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<?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
*/
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 two 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).
*
* 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 integer Callback stack depth for getWithSetCallback() */
private $callbackDepth = 0;
/** @var mixed[] Temporary warm-up cache */
private $warmupCache = [];
/** 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 integer 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',
'relayer' => new EventRelayerNull( [] )
] );
}
/**
* 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
*/
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 => $keys ) {
$prefixed = self::prefixCacheKeys( (array)$keys, 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
} else {
$wrappedValues = [];
}
$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 integer $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->warning( "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->warning( "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 integer $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 integer $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 integer $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.
$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;
}
// 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;
}
// 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 ) && $value !== false && $ttl >= 0 ) {
$tempTTL = max( 1, (int)$lockTSE ); // set() expects seconds
$newAsOf = microtime( true );
$wrapped = $this->wrap( $value, $tempTTL, $newAsOf );
// Avoid using set() to avoid pointless mcrouter broadcasting
$this->cache->merge(
self::INTERIM_KEY_PREFIX . $key,
function () use ( $wrapped ) {
return $wrapped;
},
$tempTTL,
1
);
}
if ( $value !== false && $ttl >= 0 ) {
$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;
}
/**
* 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 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()
*
* 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
* return 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', '*', [ '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 integer $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 = []
) {
$keysWarmUp = iterator_to_array( $keyedIds, true );
$checkKeys = isset( $opts['checkKeys'] ) ? $opts['checkKeys'] : [];
foreach ( $checkKeys as $i => $checkKeyOrKeys ) {
if ( is_int( $i ) ) {
$keysWarmUp[] = $checkKeyOrKeys;
} else {
$keysWarmUp = array_merge( $keysWarmUp, $checkKeyOrKeys );
}
}
$this->warmupCache = $this->cache->getMulti( $keysWarmUp );
$this->warmupCache += array_fill_keys( $keysWarmUp, false );
// Wrap $callback to match the getWithSetCallback() format while passing $id to $callback
$id = null;
$func = function ( $oldValue, &$ttl, array $setOpts, $oldAsOf ) use ( $callback, &$id ) {
return $callback( $id, $oldValue, $ttl, $setOpts, $oldAsOf );
};
$values = [];
foreach ( $keyedIds as $key => $id ) {
$values[$key] = $this->getWithSetCallback( $key, $ttl, $func, $opts );
}
$this->warmupCache = [];
return $values;
}
/**
* @see BagOStuff::makeKey()
* @param string ... Key component
* @return string
* @since 1.27
*/
public function makeKey() {
return call_user_func_array( [ $this->cache, __FUNCTION__ ], func_get_args() );
}
/**
* @see BagOStuff::makeGlobalKey()
* @param string ... Key component
* @return string
* @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 integer $flag ATTR_* class constant
* @return integer 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 integer|float $mtime UNIX timestamp
* @param integer $maxTTL Maximum TTL (seconds)
* @param integer $minTTL Minimum TTL (seconds); Default: 30
* @param float $factor Value in the range (0,1); Default: .2
* @return integer Adaptive TTL
* @since 1.28
*/
public function adaptiveTTL( $mtime, $maxTTL, $minTTL = 30, $factor = .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 ) );
}
/**
* 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 integer $ttl How long to keep the tombstone [seconds]
* @param integer $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 integer $ageNew Age of key when this might recommend refreshing (seconds)
* @param integer $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
$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 integer $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/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];
}
}