot values might still be seen if an update was made
	 * in a remote datacenter but the purge from delete() didn't relay yet.
	 *
	 * Consider using getWithSetCallback() instead of get() and set() cycles.
	 * That method has cache slam avoiding features for hot/expensive keys.
	 *
	 * @param string $key Cache key
	 * @param mixed &$curTTL Approximate TTL left on the key if present/tombstoned [returned]
	 * @param array $checkKeys List of "check" keys
	 * @param float &$asOf UNIX timestamp of cached value; null on failure [returned]
	 * @return mixed Value of cache key or false on failure
	 */
	final public function get( $key, &$curTTL = null, array $checkKeys = [], &$asOf = null ) {
		$curTTLs = [];
		$asOfs = [];
		$values = $this->getMulti( [ $key ], $curTTLs, $checkKeys, $asOfs );
		$curTTL = isset( $curTTLs[$key] ) ? $curTTLs[$key] : null;
		$asOf = isset( $asOfs[$key] ) ? $asOfs[$key] : null;

		return isset( $values[$key] ) ? $values[$key] : false;
	}

	/**
	 * Fetch the value of several keys from cache
	 *
	 * @see WANObjectCache::get()
	 *
	 * @param array $keys List of cache keys
	 * @param array &$curTTLs Map of (key => approximate TTL left) for existing keys [returned]
	 * @param array $checkKeys List of check keys to apply to all $keys. May also apply "check"
	 *  keys to specific cache keys only by using cache keys as keys in the $checkKeys array.
	 * @param float[] &$asOfs Map of (key =>  UNIX timestamp of cached value; null on failure)
	 * @return array Map of (key => value) for keys that exist and are not tombstoned
	 */
	final public function getMulti(
		array $keys, &$curTTLs = [], array $checkKeys = [], array &$asOfs = []
	) {
		$result = [];
		$curTTLs = [];
		$asOfs = [];

		$vPrefixLen = strlen( self::VALUE_KEY_PREFIX );
		$valueKeys = self::prefixCacheKeys( $keys, self::VALUE_KEY_PREFIX );

		$checkKeysForAll = [];
		$checkKeysByKey = [];
		$checkKeysFlat = [];
		foreach ( $checkKeys as $i => $checkKeyGroup ) {
			$prefixed = self::prefixCacheKeys( (array)$checkKeyGroup, self::TIME_KEY_PREFIX );
			$checkKeysFlat = array_merge( $checkKeysFlat, $prefixed );
			// Is this check keys for a specific cache key, or for all keys being fetched?
			if ( is_int( $i ) ) {
				$checkKeysForAll = array_merge( $checkKeysForAll, $prefixed );
			} else {
				$checkKeysByKey[$i] = isset( $checkKeysByKey[$i] )
					? array_merge( $checkKeysByKey[$i], $prefixed )
					: $prefixed;
			}
		}

		// Fetch all of the raw values
		$keysGet = array_merge( $valueKeys, $checkKeysFlat );
		if ( $this->warmupCache ) {
			$wrappedValues = array_intersect_key( $this->warmupCache, array_flip( $keysGet ) );
			$keysGet = array_diff( $keysGet, array_keys( $wrappedValues ) ); // keys left to fetch
			$this->warmupKeyMisses += count( $keysGet );
		} else {
			$wrappedValues = [];
		}
		if ( $keysGet ) {
			$wrappedValues += $this->cache->getMulti( $keysGet );
		}
		// Time used to compare/init "check" keys (derived after getMulti() to be pessimistic)
		$now = microtime( true );

		// Collect timestamps from all "check" keys
		$purgeValuesForAll = $this->processCheckKeys( $checkKeysForAll, $wrappedValues, $now );
		$purgeValuesByKey = [];
		foreach ( $checkKeysByKey as $cacheKey => $checks ) {
			$purgeValuesByKey[$cacheKey] =
				$this->processCheckKeys( $checks, $wrappedValues, $now );
		}

		// Get the main cache value for each key and validate them
		foreach ( $valueKeys as $vKey ) {
			if ( !isset( $wrappedValues[$vKey] ) ) {
				continue; // not found
			}

			$key = substr( $vKey, $vPrefixLen ); // unprefix

			list( $value, $curTTL ) = $this->unwrap( $wrappedValues[$vKey], $now );
			if ( $value !== false ) {
				$result[$key] = $value;

				// Force dependant keys to be invalid for a while after purging
				// to reduce race conditions involving stale data getting cached
				$purgeValues = $purgeValuesForAll;
				if ( isset( $purgeValuesByKey[$key] ) ) {
					$purgeValues = array_merge( $purgeValues, $purgeValuesByKey[$key] );
				}
				foreach ( $purgeValues as $purge ) {
					$safeTimestamp = $purge[self::FLD_TIME] + $purge[self::FLD_HOLDOFF];
					if ( $safeTimestamp >= $wrappedValues[$vKey][self::FLD_TIME] ) {
						// How long ago this value was expired by *this* check key
						$ago = min( $purge[self::FLD_TIME] - $now, self::TINY_NEGATIVE );
						// How long ago this value was expired by *any* known check key
						$curTTL = min( $curTTL, $ago );
					}
				}
			}
			$curTTLs[$key] = $curTTL;
			$asOfs[$key] = ( $value !== false ) ? $wrappedValues[$vKey][self::FLD_TIME] : null;
		}

		return $result;
	}

	/**
	 * @since 1.27
	 * @param array $timeKeys List of prefixed time check keys
	 * @param array $wrappedValues
	 * @param float $now
	 * @return array List of purge value arrays
	 */
	private function processCheckKeys( array $timeKeys, array $wrappedValues, $now ) {
		$purgeValues = [];
		foreach ( $timeKeys as $timeKey ) {
			$purge = isset( $wrappedValues[$timeKey] )
				? self::parsePurgeValue( $wrappedValues[$timeKey] )
				: false;
			if ( $purge === false ) {
				// Key is not set or invalid; regenerate
				$newVal = $this->makePurgeValue( $now, self::HOLDOFF_TTL );
				$this->cache->add( $timeKey, $newVal, self::CHECK_KEY_TTL );
				$purge = self::parsePurgeValue( $newVal );
			}
			$purgeValues[] = $purge;
		}
		return $purgeValues;
	}

	/**
	 * Set the value of a key in cache
	 *
	 * Simply calling this method when source data changes is not valid because
	 * the changes do not replicate to the other WAN sites. In that case, delete()
	 * should be used instead. This method is intended for use on cache misses.
	 *
	 * If the data was read from a snapshot-isolated transactions (e.g. the default
	 * REPEATABLE-READ in innoDB), use 'since' to avoid the following race condition:
	 *   - a) T1 starts
	 *   - b) T2 updates a row, calls delete(), and commits
	 *   - c) The HOLDOFF_TTL passes, expiring the delete() tombstone
	 *   - d) T1 reads the row and calls set() due to a cache miss
	 *   - e) Stale value is stuck in cache
	 *
	 * Setting 'lag' and 'since' help avoids keys getting stuck in stale states.
	 *
	 * Example usage:
	 * @code
	 *     $dbr = wfGetDB( DB_REPLICA );
	 *     $setOpts = Database::getCacheSetOptions( $dbr );
	 *     // Fetch the row from the DB
	 *     $row = $dbr->selectRow( ... );
	 *     $key = $cache->makeKey( 'building', $buildingId );
	 *     $cache->set( $key, $row, $cache::TTL_DAY, $setOpts );
	 * @endcode
	 *
	 * @param string $key Cache key
	 * @param mixed $value
	 * @param int $ttl Seconds to live. Special values are:
	 *   - WANObjectCache::TTL_INDEFINITE: Cache forever
	 * @param array $opts Options map:
	 *   - lag : Seconds of replica DB lag. Typically, this is either the replica DB lag
	 *      before the data was read or, if applicable, the replica DB lag before
	 *      the snapshot-isolated transaction the data was read from started.
	 *      Use false to indicate that replication is not running.
	 *      Default: 0 seconds
	 *   - since : UNIX timestamp of the data in $value. Typically, this is either
	 *      the current time the data was read or (if applicable) the time when
	 *      the snapshot-isolated transaction the data was read from started.
	 *      Default: 0 seconds
	 *   - pending : Whether this data is possibly from an uncommitted write transaction.
	 *      Generally, other threads should not see values from the future and
	 *      they certainly should not see ones that ended up getting rolled back.
	 *      Default: false
	 *   - lockTSE : if excessive replication/snapshot lag is detected, then store the value
	 *      with this TTL and flag it as stale. This is only useful if the reads for
	 *      this key use getWithSetCallback() with "lockTSE" set.
	 *      Default: WANObjectCache::TSE_NONE
	 *   - staleTTL : Seconds to keep the key around if it is stale. The get()/getMulti()
	 *      methods return such stale values with a $curTTL of 0, and getWithSetCallback()
	 *      will call the regeneration callback in such cases, passing in the old value
	 *      and its as-of time to the callback. This is useful if adaptiveTTL() is used
	 *      on the old value's as-of time when it is verified as still being correct.
	 *      Default: 0.
	 * @note Options added in 1.28: staleTTL
	 * @return bool Success
	 */
	final public function set( $key, $value, $ttl = 0, array $opts = [] ) {
		$now = microtime( true );
		$lockTSE = isset( $opts['lockTSE'] ) ? $opts['lockTSE'] : self::TSE_NONE;
		$age = isset( $opts['since'] ) ? max( 0, $now - $opts['since'] ) : 0;
		$lag = isset( $opts['lag'] ) ? $opts['lag'] : 0;
		$staleTTL = isset( $opts['staleTTL'] ) ? $opts['staleTTL'] : 0;

