3 * This program is free software; you can redistribute it and/or modify
4 * it under the terms of the GNU General Public License as published by
5 * the Free Software Foundation; either version 2 of the License, or
6 * (at your option) any later version.
8 * This program is distributed in the hope that it will be useful,
9 * but WITHOUT ANY WARRANTY; without even the implied warranty of
10 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
11 * GNU General Public License for more details.
13 * You should have received a copy of the GNU General Public License along
14 * with this program; if not, write to the Free Software Foundation, Inc.,
15 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
16 * http://www.gnu.org/copyleft/gpl.html
20 * @author Aaron Schulz
24 * Multi-datacenter aware caching interface
26 * All operations go to the local datacenter cache, except for delete(),
27 * touchCheckKey(), and resetCheckKey(), which broadcast to all datacenters.
29 * This class is intended for caching data from primary stores.
30 * If the get() method does not return a value, then the caller
31 * should query the new value and backfill the cache using set().
32 * When the source data changes, a purge method should be called.
33 * Since purges are expensive, they should be avoided. One can do so if:
34 * - a) The object cached is immutable; or
35 * - b) Validity is checked against the source after get(); or
36 * - c) Using a modest TTL is reasonably correct and performant
37 * The simplest purge method is delete().
39 * Instances of this class must be configured to point to a valid
40 * PubSub endpoint, and there must be listeners on the cache servers
41 * that subscribe to the endpoint and update the caches.
43 * Broadcasted operations like delete() and touchCheckKey() are done
44 * synchronously in the local datacenter, but are relayed asynchronously.
45 * This means that callers in other datacenters will see older values
46 * for however many milliseconds the datacenters are apart. As with
47 * any cache, this should not be relied on for cases where reads are
48 * used to determine writes to source (e.g. non-cache) data stores.
50 * All values are wrapped in metadata arrays. Keys use a "WANCache:" prefix
51 * to avoid collisions with keys that are not wrapped as metadata arrays. The
52 * prefixes are as follows:
53 * - a) "WANCache:v" : used for regular value keys
54 * - b) "WANCache:s" : used for temporarily storing values of tombstoned keys
55 * - c) "WANCache:t" : used for storing timestamp "check" keys
60 class WANObjectCache
{
61 /** @var BagOStuff The local datacenter cache */
63 /** @var string Cache pool name */
65 /** @var EventRelayer */
69 protected $lastRelayError = self
::ERR_NONE
;
71 /** Max time expected to pass between delete() and DB commit finishing */
72 const MAX_COMMIT_DELAY
= 3;
73 /** Max replication lag before applying TTL_LAGGED to set() */
74 const MAX_REPLICA_LAG
= 5;
75 /** Max time since snapshot transaction start to avoid no-op of set() */
76 const MAX_SNAPSHOT_LAG
= 5;
77 /** Seconds to tombstone keys on delete() */
78 const HOLDOFF_TTL
= 14; // MAX_COMMIT_DELAY + MAX_REPLICA_LAG + MAX_SNAPSHOT_LAG + 1
80 /** Seconds to keep dependency purge keys around */
81 const CHECK_KEY_TTL
= 31536000; // 1 year
82 /** Seconds to keep lock keys around */
84 /** Default remaining TTL at which to consider pre-emptive regeneration */
86 /** Default time-since-expiry on a miss that makes a key "hot" */
89 /** Idiom for set()/getWithSetCallback() TTL */
91 /** Idiom for getWithSetCallback() callbacks to avoid calling set() */
92 const TTL_UNCACHEABLE
= -1;
93 /** Idiom for getWithSetCallback() callbacks to 'lockTSE' logic */
95 /** Max TTL to store keys when a data sourced is lagged */
96 const TTL_LAGGED
= 30;
98 /** Cache format version number */
101 /** Fields of value holder arrays */
102 const FLD_VERSION
= 0;
107 /** Possible values for getLastError() */
108 const ERR_NONE
= 0; // no error
109 const ERR_NO_RESPONSE
= 1; // no response
110 const ERR_UNREACHABLE
= 2; // can't connect
111 const ERR_UNEXPECTED
= 3; // response gave some error
112 const ERR_RELAY
= 4; // relay broadcast failed
114 const VALUE_KEY_PREFIX
= 'WANCache:v:';
115 const STASH_KEY_PREFIX
= 'WANCache:s:';
116 const TIME_KEY_PREFIX
= 'WANCache:t:';
118 const PURGE_VAL_PREFIX
= 'PURGED:';
121 * @param array $params
122 * - cache : BagOStuff object
124 * - relayer : EventRelayer object
126 public function __construct( array $params ) {
127 $this->cache
= $params['cache'];
128 $this->pool
= $params['pool'];
129 $this->relayer
= $params['relayer'];
133 * @return WANObjectCache Cache that wraps EmptyBagOStuff
135 public static function newEmpty() {
136 return new self( array(
137 'cache' => new EmptyBagOStuff(),
139 'relayer' => new EventRelayerNull( array() )
144 * Fetch the value of a key from cache
146 * If passed in, $curTTL is set to the remaining TTL (current time left):
147 * - a) INF; if the key exists, has no TTL, and is not expired by $checkKeys
148 * - b) float (>=0); if the key exists, has a TTL, and is not expired by $checkKeys
149 * - c) float (<0); if the key is tombstoned or existing but expired by $checkKeys
150 * - d) null; if the key does not exist and is not tombstoned
152 * If a key is tombstoned, $curTTL will reflect the time since delete().
