Make WANObjectCache sets account for slave lag

[lhc/web/wiklou.git] / 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
 * @author Aaron Schulz
 */

/**
 * 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().
 * 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().
 *
 * Instances of this class must be configured to point to a valid
 * PubSub endpoint, and there must be listeners on the cache servers
 * that subscribe to the endpoint and update the caches.
 *
 * Broadcasted operations like delete() and touchCheckKey() are done
 * synchronously in the local datacenter, but are relayed asynchronously.
 * This means that callers in other datacenters will see older values
 * for however many milliseconds the datacenters are apart. 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.
 *
 * 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:s" : used for temporarily storing values of tombstoned keys
 *   - c) "WANCache:t" : used for storing timestamp "check" keys
 *
 * @ingroup Cache
 * @since 1.26
 */
class WANObjectCache {
        /** @var BagOStuff The local datacenter cache */
        protected $cache;
        /** @var string Cache pool name */
        protected $pool;
        /** @var EventRelayer */
        protected $relayer;

        /** @var int */
        protected $lastRelayError = self::ERR_NONE;

        /** Max time expected to pass between delete() and DB commit finishing */
        const MAX_COMMIT_DELAY = 3;
        /** Max replication lag before applying TTL_LAGGED to set() */
        const MAX_REPLICA_LAG = 5;
        /** Max time since snapshot transaction start to avoid no-op of set() */
        const MAX_SNAPSHOT_LAG = 5;
        /** Seconds to tombstone keys on delete() */
        const HOLDOFF_TTL = 14; // MAX_COMMIT_DELAY + MAX_REPLICA_LAG + MAX_SNAPSHOT_LAG + 1

        /** Seconds to keep dependency purge keys around */
        const CHECK_KEY_TTL = 31536000; // 1 year
        /** Seconds to keep lock keys around */
        const LOCK_TTL = 5;
        /** Default remaining TTL at which to consider pre-emptive regeneration */
        const LOW_TTL = 10;
        /** Default time-since-expiry on a miss that makes a key "hot" */
        const LOCK_TSE = 1;

        /** Idiom for set()/getWithSetCallback() TTL */
        const TTL_NONE = 0;
        /** 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;

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

        /** Fields of value holder arrays */
        const FLD_VERSION = 0;
        const FLD_VALUE = 1;
        const FLD_TTL = 2;
        const FLD_TIME = 3;

        /** Possible values for getLastError() */
        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 STASH_KEY_PREFIX = 'WANCache:s:';
        const TIME_KEY_PREFIX = 'WANCache:t:';

        const PURGE_VAL_PREFIX = 'PURGED:';

        /**
         * @param array $params
         *   - cache   : BagOStuff object
         *   - pool    : pool name
         *   - relayer : EventRelayer object
         */
        public function __construct( array $params ) {
                $this->cache = $params['cache'];
                $this->pool = $params['pool'];
                $this->relayer = $params['relayer'];
        }

        /**
         * @return WANObjectCache Cache that wraps EmptyBagOStuff
         */
        public static function newEmpty() {
                return new self( array(
                        'cache'   => new EmptyBagOStuff(),
                        'pool'    => 'empty',
                        'relayer' => new EventRelayerNull( array() )
                ) );
        }

        /**
         * Fetch the value of a key from cache
         *
         * If passed in, $curTTL is set to the remaining TTL (current time left):
         *   - a) INF; if the key exists, has no TTL, and is not expired by $checkKeys
         *   - b) float (>=0); if the key exists, has a TTL, and is not expired by $checkKeys
         *   - c) float (<0); if the key is tombstoned or existing but expired by $checkKeys
         *   - d) null; if the 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()/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 [returned]
         * @param array $checkKeys List of "check" keys
         * @return mixed Value of cache key or false on failure
         */
        final public function get( $key, &$curTTL = null, array $checkKeys = array() ) {
                $curTTLs = array();
                $values = $this->getMulti( array( $key ), $curTTLs, $checkKeys );
                $curTTL = isset( $curTTLs[$key] ) ? $curTTLs[$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
         * @return array Map of (key => value) for keys that exist
         */
        final public function getMulti(
                array $keys, &$curTTLs = array(), array $checkKeys = array()
        ) {
                $result = array();
                $curTTLs = array();