		// Do not cache potentially uncommitted data as it might get rolled back
		if ( !empty( $opts['pending'] ) ) {
			$this->logger->info( "Rejected set() for $key due to pending writes." );

			return true; // no-op the write for being unsafe
		}

		$wrapExtra = []; // additional wrapped value fields
		// Check if there's a risk of writing stale data after the purge tombstone expired
		if ( $lag === false || ( $lag + $age ) > self::MAX_READ_LAG ) {
			// Case A: read lag with "lockTSE"; save but record value as stale
			if ( $lockTSE >= 0 ) {
				$ttl = max( 1, (int)$lockTSE ); // set() expects seconds
				$wrapExtra[self::FLD_FLAGS] = self::FLG_STALE; // mark as stale
			// Case B: any long-running transaction; ignore this set()
			} elseif ( $age > self::MAX_READ_LAG ) {
				$this->logger->info( "Rejected set() for $key due to snapshot lag." );

				return true; // no-op the write for being unsafe
			// Case C: high replication lag; lower TTL instead of ignoring all set()s
			} elseif ( $lag === false || $lag > self::MAX_READ_LAG ) {
				$ttl = $ttl ? min( $ttl, self::TTL_LAGGED ) : self::TTL_LAGGED;
				$this->logger->warning( "Lowered set() TTL for $key due to replication lag." );
			// Case D: medium length request with medium replication lag; ignore this set()
			} else {
				$this->logger->info( "Rejected set() for $key due to high read lag." );

				return true; // no-op the write for being unsafe
			}
		}

		// Wrap that value with time/TTL/version metadata
		$wrapped = $this->wrap( $value, $ttl, $now ) + $wrapExtra;

		$func = function ( $cache, $key, $cWrapped ) use ( $wrapped ) {
			return ( is_string( $cWrapped ) )
				? false // key is tombstoned; do nothing
				: $wrapped;
		};

		return $this->cache->merge( self::VALUE_KEY_PREFIX . $key, $func, $ttl + $staleTTL, 1 );
	}

	/**
	 * Purge a key from all datacenters
	 *
	 * This should only be called when the underlying data (being cached)
	 * changes in a significant way. This deletes the key and starts a hold-off
	 * period where the key cannot be written to for a few seconds (HOLDOFF_TTL).
	 * This is done to avoid the following race condition:
	 *   - a) Some DB data changes and delete() is called on a corresponding key
	 *   - b) A request refills the key with a stale value from a lagged DB
	 *   - c) The stale value is stuck there until the key is expired/evicted
	 *
	 * This is implemented by storing a special "tombstone" value at the cache
	 * key that this class recognizes; get() calls will return false for the key
	 * and any set() calls will refuse to replace tombstone values at the key.
	 * For this to always avoid stale value writes, the following must hold:
	 *   - a) Replication lag is bounded to being less than HOLDOFF_TTL; or
	 *   - b) If lag is higher, the DB will have gone into read-only mode already
	 *
	 * Note that set() can also be lag-aware and lower the TTL if it's high.
	 *
	 * When using potentially long-running ACID transactions, a good pattern is
	 * to use a pre-commit hook to issue the delete. This means that immediately
	 * after commit, callers will see the tombstone in cache upon purge relay.
	 * It also avoids the following race condition:
	 *   - a) T1 begins, changes a row, and calls delete()
	 *   - b) The HOLDOFF_TTL passes, expiring the delete() tombstone
	 *   - c) T2 starts, reads the row and calls set() due to a cache miss
	 *   - d) T1 finally commits
	 *   - e) Stale value is stuck in cache
	 *
	 * Example usage:
	 * @code
	 *     $dbw->startAtomic( __METHOD__ ); // start of request
	 *     ... <execute some stuff> ...
	 *     // Update the row in the DB
	 *     $dbw->update( ... );
	 *     $key = $cache->makeKey( 'homes', $homeId );
	 *     // Purge the corresponding cache entry just before committing
	 *     $dbw->onTransactionPreCommitOrIdle( function() use ( $cache, $key ) {
	 *         $cache->delete( $key );
	 *     } );
	 *     ... <execute some stuff> ...
	 *     $dbw->endAtomic( __METHOD__ ); // end of request
	 * @endcode
	 *
	 * The $ttl parameter can be used when purging values that have not actually changed
	 * recently. For example, a cleanup script to purge cache entries does not really need
	 * a hold-off period, so it can use HOLDOFF_NONE. Likewise for user-requested purge.
	 * Note that $ttl limits the effective range of 'lockTSE' for getWithSetCallback().
	 *
	 * If called twice on the same key, then the last hold-off TTL takes precedence. For
	 * idempotence, the $ttl should not vary for different delete() calls on the same key.
	 *
	 * @param string $key Cache key
	 * @param int $ttl Tombstone TTL; Default: WANObjectCache::HOLDOFF_TTL
	 * @return bool True if the item was purged or not found, false on failure
	 */
	final public function delete( $key, $ttl = self::HOLDOFF_TTL ) {
		$key = self::VALUE_KEY_PREFIX . $key;

		if ( $ttl <= 0 ) {
			// Publish the purge to all datacenters
			$ok = $this->relayDelete( $key );
		} else {
			// Publish the purge to all datacenters
			$ok = $this->relayPurge( $key, $ttl, self::HOLDOFF_NONE );
		}

		return $ok;
	}

	/**
	 * Fetch the value of a timestamp "check" key
	 *
	 * The key will be *initialized* to the current time if not set,
	 * so only call this method if this behavior is actually desired
	 *
	 * The timestamp can be used to check whether a cached value is valid.
	 * Callers should not assume that this returns the same timestamp in
	 * all datacenters due to relay delays.
	 *
	 * The level of staleness can roughly be estimated from this key, but
	 * if the key was evicted from cache, such calculations may show the
	 * time since expiry as ~0 seconds.
	 *
	 * Note that "check" keys won't collide with other regular keys.
	 *
	 * @param string $key
	 * @return float UNIX timestamp of the check key
	 */
	final public function getCheckKeyTime( $key ) {
		$key = self::TIME_KEY_PREFIX . $key;