154 * The timestamp of $key will be checked against the last-purge timestamp
155 * of each of $checkKeys. Those $checkKeys not in cache will have the last-purge
156 * initialized to the current timestamp. If any of $checkKeys have a timestamp
157 * greater than that of $key, then $curTTL will reflect how long ago $key
158 * became invalid. Callers can use $curTTL to know when the value is stale.
159 * The $checkKeys parameter allow mass invalidations by updating a single key:
160 * - a) Each "check" key represents "last purged" of some source data
161 * - b) Callers pass in relevant "check" keys as $checkKeys in get()
162 * - c) When the source data that "check" keys represent changes,
163 * the touchCheckKey() method is called on them
165 * Source data entities might exists in a DB that uses snapshot isolation
166 * (e.g. the default REPEATABLE-READ in innoDB). Even for mutable data, that
167 * isolation can largely be maintained by doing the following:
168 * - a) Calling delete() on entity change *and* creation, before DB commit
169 * - b) Keeping transaction duration shorter than delete() hold-off TTL
171 * However, pre-snapshot values might still be seen if an update was made
172 * in a remote datacenter but the purge from delete() didn't relay yet.
174 * Consider using getWithSetCallback() instead of get()/set() cycles.
175 * That method has cache slam avoiding features for hot/expensive keys.
177 * @param string $key Cache key
178 * @param mixed $curTTL Approximate TTL left on the key if present [returned]
179 * @param array $checkKeys List of "check" keys
180 * @return mixed Value of cache key or false on failure
182 final public function get( $key, &$curTTL = null, array $checkKeys = array() ) {
184 $values = $this->getMulti( array( $key ), $curTTLs, $checkKeys );
185 $curTTL = isset( $curTTLs[$key] ) ?
$curTTLs[$key] : null;
187 return isset( $values[$key] ) ?
$values[$key] : false;
191 * Fetch the value of several keys from cache
193 * @see WANObjectCache::get()
195 * @param array $keys List of cache keys
196 * @param array $curTTLs Map of (key => approximate TTL left) for existing keys [returned]
197 * @param array $checkKeys List of "check" keys
198 * @return array Map of (key => value) for keys that exist
200 final public function getMulti(
201 array $keys, &$curTTLs = array(), array $checkKeys = array()
206 $vPrefixLen = strlen( self
::VALUE_KEY_PREFIX
);
207 $valueKeys = self
::prefixCacheKeys( $keys, self
::VALUE_KEY_PREFIX
);
208 $checkKeys = self
::prefixCacheKeys( $checkKeys, self
::TIME_KEY_PREFIX
);
210 // Fetch all of the raw values
211 $wrappedValues = $this->cache
->getMulti( array_merge( $valueKeys, $checkKeys ) );
212 $now = microtime( true );
214 // Get/initialize the timestamp of all the "check" keys
215 $checkKeyTimes = array();
216 foreach ( $checkKeys as $checkKey ) {
217 $timestamp = isset( $wrappedValues[$checkKey] )
218 ? self
::parsePurgeValue( $wrappedValues[$checkKey] )
220 if ( !is_float( $timestamp ) ) {
221 // Key is not set or invalid; regenerate
222 $this->cache
->add( $checkKey,
223 self
::PURGE_VAL_PREFIX
. $now, self
::CHECK_KEY_TTL
);
227 $checkKeyTimes[] = $timestamp;
230 // Get the main cache value for each key and validate them
231 foreach ( $valueKeys as $vKey ) {
232 if ( !isset( $wrappedValues[$vKey] ) ) {
233 continue; // not found
236 $key = substr( $vKey, $vPrefixLen ); // unprefix
238 list( $value, $curTTL ) = $this->unwrap( $wrappedValues[$vKey], $now );
239 if ( $value !== false ) {
240 $result[$key] = $value;
241 foreach ( $checkKeyTimes as $checkKeyTime ) {
242 // Force dependant keys to be invalid for a while after purging
243 // to reduce race conditions involving stale data getting cached
244 $safeTimestamp = $checkKeyTime + self
::HOLDOFF_TTL
;
245 if ( $safeTimestamp >= $wrappedValues[$vKey][self
::FLD_TIME
] ) {
246 $curTTL = min( $curTTL, $checkKeyTime - $now );
251 $curTTLs[$key] = $curTTL;
258 * Set the value of a key from cache
260 * Simply calling this method when source data changes is not valid because
261 * the changes do not replicate to the other WAN sites. In that case, delete()
262 * should be used instead. This method is intended for use on cache misses.