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

                // Fetch all of the raw values
                $wrappedValues = $this->cache->getMulti( array_merge( $valueKeys, $checkKeys ) );
                $now = microtime( true );

                // Get/initialize the timestamp of all the "check" keys
                $checkKeyTimes = array();
                foreach ( $checkKeys as $checkKey ) {
                        $timestamp = isset( $wrappedValues[$checkKey] )
                                ? self::parsePurgeValue( $wrappedValues[$checkKey] )
                                : false;
                        if ( !is_float( $timestamp ) ) {
                                // Key is not set or invalid; regenerate
                                $this->cache->add( $checkKey,
                                        self::PURGE_VAL_PREFIX . $now, self::CHECK_KEY_TTL );
                                $timestamp = $now;
                        }

                        $checkKeyTimes[] = $timestamp;
                }

                // 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;
                                foreach ( $checkKeyTimes as $checkKeyTime ) {
                                        // Force dependant keys to be invalid for a while after purging
                                        // to reduce race conditions involving stale data getting cached
                                        $safeTimestamp = $checkKeyTime + self::HOLDOFF_TTL;
                                        if ( $safeTimestamp >= $wrappedValues[$vKey][self::FLD_TIME] ) {
                                                $curTTL = min( $curTTL, $checkKeyTime - $now );
                                        }
                                }
                        }

                        $curTTLs[$key] = $curTTL;
                }

                return $result;
        }

        /**
         * Set the value of a key from 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
         *
         * Example usage:
         * @code
         *     $dbr = wfGetDB( DB_SLAVE );
         *     $setOpts = DatabaseBase::getCacheSetOptions( $dbr );
         *     // Fetch the row from the DB
         *     $row = $dbr->selectRow( ... );
         *     $key = wfMemcKey( 'building', $buildingId );
         *     $cache->set( $key, $row, 86400, $setOpts );
         * @endcode
         *
         * @param string $key Cache key
         * @param mixed $value
         * @param integer $ttl Seconds to live [0=forever]
         * @param array $opts Options map:
         *   - lag     : Seconds of slave lag. Typically, this is either the slave lag
         *               before the data was read or, if applicable, the slave lag before
         *               the snapshot-isolated transaction the data was read from started.
         *               [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]
         *   - lockTSE : if excessive possible snapshot lag is detected,
         *               then stash the value into a temporary location
         *               with this TTL. This is only useful if the reads
         *               use getWithSetCallback() with "lockTSE" set.
         *               [Default: WANObjectCache::TSE_NONE]
         * @return bool Success
         */
        final public function set( $key, $value, $ttl = 0, array $opts = array() ) {
                $lockTSE = isset( $opts['lockTSE'] ) ? $opts['lockTSE'] : self::TSE_NONE;
                $age = isset( $opts['since'] ) ? max( 0, microtime( true ) - $opts['since'] ) : 0;
                $lag = isset( $opts['lag'] ) ? $opts['lag'] : 0;

                if ( $lag > self::MAX_REPLICA_LAG ) {
                        // Too much lag detected; lower TTL so it converges faster
                        $ttl = $ttl ? min( $ttl, self::TTL_LAGGED ) : self::TTL_LAGGED;
                }

                if ( $age > self::MAX_SNAPSHOT_LAG ) {
                        if ( $lockTSE >= 0 ) {
                                $tempTTL = max( 1, (int)$lockTSE ); // set() expects seconds
                                $this->cache->set( self::STASH_KEY_PREFIX . $key, $value, $tempTTL );
                        }

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

                $wrapped = $this->wrap( $value, $ttl );