		$purge = self::parsePurgeValue( $this->cache->get( $key ) );
		if ( $purge !== false ) {
			$time = $purge[self::FLD_TIME];
		} else {
			// Casting assures identical floats for the next getCheckKeyTime() calls
			$now = (string)microtime( true );
			$this->cache->add( $key,
				$this->makePurgeValue( $now, self::HOLDOFF_TTL ),
				self::CHECK_KEY_TTL
			);
			$time = (float)$now;
		}

		return $time;
	}

	/**
	 * Purge a "check" key from all datacenters, invalidating keys that use it
	 *
	 * This should only be called when the underlying data (being cached)
	 * changes in a significant way, and it is impractical to call delete()
	 * on all keys that should be changed. When get() is called on those
	 * keys, the relevant "check" keys must be supplied for this to work.
	 *
	 * The "check" key essentially represents a last-modified field.
	 * When touched, the field will be updated on all cache servers.
	 * Keys using it via get(), getMulti(), or getWithSetCallback() will
	 * be invalidated. It is treated as being HOLDOFF_TTL seconds in the future
	 * by those methods to avoid race conditions where dependent keys get updated
	 * with stale values (e.g. from a DB replica DB).
	 *
	 * This is typically useful for keys with hardcoded names or in some cases
	 * dynamically generated names where a low number of combinations exist.
	 * When a few important keys get a large number of hits, a high cache
	 * time is usually desired as well as "lockTSE" logic. The resetCheckKey()
	 * method is less appropriate in such cases since the "time since expiry"
	 * cannot be inferred, causing any get() after the reset to treat the key
	 * as being "hot", resulting in more stale value usage.
	 *
	 * Note that "check" keys won't collide with other regular keys.
	 *
	 * @see WANObjectCache::get()
	 * @see WANObjectCache::getWithSetCallback()
	 * @see WANObjectCache::resetCheckKey()
	 *
	 * @param string $key Cache key
	 * @param int $holdoff HOLDOFF_TTL or HOLDOFF_NONE constant
	 * @return bool True if the item was purged or not found, false on failure
	 */
	final public function touchCheckKey( $key, $holdoff = self::HOLDOFF_TTL ) {
		// Publish the purge to all datacenters
		return $this->relayPurge( self::TIME_KEY_PREFIX . $key, self::CHECK_KEY_TTL, $holdoff );
	}

	/**
	 * Delete a "check" key from all datacenters, invalidating keys that use it
	 *
	 * This is similar to touchCheckKey() in that keys using it via get(), getMulti(),
	 * or getWithSetCallback() will be invalidated. The differences are:
	 *   - a) The "check" key will be deleted from all caches and lazily
	 *        re-initialized when accessed (rather than set everywhere)
	 *   - b) Thus, dependent keys will be known to be invalid, but not
	 *        for how long (they are treated as "just" purged), which
	 *        effects any lockTSE logic in getWithSetCallback()
	 *   - c) Since "check" keys are initialized only on the server the key hashes
	 *        to, any temporary ejection of that server will cause the value to be
	 *        seen as purged as a new server will initialize the "check" key.
	 *
	 * The advantage is that this does not place high TTL keys on every cache
	 * server, making it better for code that will cache many different keys
	 * and either does not use lockTSE or uses a low enough TTL anyway.
	 *
	 * This is typically useful for keys with dynamically generated names
	 * where a high number of combinations exist.
	 *
	 * Note that "check" keys won't collide with other regular keys.
	 *
	 * @see WANObjectCache::get()
	 * @see WANObjectCache::getWithSetCallback()
	 * @see WANObjectCache::touchCheckKey()
	 *
	 * @param string $key Cache key
	 * @return bool True if the item was purged or not found, false on failure
	 */
	final public function resetCheckKey( $key ) {
		// Publish the purge to all datacenters
		return $this->relayDelete( self::TIME_KEY_PREFIX . $key );
	}

	/**
	 * Method to fetch/regenerate cache keys
	 *
	 * On cache miss, the key will be set to the callback result via set()
	 * (unless the callback returns false) and that result will be returned.
	 * The arguments supplied to the callback are:
	 *   - $oldValue : current cache value or false if not present
	 *   - &$ttl : a reference to the TTL which can be altered
	 *   - &$setOpts : a reference to options for set() which can be altered
	 *   - $oldAsOf : generation UNIX timestamp of $oldValue or null if not present (since 1.28)
	 *
	 * It is strongly recommended to set the 'lag' and 'since' fields to avoid race conditions
	 * that can cause stale values to get stuck at keys. Usually, callbacks ignore the current
	 * value, but it can be used to maintain "most recent X" values that come from time or
	 * sequence based source data, provided that the "as of" id/time is tracked. Note that
	 * preemptive regeneration and $checkKeys can result in a non-false current value.
	 *
	 * Usage of $checkKeys is similar to get() and getMulti(). However, rather than the caller
	 * having to inspect a "current time left" variable (e.g. $curTTL, $curTTLs), a cache
	 * regeneration will automatically be triggered using the callback.
	 *
	 * The simplest way to avoid stampedes for hot keys is to use
	 * the 'lockTSE' option in $opts. If cache purges are needed, also:
	 *   - a) Pass $key into $checkKeys
	 *   - b) Use touchCheckKey( $key ) instead of delete( $key )
	 *
	 * Example usage (typical key):
	 * @code
	 *     $catInfo = $cache->getWithSetCallback(
	 *         // Key to store the cached value under
	 *         $cache->makeKey( 'cat-attributes', $catId ),
	 *         // Time-to-live (in seconds)
	 *         $cache::TTL_MINUTE,
	 *         // Function that derives the new key value
	 *         function ( $oldValue, &$ttl, array &$setOpts ) {
	 *             $dbr = wfGetDB( DB_REPLICA );
	 *             // Account for any snapshot/replica DB lag
	 *             $setOpts += Database::getCacheSetOptions( $dbr );
	 *
	 *             return $dbr->selectRow( ... );
	 *        }
	 *     );
	 * @endcode
	 *
	 * Example usage (key that is expensive and hot):
	 * @code
	 *     $catConfig = $cache->getWithSetCallback(
	 *         // Key to store the cached value under
	 *         $cache->makeKey( 'site-cat-config' ),
	 *         // Time-to-live (in seconds)
	 *         $cache::TTL_DAY,
	 *         // Function that derives the new key value
	 *         function ( $oldValue, &$ttl, array &$setOpts ) {
	 *             $dbr = wfGetDB( DB_REPLICA );
	 *             // Account for any snapshot/replica DB lag
	 *             $setOpts += Database::getCacheSetOptions( $dbr );
	 *
	 *             return CatConfig::newFromRow( $dbr->selectRow( ... ) );
	 *         },
	 *         [
	 *             // Calling touchCheckKey() on this key invalidates the cache
	 *             'checkKeys' => [ $cache->makeKey( 'site-cat-config' ) ],
	 *             // Try to only let one datacenter thread manage cache updates at a time
	 *             'lockTSE' => 30,
	 *             // Avoid querying cache servers multiple times in a web request
	 *             'pcTTL' => $cache::TTL_PROC_LONG
	 *         ]
	 *     );
	 * @endcode
	 *
	 * Example usage (key with dynamic dependencies):
	 * @code
	 *     $catState = $cache->getWithSetCallback(
	 *         // Key to store the cached value under
	 *         $cache->makeKey( 'cat-state', $cat->getId() ),
	 *         // Time-to-live (seconds)
	 *         $cache::TTL_HOUR,
	 *         // Function that derives the new key value
	 *         function ( $oldValue, &$ttl, array &$setOpts ) {
	 *             // Determine new value from the DB
	 *             $dbr = wfGetDB( DB_REPLICA );
	 *             // Account for any snapshot/replica DB lag
	 *             $setOpts += Database::getCacheSetOptions( $dbr );
	 *
	 *             return CatState::newFromResults( $dbr->select( ... ) );
	 *         },
	 *         [
	 *              // The "check" keys that represent things the value depends on;
	 *              // Calling touchCheckKey() on any of them invalidates the cache
	 *             'checkKeys' => [
	 *                 $cache->makeKey( 'sustenance-bowls', $cat->getRoomId() ),
	 *                 $cache->makeKey( 'people-present', $cat->getHouseId() ),
	 *                 $cache->makeKey( 'cat-laws', $cat->getCityId() ),
	 *             ]
	 *         ]
	 *     );
	 * @endcode
	 *
	 * Example usage (hot key holding most recent 100 events):
	 * @code
	 *     $lastCatActions = $cache->getWithSetCallback(
	 *         // Key to store the cached value under
	 *         $cache->makeKey( 'cat-last-actions', 100 ),
	 *         // Time-to-live (in seconds)
	 *         10,
	 *         // Function that derives the new key value
	 *         function ( $oldValue, &$ttl, array &$setOpts ) {
	 *             $dbr = wfGetDB( DB_REPLICA );
	 *             // Account for any snapshot/replica DB lag
	 *             $setOpts += Database::getCacheSetOptions( $dbr );
	 *
	 *             // Start off with the last cached list
	 *             $list = $oldValue ?: [];
	 *             // Fetch the last 100 relevant rows in descending order;
	 *             // only fetch rows newer than $list[0] to reduce scanning
	 *             $rows = iterator_to_array( $dbr->select( ... ) );
	 *             // Merge them and get the new "last 100" rows
	 *             return array_slice( array_merge( $new, $list ), 0, 100 );
	 *        },
	 *        [
	 *             // Try to only let one datacenter thread manage cache updates at a time
	 *             'lockTSE' => 30,
	 *             // Use a magic value when no cache value is ready rather than stampeding
	 *             'busyValue' => 'computing'
	 *        ]
	 *     );
	 * @endcode
	 *
	 * @see WANObjectCache::get()
	 * @see WANObjectCache::set()
	 *
	 * @param string $key Cache key
	 * @param int $ttl Seconds to live for key updates. Special values are:
	 *   - WANObjectCache::TTL_INDEFINITE: Cache forever
	 *   - WANObjectCache::TTL_UNCACHEABLE: Do not cache at all
	 * @param callable $callback Value generation function
	 * @param array $opts Options map:
	 *   - checkKeys: List of "check" keys. The key at $key will be seen as invalid when either
	 *      touchCheckKey() or resetCheckKey() is called on any of these keys.
	 *      Default: [].
	 *   - lockTSE: If the key is tombstoned or expired (by checkKeys) less than this many seconds
	 *      ago, then try to have a single thread handle cache regeneration at any given time.
	 *      Other threads will try to use stale values if possible. If, on miss, the time since
	 *      expiration is low, the assumption is that the key is hot and that a stampede is worth
	 *      avoiding. Setting this above WANObjectCache::HOLDOFF_TTL makes no difference. The
	 *      higher this is set, the higher the worst-case staleness can be.
	 *      Use WANObjectCache::TSE_NONE to disable this logic.
	 *      Default: WANObjectCache::TSE_NONE.
	 *   - busyValue: If no value exists and another thread is currently regenerating it, use this
	 *      as a fallback value (or a callback to generate such a value). This assures that cache
	 *      stampedes cannot happen if the value falls out of cache. This can be used as insurance
	 *      against cache regeneration becoming very slow for some reason (greater than the TTL).
	 *      Default: null.
	 *   - pcTTL: Process cache the value in this PHP instance for this many seconds. This avoids
	 *      network I/O when a key is read several times. This will not cache when the callback
	 *      returns false, however. Note that any purges will not be seen while process cached;
	 *      since the callback should use replica DBs and they may be lagged or have snapshot
	 *      isolation anyway, this should not typically matter.
	 *      Default: WANObjectCache::TTL_UNCACHEABLE.
	 *   - pcGroup: Process cache group to use instead of the primary one. If set, this must be
	 *      of the format ALPHANUMERIC_NAME:MAX_KEY_SIZE, e.g. "mydata:10". Use this for storing
	 *      large values, small yet numerous values, or some values with a high cost of eviction.
	 *      It is generally preferable to use a class constant when setting this value.
	 *      This has no effect unless pcTTL is used.
	 *      Default: WANObjectCache::PC_PRIMARY.
	 *   - version: Integer version number. This allows for callers to make breaking changes to
	 *      how values are stored while maintaining compatability and correct cache purges. New
	 *      versions are stored alongside older versions concurrently. Avoid storing class objects
	 *      however, as this reduces compatibility (due to serialization).
	 *      Default: null.
	 *   - minAsOf: Reject values if they were generated before this UNIX timestamp.
	 *      This is useful if the source of a key is suspected of having possibly changed
	 *      recently, and the caller wants any such changes to be reflected.
	 *      Default: WANObjectCache::MIN_TIMESTAMP_NONE.
	 *   - hotTTR: Expected time-till-refresh for keys that average ~1 hit/second.
	 *      This should be greater than "ageNew". Keys with higher hit rates will regenerate
	 *      more often. This is useful when a popular key is changed but the cache purge was
	 *      delayed or lost. Seldom used keys are rarely affected by this setting, unless an
	 *      extremely low "hotTTR" value is passed in.
	 *      Default: WANObjectCache::HOT_TTR.
	 *   - lowTTL: Consider pre-emptive updates when the current TTL (seconds) of the key is less
	 *      than this. It becomes more likely over time, becoming certain once the key is expired.
	 *      Default: WANObjectCache::LOW_TTL.
	 *   - ageNew: Consider popularity refreshes only once a key reaches this age in seconds.
	 *      Default: WANObjectCache::AGE_NEW.
	 * @return mixed Value found or written to the key
	 * @note Options added in 1.28: version, busyValue, hotTTR, ageNew, pcGroup, minAsOf
	 * @note Callable type hints are not used to avoid class-autoloading
	 */
	final public function getWithSetCallback( $key, $ttl, $callback, array $opts = [] ) {
		$pcTTL = isset( $opts['pcTTL'] ) ? $opts['pcTTL'] : self::TTL_UNCACHEABLE;

		// Try the process cache if enabled and the cache callback is not within a cache callback.
		// Process cache use in nested callbacks is not lag-safe with regard to HOLDOFF_TTL since
		// the in-memory value is further lagged than the shared one since it uses a blind TTL.
		if ( $pcTTL >= 0 && $this->callbackDepth == 0 ) {
			$group = isset( $opts['pcGroup'] ) ? $opts['pcGroup'] : self::PC_PRIMARY;
			$procCache = $this->getProcessCache( $group );
			$value = $procCache->get( $key );
		} else {
			$procCache = false;
			$value = false;
		}

		if ( $value === false ) {
			// Fetch the value over the network
			if ( isset( $opts['version'] ) ) {
				$version = $opts['version'];
				$asOf = null;
				$cur = $this->doGetWithSetCallback(
					$key,
					$ttl,
					function ( $oldValue, &$ttl, &$setOpts, $oldAsOf )
					use ( $callback, $version ) {
						if ( is_array( $oldValue )
							&& array_key_exists( self::VFLD_DATA, $oldValue )
						) {
							$oldData = $oldValue[self::VFLD_DATA];
						} else {
							// VFLD_DATA is not set if an old, unversioned, key is present
							$oldData = false;
						}

						return [
							self::VFLD_DATA => $callback( $oldData, $ttl, $setOpts, $oldAsOf ),
							self::VFLD_VERSION => $version
						];
					},
					$opts,
					$asOf
				);
				if ( $cur[self::VFLD_VERSION] === $version ) {
					// Value created or existed before with version; use it
					$value = $cur[self::VFLD_DATA];
				} else {
					// Value existed before with a different version; use variant key.
					// Reflect purges to $key by requiring that this key value be newer.
					$value = $this->doGetWithSetCallback(
						'cache-variant:' . md5( $key ) . ":$version",
						$ttl,
						$callback,
						// Regenerate value if not newer than $key
						[ 'version' => null, 'minAsOf' => $asOf ] + $opts
					);
				}
			} else {
				$value = $this->doGetWithSetCallback( $key, $ttl, $callback, $opts );
			}

			// Update the process cache if enabled
			if ( $procCache && $value !== false ) {
				$procCache->set( $key, $value, $pcTTL );
			}
		}

		return $value;
	}

	/**
	 * Do the actual I/O for getWithSetCallback() when needed
	 *
	 * @see WANObjectCache::getWithSetCallback()
	 *
	 * @param string $key
	 * @param int $ttl
	 * @param callback $callback
	 * @param array $opts Options map for getWithSetCallback()
	 * @param float &$asOf Cache generation timestamp of returned value [returned]
	 * @return mixed
	 * @note Callable type hints are not used to avoid class-autoloading
	 */
	protected function doGetWithSetCallback( $key, $ttl, $callback, array $opts, &$asOf = null ) {
		$lowTTL = isset( $opts['lowTTL'] ) ? $opts['lowTTL'] : min( self::LOW_TTL, $ttl );
		$lockTSE = isset( $opts['lockTSE'] ) ? $opts['lockTSE'] : self::TSE_NONE;
		$checkKeys = isset( $opts['checkKeys'] ) ? $opts['checkKeys'] : [];
		$busyValue = isset( $opts['busyValue'] ) ? $opts['busyValue'] : null;
		$popWindow = isset( $opts['hotTTR'] ) ? $opts['hotTTR'] : self::HOT_TTR;
		$ageNew = isset( $opts['ageNew'] ) ? $opts['ageNew'] : self::AGE_NEW;
		$minTime = isset( $opts['minAsOf'] ) ? $opts['minAsOf'] : self::MIN_TIMESTAMP_NONE;
		$versioned = isset( $opts['version'] );