                $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, 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 writing stale values, 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
         *
         * 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 in the local datacenter
         * and in the others upon 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->begin(); // start of request
         *     ... <execute some stuff> ...
         *     // Update the row in the DB
         *     $dbw->update( ... );
         *     $key = wfMemcKey( 'homes', $homeId );
         *     // Purge the corresponding cache entry just before committing
         *     $dbw->onTransactionPreCommitOrIdle( function() use ( $cache, $key ) {
         *         $cache->delete( $key );
         *     } );
         *     ... <execute some stuff> ...
         *     $dbw->commit(); // end of request
         * @endcode
         *
         * 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. Also note that lowering $ttl reduces
         * the effective range of the 'lockTSE' parameter to getWithSetCallback().
         *
         * @param string $key Cache key
         * @param integer $ttl How long to block writes to the key [seconds]
         * @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;
                // Avoid indefinite key salting for sanity
                $ttl = max( $ttl, 1 );
                // Update the local datacenter immediately
                $ok = $this->cache->set( $key, self::PURGE_VAL_PREFIX . microtime( true ), $ttl );
                // Publish the purge to all datacenters
                return $this->relayPurge( $key, $ttl ) && $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 key
         */
        final public function getCheckKeyTime( $key ) {
                $key = self::TIME_KEY_PREFIX . $key;

                $time = self::parsePurgeValue( $this->cache->get( $key ) );
                if ( $time === false ) {
                        // Casting assures identical floats for the next getCheckKeyTime() calls
                        $time = (string)microtime( true );
                        $this->cache->add( $key, self::PURGE_VAL_PREFIX . $time, self::CHECK_KEY_TTL );
                        $time = (float)$time;
                }

                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.
         * It is set in the future a few seconds when this is called, to
         * avoid race conditions where dependent keys get updated with a
         * stale value (e.g. from a DB slave).
         *
         * This is typically useful for keys with static names or 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.
         *
         * Note that "check" keys won't collide with other regular keys
         *
         * @see WANObjectCache::get()
         *
         * @param string $key Cache key
         * @return bool True if the item was purged or not found, false on failure
         */
        final public function touchCheckKey( $key ) {
                $key = self::TIME_KEY_PREFIX . $key;
                // Update the local datacenter immediately
                $ok = $this->cache->set( $key,
                        self::PURGE_VAL_PREFIX . microtime( true ), self::CHECK_KEY_TTL );
                // Publish the purge to all datacenters
                return $this->relayPurge( $key, self::CHECK_KEY_TTL ) && $ok;
        }

        /**
         * Delete a "check" key from all datacenters, invalidating keys that use it
         *
         * This is similar to touchCheckKey() in that keys using it via
         * getWithSetCallback() will be invalidated. The differences are:
         *   - a) The timestamp 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()
         *
         * 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::touchCheckKey()
         * @see WANObjectCache::get()
         *
         * @param string $key Cache key
         * @return bool True if the item was purged or not found, false on failure
         */
        final public function resetCheckKey( $key ) {
                $key = self::TIME_KEY_PREFIX . $key;
                // Update the local datacenter immediately
                $ok = $this->cache->delete( $key );
                // Publish the purge to all datacenters
                return $this->relayDelete( $key ) && $ok;
        }