		// Get the current key value
		$curTTL = null;
		$cValue = $this->get( $key, $curTTL, $checkKeys, $asOf ); // current value
		$value = $cValue; // return value

		$preCallbackTime = microtime( true );
		// Determine if a cached value regeneration is needed or desired
		if ( $value !== false
			&& $curTTL > 0
			&& $this->isValid( $value, $versioned, $asOf, $minTime )
			&& !$this->worthRefreshExpiring( $curTTL, $lowTTL )
			&& !$this->worthRefreshPopular( $asOf, $ageNew, $popWindow, $preCallbackTime )
		) {
			return $value;
		}

		// A deleted key with a negative TTL left must be tombstoned
		$isTombstone = ( $curTTL !== null && $value === false );
		// Assume a key is hot if requested soon after invalidation
		$isHot = ( $curTTL !== null && $curTTL <= 0 && abs( $curTTL ) <= $lockTSE );
		// Use the mutex if there is no value and a busy fallback is given
		$checkBusy = ( $busyValue !== null && $value === false );
		// Decide whether a single thread should handle regenerations.
		// This avoids stampedes when $checkKeys are bumped and when preemptive
		// renegerations take too long. It also reduces regenerations while $key
		// is tombstoned. This balances cache freshness with avoiding DB load.
		$useMutex = ( $isHot || ( $isTombstone && $lockTSE > 0 ) || $checkBusy );

		$lockAcquired = false;
		if ( $useMutex ) {
			// Acquire a datacenter-local non-blocking lock
			if ( $this->cache->add( self::MUTEX_KEY_PREFIX . $key, 1, self::LOCK_TTL ) ) {
				// Lock acquired; this thread should update the key
				$lockAcquired = true;
			} elseif ( $value !== false && $this->isValid( $value, $versioned, $asOf, $minTime ) ) {
				// If it cannot be acquired; then the stale value can be used
				return $value;
			} else {
				// Use the INTERIM value for tombstoned keys to reduce regeneration load.
				// For hot keys, either another thread has the lock or the lock failed;
				// use the INTERIM value from the last thread that regenerated it.
				$value = $this->getInterimValue( $key, $versioned, $minTime, $asOf );
				if ( $value !== false ) {
					return $value;
				}
				// Use the busy fallback value if nothing else
				if ( $busyValue !== null ) {
					return is_callable( $busyValue ) ? $busyValue() : $busyValue;
				}
			}
		}

		if ( !is_callable( $callback ) ) {
			throw new InvalidArgumentException( "Invalid cache miss callback provided." );
		}

		// Generate the new value from the callback...
		$setOpts = [];
		++$this->callbackDepth;
		try {
			$value = call_user_func_array( $callback, [ $cValue, &$ttl, &$setOpts, $asOf ] );
		} finally {
			--$this->callbackDepth;
		}
		$valueIsCacheable = ( $value !== false && $ttl >= 0 );

		// When delete() is called, writes are write-holed by the tombstone,
		// so use a special INTERIM key to pass the new value around threads.
		if ( ( $isTombstone && $lockTSE > 0 ) && $valueIsCacheable ) {
			$tempTTL = max( 1, (int)$lockTSE ); // set() expects seconds
			$newAsOf = microtime( true );
			$wrapped = $this->wrap( $value, $tempTTL, $newAsOf );
			// Avoid using set() to avoid pointless mcrouter broadcasting
			$this->setInterimValue( $key, $wrapped, $tempTTL );
		}

		if ( $valueIsCacheable ) {
			$setOpts['lockTSE'] = $lockTSE;
			// Use best known "since" timestamp if not provided
			$setOpts += [ 'since' => $preCallbackTime ];
			// Update the cache; this will fail if the key is tombstoned
			$this->set( $key, $value, $ttl, $setOpts );
		}

		if ( $lockAcquired ) {
			// Avoid using delete() to avoid pointless mcrouter broadcasting
			$this->cache->changeTTL( self::MUTEX_KEY_PREFIX . $key, 1 );
		}

		return $value;
	}

	/**
	 * @param string $key
	 * @param bool $versioned
	 * @param float $minTime
	 * @param mixed $asOf
	 * @return mixed
	 */
	protected function getInterimValue( $key, $versioned, $minTime, &$asOf ) {
		$wrapped = $this->cache->get( self::INTERIM_KEY_PREFIX . $key );
		list( $value ) = $this->unwrap( $wrapped, microtime( true ) );
		if ( $value !== false && $this->isValid( $value, $versioned, $asOf, $minTime ) ) {
			$asOf = $wrapped[self::FLD_TIME];

			return $value;
		}

		return false;
	}

	/**
	 * @param string $key
	 * @param array $wrapped
	 * @param int $tempTTL
	 */
	protected function setInterimValue( $key, $wrapped, $tempTTL ) {
		$this->cache->merge(
			self::INTERIM_KEY_PREFIX . $key,
			function () use ( $wrapped ) {
				return $wrapped;
			},
			$tempTTL,
			1
		);
	}

	/**
	 * Method to fetch multiple cache keys at once with regeneration
	 *
	 * This works the same as getWithSetCallback() except:
	 *   - a) The $keys argument expects the result of WANObjectCache::makeMultiKeys()
	 *   - b) The $callback argument expects a callback taking the following arguments:
	 *         - $id: ID of an entity to query
	 *         - $oldValue : the prior cache value or false if none was present
	 *         - &$ttl : a reference to the new value TTL in seconds
	 *         - &$setOpts : a reference to options for set() which can be altered
	 *         - $oldAsOf : generation UNIX timestamp of $oldValue or null if not present
	 *        Aside from the additional $id argument, the other arguments function the same
	 *        way they do in getWithSetCallback().
	 *   - c) The return value is a map of (cache key => value) in the order of $keyedIds
	 *
	 * @see WANObjectCache::getWithSetCallback()
	 * @see WANObjectCache::getMultiWithUnionSetCallback()
	 *
	 * Example usage:
	 * @code
	 *     $rows = $cache->getMultiWithSetCallback(
	 *         // Map of cache keys to entity IDs
	 *         $cache->makeMultiKeys(
	 *             $this->fileVersionIds(),
	 *             function ( $id, WANObjectCache $cache ) {
	 *                 return $cache->makeKey( 'file-version', $id );
	 *             }
	 *         ),
	 *         // Time-to-live (in seconds)
	 *         $cache::TTL_DAY,
	 *         // Function that derives the new key value
	 *         function ( $id, $oldValue, &$ttl, array &$setOpts ) {
	 *             $dbr = wfGetDB( DB_REPLICA );
	 *             // Account for any snapshot/replica DB lag
	 *             $setOpts += Database::getCacheSetOptions( $dbr );
	 *
	 *             // Load the row for this file
	 *             $row = $dbr->selectRow( 'file', File::selectFields(), [ 'id' => $id ], __METHOD__ );
	 *
	 *             return $row ? (array)$row : false;
	 *         },
	 *         [
	 *             // Process cache for 30 seconds
	 *             'pcTTL' => 30,
	 *             // Use a dedicated 500 item cache (initialized on-the-fly)
	 *             'pcGroup' => 'file-versions:500'
	 *         ]
	 *     );
	 *     $files = array_map( [ __CLASS__, 'newFromRow' ], $rows );
	 * @endcode
	 *
	 * @param ArrayIterator $keyedIds Result of WANObjectCache::makeMultiKeys()
	 * @param int $ttl Seconds to live for key updates
	 * @param callable $callback Callback the yields entity regeneration callbacks
	 * @param array $opts Options map
	 * @return array Map of (cache key => value) in the same order as $keyedIds
	 * @since 1.28
	 */
	final public function getMultiWithSetCallback(
		ArrayIterator $keyedIds, $ttl, callable $callback, array $opts = []
	) {
		$valueKeys = array_keys( $keyedIds->getArrayCopy() );
		$checkKeys = isset( $opts['checkKeys'] ) ? $opts['checkKeys'] : [];