        /**
         * 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. The arguments supplied to it are:
         *     (current value or false, &$ttl, &$setOpts)
         * The callback function returns the new value given the current
         * value (false if not present). Preemptive re-caching and $checkKeys
         * can result in a non-false current value. The TTL of the new value
         * can be set dynamically by altering $ttl in the callback (by reference).
         * The $setOpts array can be altered and is given to set() when called;
         * it is recommended to set the 'since' field to avoid race conditions.
         *
         * 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.
         *
         * 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 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
         *         wfMemcKey( 'cat-attributes', $catId ),
         *         // Function that derives the new key value
         *         function ( $oldValue, &$ttl, array &$setOpts ) {
         *             $dbr = wfGetDB( DB_SLAVE );
         *             // Account for any snapshot/slave lag
         *             $setOpts += DatabaseBase::getCacheSetOptions( $dbr );
         *
         *             return $dbr->selectRow( ... );
         *        },
         *        // Time-to-live (seconds)
         *        60
         *     );
         * @endcode
         *
         * Example usage (key that is expensive and hot):
         * @code
         *     $catConfig = $cache->getWithSetCallback(
         *         // Key to store the cached value under
         *         wfMemcKey( 'site-cat-config' ),
         *         // Function that derives the new key value
         *         function ( $oldValue, &$ttl, array &$setOpts ) {
         *             $dbr = wfGetDB( DB_SLAVE );
         *             // Account for any snapshot/slave lag
         *             $setOpts += DatabaseBase::getCacheSetOptions( $dbr );
         *
         *             return CatConfig::newFromRow( $dbr->selectRow( ... ) );
         *        },
         *        // Time-to-live (seconds)
         *        86400,
         *        // Calling touchCheckKey() on this key invalidates the cache
         *        wfMemcKey( 'site-cat-config' ),
         *        // Try to only let one datacenter thread manage cache updates at a time
         *        array( 'lockTSE' => 30 )
         *     );
         * @endcode
         *
         * Example usage (key with dynamic dependencies):
         * @code
         *     $catState = $cache->getWithSetCallback(
         *         // Key to store the cached value under
         *         wfMemcKey( 'cat-state', $cat->getId() ),
         *         // Function that derives the new key value
         *         function ( $oldValue, &$ttl, array &$setOpts ) {
         *             // Determine new value from the DB
         *             $dbr = wfGetDB( DB_SLAVE );
         *             // Account for any snapshot/slave lag
         *             $setOpts += DatabaseBase::getCacheSetOptions( $dbr );
         *
         *             return CatState::newFromResults( $dbr->select( ... ) );
         *        },
         *        // Time-to-live (seconds)
         *        900,
         *        // The "check" keys that represent things the value depends on;
         *        // Calling touchCheckKey() on any of them invalidates the cache
         *        array(
         *             wfMemcKey( 'sustenance-bowls', $cat->getRoomId() ),
         *             wfMemcKey( 'people-present', $cat->getHouseId() ),
         *             wfMemcKey( '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
         *         wfMemcKey( 'cat-last-actions', 100 ),
         *         // Function that derives the new key value
         *         function ( $oldValue, &$ttl, array &$setOpts ) {
         *             $dbr = wfGetDB( DB_SLAVE );
         *             // Account for any snapshot/slave lag
         *             $setOpts += DatabaseBase::getCacheSetOptions( $dbr );
         *
         *             // Start off with the last cached list
         *             $list = $oldValue ?: array();
         *             // 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 );
         *        },
         *        // Time-to-live (seconds)
         *        10,
         *        // No "check" keys
         *        array(),
         *        // Try to only let one datacenter thread manage cache updates at a time
         *        array( 'lockTSE' => 30 )
         *     );
         * @endcode
         *
         * @see WANObjectCache::get()
         * @see WANObjectCache::set()
         *
         * @param string $key Cache key
         * @param callable $callback Value generation function
         * @param integer $ttl Seconds to live for key updates. Special values are:
         *   - WANObjectCache::TTL_NONE        : cache forever
         *   - WANObjectCache::TTL_UNCACHEABLE : do not cache at all
         * @param array $checkKeys List of "check" keys
         * @param array $opts Options map:
         *   - lowTTL  : consider pre-emptive updates when the current TTL (sec)
         *               of the key is less than this. It becomes more likely
         *               over time, becoming a certainty once the key is expired.
         *               [Default: WANObjectCache::LOW_TTL seconds]
         *   - 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]
         * @return mixed Value to use for the key
         */
        final public function getWithSetCallback(
                $key, $callback, $ttl, array $checkKeys = array(), array $opts = array()
        ) {
                $lowTTL = isset( $opts['lowTTL'] ) ? $opts['lowTTL'] : min( self::LOW_TTL, $ttl );
                $lockTSE = isset( $opts['lockTSE'] ) ? $opts['lockTSE'] : self::TSE_NONE;

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

                // Determine if a regeneration is desired
                if ( $value !== false && $curTTL > 0 && !$this->worthRefresh( $curTTL, $lowTTL ) ) {
                        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 );
                // 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 ) );

                $lockAcquired = false;
                if ( $useMutex ) {
                        // Acquire a datacenter-local non-blocking lock
                        if ( $this->cache->lock( $key, 0, self::LOCK_TTL ) ) {
                                // Lock acquired; this thread should update the key
                                $lockAcquired = true;
                        } elseif ( $value !== false ) {
                                // If it cannot be acquired; then the stale value can be used
                                return $value;
                        } else {
                                // Use the stash value for tombstoned keys to reduce regeneration load.
                                // For hot keys, either another thread has the lock or the lock failed;
                                // use the stash value from the last thread that regenerated it.
                                $value = $this->cache->get( self::STASH_KEY_PREFIX . $key );
                                if ( $value !== false ) {
                                        return $value;
                                }
                        }
                }