		// Load required keys into process cache in one go
		$this->warmupCache = $this->getRawKeysForWarmup(
			$this->getNonProcessCachedKeys( $valueKeys, $opts ),
			$checkKeys
		);
		$this->warmupKeyMisses = 0;

		// Wrap $callback to match the getWithSetCallback() format while passing $id to $callback
		$id = null; // current entity ID
		$func = function ( $oldValue, &$ttl, &$setOpts, $oldAsOf ) use ( $callback, &$id ) {
			return $callback( $id, $oldValue, $ttl, $setOpts, $oldAsOf );
		};

		$values = [];
		foreach ( $keyedIds as $key => $id ) { // preserve order
			$values[$key] = $this->getWithSetCallback( $key, $ttl, $func, $opts );
		}

		$this->warmupCache = [];

		return $values;
	}

	/**
	 * Method to fetch/regenerate multiple cache keys at once
	 *
	 * This works the same as getWithSetCallback() except:
	 *   - a) The $keys argument expects the result of WANObjectCache::makeMultiKeys()
	 *   - b) The $callback argument expects a callback returning a map of (ID => new value)
	 *        for all entity IDs in $regenById and it takes the following arguments:
	 *          - $ids: a list of entity IDs to regenerate
	 *          - &$ttls: a reference to the (entity ID => new TTL) map
	 *          - &$setOpts: a reference to options for set() which can be altered
	 *   - c) The return value is a map of (cache key => value) in the order of $keyedIds
	 *   - d) The "lockTSE" and "busyValue" options are ignored
	 *
	 * @see WANObjectCache::getWithSetCallback()
	 * @see WANObjectCache::getMultiWithSetCallback()
	 *
	 * Example usage:
	 * @code
	 *     $rows = $cache->getMultiWithUnionSetCallback(
	 *         // Map of cache keys to entity IDs
	 *         $cache->makeMultiKeys(
	 *             $this->fileVersionIds(),
	 *             function ( $id, WANObjectCache $cache ) {
	 *                 return $cache->makeKey( 'file-version', $id );
	 *             }
	 *         ),
	 *         // Time-to-live (in seconds)
	 *         $cache::TTL_DAY,
	 *         // Function that derives the new key value
	 *         function ( array $ids, array &$ttls, array &$setOpts ) {
	 *             $dbr = wfGetDB( DB_REPLICA );
	 *             // Account for any snapshot/replica DB lag
	 *             $setOpts += Database::getCacheSetOptions( $dbr );
	 *
	 *             // Load the rows for these files
	 *             $rows = [];
	 *             $res = $dbr->select( 'file', File::selectFields(), [ 'id' => $ids ], __METHOD__ );
	 *             foreach ( $res as $row ) {
	 *                 $rows[$row->id] = $row;
	 *                 $mtime = wfTimestamp( TS_UNIX, $row->timestamp );
	 *                 $ttls[$row->id] = $this->adaptiveTTL( $mtime, $ttls[$row->id] );
	 *             }
	 *
	 *             return $rows;
	 *         },
	 *         ]
	 *     );
	 *     $files = array_map( [ __CLASS__, 'newFromRow' ], $rows );
	 * @endcode
	 *
	 * @param ArrayIterator $keyedIds Result of WANObjectCache::makeMultiKeys()
	 * @param int $ttl Seconds to live for key updates
	 * @param callable $callback Callback the yields entity regeneration callbacks
	 * @param array $opts Options map
	 * @return array Map of (cache key => value) in the same order as $keyedIds
	 * @since 1.30
	 */
	final public function getMultiWithUnionSetCallback(
		ArrayIterator $keyedIds, $ttl, callable $callback, array $opts = []
	) {
		$idsByValueKey = $keyedIds->getArrayCopy();
		$valueKeys = array_keys( $idsByValueKey );
		$checkKeys = isset( $opts['checkKeys'] ) ? $opts['checkKeys'] : [];
		unset( $opts['lockTSE'] ); // incompatible
		unset( $opts['busyValue'] ); // incompatible

		// Load required keys into process cache in one go
		$keysGet = $this->getNonProcessCachedKeys( $valueKeys, $opts );
		$this->warmupCache = $this->getRawKeysForWarmup( $keysGet, $checkKeys );
		$this->warmupKeyMisses = 0;

		// IDs of entities known to be in need of regeneration
		$idsRegen = [];

		// Find out which keys are missing/deleted/stale
		$curTTLs = [];
		$asOfs = [];
		$curByKey = $this->getMulti( $keysGet, $curTTLs, $checkKeys, $asOfs );
		foreach ( $keysGet as $key ) {
			if ( !array_key_exists( $key, $curByKey ) || $curTTLs[$key] < 0 ) {
				$idsRegen[] = $idsByValueKey[$key];
			}
		}

		// Run the callback to populate the regeneration value map for all required IDs
		$newSetOpts = [];
		$newTTLsById = array_fill_keys( $idsRegen, $ttl );
		$newValsById = $idsRegen ? $callback( $idsRegen, $newTTLsById, $newSetOpts ) : [];

		// Wrap $callback to match the getWithSetCallback() format while passing $id to $callback
		$id = null; // current entity ID
		$func = function ( $oldValue, &$ttl, &$setOpts, $oldAsOf )
			use ( $callback, &$id, $newValsById, $newTTLsById, $newSetOpts )
		{
			if ( array_key_exists( $id, $newValsById ) ) {
				// Value was already regerated as expected, so use the value in $newValsById
				$newValue = $newValsById[$id];
				$ttl = $newTTLsById[$id];
				$setOpts = $newSetOpts;
			} else {
				// Pre-emptive/popularity refresh and version mismatch cases are not detected
				// above and thus $newValsById has no entry. Run $callback on this single entity.
				$ttls = [ $id => $ttl ];
				$newValue = $callback( [ $id ], $ttls, $setOpts )[$id];
				$ttl = $ttls[$id];
			}

			return $newValue;
		};

		// Run the cache-aside logic using warmupCache instead of persistent cache queries
		$values = [];
		foreach ( $idsByValueKey as $key => $id ) { // preserve order
			$values[$key] = $this->getWithSetCallback( $key, $ttl, $func, $opts );
		}

		$this->warmupCache = [];

		return $values;
	}

	/**
	 * Locally set a key to expire soon if it is stale based on $purgeTimestamp
	 *
	 * This sets stale keys' time-to-live at HOLDOFF_TTL seconds, which both avoids
	 * broadcasting in mcrouter setups and also avoids races with new tombstones.
	 *
	 * @param string $key Cache key
	 * @param int $purgeTimestamp UNIX timestamp of purge
	 * @param bool &$isStale Whether the key is stale
	 * @return bool Success
	 * @since 1.28
	 */
	public function reap( $key, $purgeTimestamp, &$isStale = false ) {
		$minAsOf = $purgeTimestamp + self::HOLDOFF_TTL;
		$wrapped = $this->cache->get( self::VALUE_KEY_PREFIX . $key );
		if ( is_array( $wrapped ) && $wrapped[self::FLD_TIME] < $minAsOf ) {
			$isStale = true;
			$this->logger->warning( "Reaping stale value key '$key'." );
			$ttlReap = self::HOLDOFF_TTL; // avoids races with tombstone creation
			$ok = $this->cache->changeTTL( self::VALUE_KEY_PREFIX . $key, $ttlReap );
			if ( !$ok ) {
				$this->logger->error( "Could not complete reap of key '$key'." );
			}

			return $ok;
		}

		$isStale = false;

		return true;
	}

	/**
	 * Locally set a "check" key to expire soon if it is stale based on $purgeTimestamp
	 *
	 * @param string $key Cache key
	 * @param int $purgeTimestamp UNIX timestamp of purge
	 * @param bool &$isStale Whether the key is stale
	 * @return bool Success
	 * @since 1.28
	 */
	public function reapCheckKey( $key, $purgeTimestamp, &$isStale = false ) {
		$purge = $this->parsePurgeValue( $this->cache->get( self::TIME_KEY_PREFIX . $key ) );
		if ( $purge && $purge[self::FLD_TIME] < $purgeTimestamp ) {
			$isStale = true;
			$this->logger->warning( "Reaping stale check key '$key'." );
			$ok = $this->cache->changeTTL( self::TIME_KEY_PREFIX . $key, 1 );
			if ( !$ok ) {
				$this->logger->error( "Could not complete reap of check key '$key'." );
			}

			return $ok;
		}

		$isStale = false;

		return false;
	}

	/**
	 * @see BagOStuff::makeKey()
	 * @param string $keys,... Key component (starting with a key collection name)
	 * @return string Colon-delimited list of $keyspace followed by escaped components of $args
	 * @since 1.27
	 */
	public function makeKey() {
		return call_user_func_array( [ $this->cache, __FUNCTION__ ], func_get_args() );
	}

	/**
	 * @see BagOStuff::makeGlobalKey()
	 * @param string $keys,... Key component (starting with a key collection name)
	 * @return string Colon-delimited list of $keyspace followed by escaped components of $args
	 * @since 1.27
	 */
	public function makeGlobalKey() {
		return call_user_func_array( [ $this->cache, __FUNCTION__ ], func_get_args() );
	}

	/**
	 * @param array $entities List of entity IDs
	 * @param callable $keyFunc Callback yielding a key from (entity ID, this WANObjectCache)
	 * @return ArrayIterator Iterator yielding (cache key => entity ID) in $entities order
	 * @since 1.28
	 */
	public function makeMultiKeys( array $entities, callable $keyFunc ) {
		$map = [];
		foreach ( $entities as $entity ) {
			$map[$keyFunc( $entity, $this )] = $entity;
		}

		return new ArrayIterator( $map );
	}

	/**
	 * Get the "last error" registered; clearLastError() should be called manually
	 * @return int ERR_* class constant for the "last error" registry
	 */
	final public function getLastError() {
		if ( $this->lastRelayError ) {
			// If the cache and the relayer failed, focus on the latter.
			// An update not making it to the relayer means it won't show up
			// in other DCs (nor will consistent re-hashing see up-to-date values).
			// On the other hand, if just the cache update failed, then it should
			// eventually be applied by the relayer.
			return $this->lastRelayError;
		}

		$code = $this->cache->getLastError();
		switch ( $code ) {
			case BagOStuff::ERR_NONE:
				return self::ERR_NONE;
			case BagOStuff::ERR_NO_RESPONSE:
				return self::ERR_NO_RESPONSE;
			case BagOStuff::ERR_UNREACHABLE:
				return self::ERR_UNREACHABLE;
			default:
				return self::ERR_UNEXPECTED;
		}
	}

	/**
	 * Clear the "last error" registry
	 */
	final public function clearLastError() {
		$this->cache->clearLastError();
		$this->lastRelayError = self::ERR_NONE;
	}

	/**
	 * Clear the in-process caches; useful for testing
	 *
	 * @since 1.27
	 */
	public function clearProcessCache() {
		$this->processCaches = [];
	}

	/**
	 * @param int $flag ATTR_* class constant
	 * @return int QOS_* class constant
	 * @since 1.28
	 */
	public function getQoS( $flag ) {
		return $this->cache->getQoS( $flag );
	}

	/**
	 * Get a TTL that is higher for objects that have not changed recently
	 *
	 * This is useful for keys that get explicit purges and DB or purge relay
	 * lag is a potential concern (especially how it interacts with CDN cache)
	 *
	 * Example usage:
	 * @code
	 *     // Last-modified time of page
	 *     $mtime = wfTimestamp( TS_UNIX, $page->getTimestamp() );
	 *     // Get adjusted TTL. If $mtime is 3600 seconds ago and $minTTL/$factor left at
	 *     // defaults, then $ttl is 3600 * .2 = 720. If $minTTL was greater than 720, then
	 *     // $ttl would be $minTTL. If $maxTTL was smaller than 720, $ttl would be $maxTTL.
	 *     $ttl = $cache->adaptiveTTL( $mtime, $cache::TTL_DAY );
	 * @endcode
	 *
	 * @param int|float $mtime UNIX timestamp
	 * @param int $maxTTL Maximum TTL (seconds)
	 * @param int $minTTL Minimum TTL (seconds); Default: 30
	 * @param float $factor Value in the range (0,1); Default: .2
	 * @return int Adaptive TTL
	 * @since 1.28
	 */
	public function adaptiveTTL( $mtime, $maxTTL, $minTTL = 30, $factor = 0.2 ) {
		if ( is_float( $mtime ) || ctype_digit( $mtime ) ) {
			$mtime = (int)$mtime; // handle fractional seconds and string integers
		}

		if ( !is_int( $mtime ) || $mtime <= 0 ) {
			return $minTTL; // no last-modified time provided
		}

		$age = time() - $mtime;

		return (int)min( $maxTTL, max( $minTTL, $factor * $age ) );
	}

	/**
	 * @return int Number of warmup key cache misses last round
	 * @since 1.30
	 */
	public function getWarmupKeyMisses() {
		return $this->warmupKeyMisses;
	}

	/**
	 * Do the actual async bus purge of a key
	 *
	 * This must set the key to "PURGED:<UNIX timestamp>:<holdoff>"
	 *
	 * @param string $key Cache key
	 * @param int $ttl How long to keep the tombstone [seconds]
	 * @param int $holdoff HOLDOFF_* constant controlling how long to ignore sets for this key
	 * @return bool Success
	 */
	protected function relayPurge( $key, $ttl, $holdoff ) {
		if ( $this->purgeRelayer instanceof EventRelayerNull ) {
			// This handles the mcrouter and the single-DC case
			$ok = $this->cache->set( $key,
				$this->makePurgeValue( microtime( true ), self::HOLDOFF_NONE ),
				$ttl
			);
		} else {
			$event = $this->cache->modifySimpleRelayEvent( [
				'cmd' => 'set',
				'key' => $key,
				'val' => 'PURGED:$UNIXTIME$:' . (int)$holdoff,
				'ttl' => max( $ttl, 1 ),
				'sbt' => true, // substitute $UNIXTIME$ with actual microtime
			] );

			$ok = $this->purgeRelayer->notify( $this->purgeChannel, $event );
			if ( !$ok ) {
				$this->lastRelayError = self::ERR_RELAY;
			}
		}

		return $ok;
	}

	/**
	 * Do the actual async bus delete of a key
	 *
	 * @param string $key Cache key
	 * @return bool Success
	 */
	protected function relayDelete( $key ) {
		if ( $this->purgeRelayer instanceof EventRelayerNull ) {
			// This handles the mcrouter and the single-DC case
			$ok = $this->cache->delete( $key );
		} else {
			$event = $this->cache->modifySimpleRelayEvent( [
				'cmd' => 'delete',
				'key' => $key,
			] );

			$ok = $this->purgeRelayer->notify( $this->purgeChannel, $event );
			if ( !$ok ) {
				$this->lastRelayError = self::ERR_RELAY;
			}
		}

		return $ok;
	}

	/**
	 * Check if a key should be regenerated (using random probability)
	 *
	 * This returns false if $curTTL >= $lowTTL. Otherwise, the chance
	 * of returning true increases steadily from 0% to 100% as the $curTTL
	 * moves from $lowTTL to 0 seconds. This handles widely varying
	 * levels of cache access traffic.
	 *
	 * @param float $curTTL Approximate TTL left on the key if present
	 * @param float $lowTTL Consider a refresh when $curTTL is less than this
	 * @return bool
	 */
	protected function worthRefreshExpiring( $curTTL, $lowTTL ) {
		if ( $curTTL >= $lowTTL ) {
			return false;
		} elseif ( $curTTL <= 0 ) {
			return true;
		}