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

                // Generate the new value from the callback...
                $setOpts = array();
                $value = call_user_func_array( $callback, array( $cValue, &$ttl, &$setOpts ) );
                // When delete() is called, writes are write-holed by the tombstone,
                // so use a special stash key to pass the new value around threads.
                if ( $useMutex && $value !== false && $ttl >= 0 ) {
                        $tempTTL = max( 1, (int)$lockTSE ); // set() expects seconds
                        $this->cache->set( self::STASH_KEY_PREFIX . $key, $value, $tempTTL );
                }

                if ( $lockAcquired ) {
                        $this->cache->unlock( $key );
                }

                if ( $value !== false && $ttl >= 0 ) {
                        // Update the cache; this will fail if the key is tombstoned
                        $setOpts['lockTSE'] = $lockTSE;
                        $this->set( $key, $value, $ttl, $setOpts );
                }

                return $value;
        }

        /**
         * Get the "last error" registered; clearLastError() should be called manually
         * @return int ERR_* constant for the "last error" registry
         */
        final public function getLastError() {
                if ( $this->lastRelayError ) {
                        // If the cache and the relayer failed, focus on the later.
                        // 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;
        }

        /**
         * Do the actual async bus purge of a key
         *
         * This must set the key to "PURGED:<UNIX timestamp>"
         *
         * @param string $key Cache key
         * @param integer $ttl How long to keep the tombstone [seconds]
         * @return bool Success
         */
        protected function relayPurge( $key, $ttl ) {
                $event = $this->cache->modifySimpleRelayEvent( array(
                        'cmd' => 'set',
                        'key' => $key,
                        'val' => 'PURGED:$UNIXTIME$',
                        'ttl' => max( $ttl, 1 ),
                        'sbt' => true, // substitute $UNIXTIME$ with actual microtime
                ) );

                $ok = $this->relayer->notify( "{$this->pool}:purge", $event );
                if ( !$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 ) {
                $event = $this->cache->modifySimpleRelayEvent( array(
                        'cmd' => 'delete',
                        'key' => $key,
                ) );

                $ok = $this->relayer->notify( "{$this->pool}:purge", $event );
                if ( !$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 worthRefresh( $curTTL, $lowTTL ) {
                if ( $curTTL >= $lowTTL ) {
                        return false;
                } elseif ( $curTTL <= 0 ) {
                        return true;
                }

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

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

        /**
         * Do not use this method outside WANObjectCache
         *
         * @param mixed $value
         * @param integer $ttl [0=forever]
         * @return string
         */
        protected function wrap( $value, $ttl ) {
                return array(
                        self::FLD_VERSION => self::VERSION,
                        self::FLD_VALUE => $value,
                        self::FLD_TTL => $ttl,
                        self::FLD_TIME => microtime( true )
                );
        }

        /**
         * Do not use this method outside WANObjectCache
         *
         * @param array|string|bool $wrapped
         * @param float $now Unix Current timestamp (preferrable pre-query)
         * @return array (mixed; false if absent/invalid, current time left)
         */
        protected function unwrap( $wrapped, $now ) {
                // Check if the value is a tombstone
                $purgeTimestamp = self::parsePurgeValue( $wrapped );
                if ( is_float( $purgeTimestamp ) ) {
                        // Purged values should always have a negative current $ttl
                        $curTTL = min( -0.000001, $purgeTimestamp - $now );
                        return array( false, $curTTL );
                }

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

                if ( $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 array( $wrapped[self::FLD_VALUE], $curTTL );
        }

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

                return $res;
        }

        /**
         * @param string $value String like "PURGED:<timestamp>"
         * @return float|bool UNIX timestamp or false on failure
         */
        protected static function parsePurgeValue( $value ) {
                $m = array();
                if ( is_string( $value ) &&
                        preg_match( '/^' . self::PURGE_VAL_PREFIX . '([^:]+)$/', $value, $m )
                ) {
                        return (float)$m[1];
                } else {
                        return false;
                }
        }
}