		$chance = ( 1 - $curTTL / $lowTTL );

		return mt_rand( 1, 1e9 ) <= 1e9 * $chance;
	}

	/**
	 * Check if a key is due for randomized regeneration due to its popularity
	 *
	 * This is used so that popular keys can preemptively refresh themselves for higher
	 * consistency (especially in the case of purge loss/delay). Unpopular keys can remain
	 * in cache with their high nominal TTL. This means popular keys keep good consistency,
	 * whether the data changes frequently or not, and long-tail keys get to stay in cache
	 * and get hits too. Similar to worthRefreshExpiring(), randomization is used.
	 *
	 * @param float $asOf UNIX timestamp of the value
	 * @param int $ageNew Age of key when this might recommend refreshing (seconds)
	 * @param int $timeTillRefresh Age of key when it should be refreshed if popular (seconds)
	 * @param float $now The current UNIX timestamp
	 * @return bool
	 */
	protected function worthRefreshPopular( $asOf, $ageNew, $timeTillRefresh, $now ) {
		$age = $now - $asOf;
		$timeOld = $age - $ageNew;
		if ( $timeOld <= 0 ) {
			return false;
		}

		// Lifecycle is: new, ramp-up refresh chance, full refresh chance.
		// Note that the "expected # of refreshes" for the ramp-up time range is half of what it
		// would be if P(refresh) was at its full value during that time range.
		$refreshWindowSec = max( $timeTillRefresh - $ageNew - self::RAMPUP_TTL / 2, 1 );
		// P(refresh) * (# hits in $refreshWindowSec) = (expected # of refreshes)
		// P(refresh) * ($refreshWindowSec * $popularHitsPerSec) = 1
		// P(refresh) = 1/($refreshWindowSec * $popularHitsPerSec)
		$chance = 1 / ( self::HIT_RATE_HIGH * $refreshWindowSec );

		// Ramp up $chance from 0 to its nominal value over RAMPUP_TTL seconds to avoid stampedes
		$chance *= ( $timeOld <= self::RAMPUP_TTL ) ? $timeOld / self::RAMPUP_TTL : 1;

		return mt_rand( 1, 1e9 ) <= 1e9 * $chance;
	}

	/**
	 * Check whether $value is appropriately versioned and not older than $minTime (if set)
	 *
	 * @param array $value
	 * @param bool $versioned
	 * @param float $asOf The time $value was generated
	 * @param float $minTime The last time the main value was generated (0.0 if unknown)
	 * @return bool
	 */
	protected function isValid( $value, $versioned, $asOf, $minTime ) {
		if ( $versioned && !isset( $value[self::VFLD_VERSION] ) ) {
			return false;
		} elseif ( $minTime > 0 && $asOf < $minTime ) {
			return false;
		}

		return true;
	}

	/**
	 * Do not use this method outside WANObjectCache
	 *
	 * @param mixed $value
	 * @param int $ttl [0=forever]
	 * @param float $now Unix Current timestamp just before calling set()
	 * @return array
	 */
	protected function wrap( $value, $ttl, $now ) {
		return [
			self::FLD_VERSION => self::VERSION,
			self::FLD_VALUE => $value,
			self::FLD_TTL => $ttl,
			self::FLD_TIME => $now
		];
	}

	/**
	 * Do not use this method outside WANObjectCache
	 *
	 * @param array|string|bool $wrapped
	 * @param float $now Unix Current timestamp (preferrably pre-query)
	 * @return array (mixed; false if absent/tombstoned/invalid, current time left)
	 */
	protected function unwrap( $wrapped, $now ) {
		// Check if the value is a tombstone
		$purge = self::parsePurgeValue( $wrapped );
		if ( $purge !== false ) {
			// Purged values should always have a negative current $ttl
			$curTTL = min( $purge[self::FLD_TIME] - $now, self::TINY_NEGATIVE );
			return [ false, $curTTL ];
		}

		if ( !is_array( $wrapped ) // not found
			|| !isset( $wrapped[self::FLD_VERSION] ) // wrong format
			|| $wrapped[self::FLD_VERSION] !== self::VERSION // wrong version
		) {
			return [ false, null ];
		}

		$flags = isset( $wrapped[self::FLD_FLAGS] ) ? $wrapped[self::FLD_FLAGS] : 0;
		if ( ( $flags & self::FLG_STALE ) == self::FLG_STALE ) {
			// Treat as expired, with the cache time as the expiration
			$age = $now - $wrapped[self::FLD_TIME];
			$curTTL = min( -$age, self::TINY_NEGATIVE );
		} elseif ( $wrapped[self::FLD_TTL] > 0 ) {
			// Get the approximate time left on the key
			$age = $now - $wrapped[self::FLD_TIME];
			$curTTL = max( $wrapped[self::FLD_TTL] - $age, 0.0 );
		} else {
			// Key had no TTL, so the time left is unbounded
			$curTTL = INF;
		}

		return [ $wrapped[self::FLD_VALUE], $curTTL ];
	}

	/**
	 * @param array $keys
	 * @param string $prefix
	 * @return string[]
	 */
	protected static function prefixCacheKeys( array $keys, $prefix ) {
		$res = [];
		foreach ( $keys as $key ) {
			$res[] = $prefix . $key;
		}

		return $res;
	}

	/**
	 * @param string $value Wrapped value like "PURGED:<timestamp>:<holdoff>"
	 * @return array|bool Array containing a UNIX timestamp (float) and holdoff period (integer),
	 *  or false if value isn't a valid purge value
	 */
	protected static function parsePurgeValue( $value ) {
		if ( !is_string( $value ) ) {
			return false;
		}
		$segments = explode( ':', $value, 3 );
		if ( !isset( $segments[0] ) || !isset( $segments[1] )
			|| "{$segments[0]}:" !== self::PURGE_VAL_PREFIX
		) {
			return false;
		}
		if ( !isset( $segments[2] ) ) {
			// Back-compat with old purge values without holdoff
			$segments[2] = self::HOLDOFF_TTL;
		}
		return [
			self::FLD_TIME => (float)$segments[1],
			self::FLD_HOLDOFF => (int)$segments[2],
		];
	}

	/**
	 * @param float $timestamp
	 * @param int $holdoff In seconds
	 * @return string Wrapped purge value
	 */
	protected function makePurgeValue( $timestamp, $holdoff ) {
		return self::PURGE_VAL_PREFIX . (float)$timestamp . ':' . (int)$holdoff;
	}

	/**
	 * @param string $group
	 * @return HashBagOStuff
	 */
	protected function getProcessCache( $group ) {
		if ( !isset( $this->processCaches[$group] ) ) {
			list( , $n ) = explode( ':', $group );
			$this->processCaches[$group] = new HashBagOStuff( [ 'maxKeys' => (int)$n ] );
		}

		return $this->processCaches[$group];
	}

	/**
	 * @param array $keys
	 * @param array $opts
	 * @return array List of keys
	 */
	private function getNonProcessCachedKeys( array $keys, array $opts ) {
		$keysFound = [];
		if ( isset( $opts['pcTTL'] ) && $opts['pcTTL'] > 0 && $this->callbackDepth == 0 ) {
			$pcGroup = isset( $opts['pcGroup'] ) ? $opts['pcGroup'] : self::PC_PRIMARY;
			$procCache = $this->getProcessCache( $pcGroup );
			foreach ( $keys as $key ) {
				if ( $procCache->get( $key ) !== false ) {
					$keysFound[] = $key;
				}
			}
		}

		return array_diff( $keys, $keysFound );
	}

		/**
	 * @param array $keys
	 * @param array $checkKeys
	 * @return array Map of (cache key => mixed)
	 */
	private function getRawKeysForWarmup( array $keys, array $checkKeys ) {
		if ( !$keys ) {
			return [];
		}

		$keysWarmUp = [];
		// Get all the value keys to fetch...
		foreach ( $keys as $key ) {
			$keysWarmUp[] = self::VALUE_KEY_PREFIX . $key;
		}
		// Get all the check keys to fetch...
		foreach ( $checkKeys as $i => $checkKeyOrKeys ) {
			if ( is_int( $i ) ) {
				// Single check key that applies to all value keys
				$keysWarmUp[] = self::TIME_KEY_PREFIX . $checkKeyOrKeys;
			} else {
				// List of check keys that apply to value key $i
				$keysWarmUp = array_merge(
					$keysWarmUp,
					self::prefixCacheKeys( $checkKeyOrKeys, self::TIME_KEY_PREFIX )
				);
			}
		}

		$warmupCache = $this->cache->getMulti( $keysWarmUp );
		$warmupCache += array_fill_keys( $keysWarmUp, false );

		return $warmupCache;
	}
}