264 * If the data was read from a snapshot-isolated transactions (e.g. the default
265 * REPEATABLE-READ in innoDB), use 'since' to avoid the following race condition:
267 * - b) T2 updates a row, calls delete(), and commits
268 * - c) The HOLDOFF_TTL passes, expiring the delete() tombstone
269 * - d) T1 reads the row and calls set() due to a cache miss
270 * - e) Stale value is stuck in cache
272 * Setting 'lag' helps avoids keys getting stuck in long-term stale states.
276 * $dbr = wfGetDB( DB_SLAVE );
277 * $setOpts = Database::getCacheSetOptions( $dbr );
278 * // Fetch the row from the DB
279 * $row = $dbr->selectRow( ... );
280 * $key = wfMemcKey( 'building', $buildingId );
281 * $cache->set( $key, $row, 86400, $setOpts );
284 * @param string $key Cache key
285 * @param mixed $value
286 * @param integer $ttl Seconds to live [0=forever]
287 * @param array $opts Options map:
288 * - lag : Seconds of slave lag. Typically, this is either the slave lag
289 * before the data was read or, if applicable, the slave lag before
290 * the snapshot-isolated transaction the data was read from started.
291 * [Default: 0 seconds]
292 * - since : UNIX timestamp of the data in $value. Typically, this is either
293 * the current time the data was read or (if applicable) the time when
294 * the snapshot-isolated transaction the data was read from started.
295 * [Default: 0 seconds]
296 * - lockTSE : if excessive possible snapshot lag is detected,
297 * then stash the value into a temporary location
298 * with this TTL. This is only useful if the reads
299 * use getWithSetCallback() with "lockTSE" set.
300 * [Default: WANObjectCache::TSE_NONE]
301 * @return bool Success
303 final public function set( $key, $value, $ttl = 0, array $opts = array() ) {
304 $lockTSE = isset( $opts['lockTSE'] ) ?
$opts['lockTSE'] : self
::TSE_NONE
;
305 $age = isset( $opts['since'] ) ?
max( 0, microtime( true ) - $opts['since'] ) : 0;
306 $lag = isset( $opts['lag'] ) ?
$opts['lag'] : 0;
308 if ( $lag > self
::MAX_REPLICA_LAG
) {
309 // Too much lag detected; lower TTL so it converges faster
310 $ttl = $ttl ?
min( $ttl, self
::TTL_LAGGED
) : self
::TTL_LAGGED
;
313 if ( $age > self
::MAX_SNAPSHOT_LAG
) {
314 if ( $lockTSE >= 0 ) {
315 $tempTTL = max( 1, (int)$lockTSE ); // set() expects seconds
316 $this->cache
->set( self
::STASH_KEY_PREFIX
. $key, $value, $tempTTL );
319 return true; // no-op the write for being unsafe
322 $wrapped = $this->wrap( $value, $ttl );
324 $func = function ( $cache, $key, $cWrapped ) use ( $wrapped ) {
325 return ( is_string( $cWrapped ) )
326 ?
false // key is tombstoned; do nothing
330 return $this->cache
->merge( self
::VALUE_KEY_PREFIX
. $key, $func, $ttl, 1 );
334 * Purge a key from all datacenters
336 * This should only be called when the underlying data (being cached)
337 * changes in a significant way. This deletes the key and starts a hold-off
338 * period where the key cannot be written to for a few seconds (HOLDOFF_TTL).
339 * This is done to avoid the following race condition:
340 * - a) Some DB data changes and delete() is called on a corresponding key
341 * - b) A request refills the key with a stale value from a lagged DB
342 * - c) The stale value is stuck there until the key is expired/evicted
344 * This is implemented by storing a special "tombstone" value at the cache
345 * key that this class recognizes; get() calls will return false for the key
346 * and any set() calls will refuse to replace tombstone values at the key.
347 * For this to always avoid writing stale values, the following must hold:
348 * - a) Replication lag is bounded to being less than HOLDOFF_TTL; or
349 * - b) If lag is higher, the DB will have gone into read-only mode already
351 * When using potentially long-running ACID transactions, a good pattern is
352 * to use a pre-commit hook to issue the delete. This means that immediately
353 * after commit, callers will see the tombstone in cache in the local datacenter
354 * and in the others upon relay. It also avoids the following race condition:
355 * - a) T1 begins, changes a row, and calls delete()
356 * - b) The HOLDOFF_TTL passes, expiring the delete() tombstone
357 * - c) T2 starts, reads the row and calls set() due to a cache miss
358 * - d) T1 finally commits
359 * - e) Stale value is stuck in cache
363 * $dbw->begin(); // start of request
364 * ... <execute some stuff> ...
365 * // Update the row in the DB
366 * $dbw->update( ... );
367 * $key = wfMemcKey( 'homes', $homeId );
368 * // Purge the corresponding cache entry just before committing
369 * $dbw->onTransactionPreCommitOrIdle( function() use ( $cache, $key ) {
370 * $cache->delete( $key );
372 * ... <execute some stuff> ...
373 * $dbw->commit(); // end of request
376 * If called twice on the same key, then the last hold-off TTL takes
377 * precedence. For idempotence, the $ttl should not vary for different
378 * delete() calls on the same key. Also note that lowering $ttl reduces
379 * the effective range of the 'lockTSE' parameter to getWithSetCallback().
381 * @param string $key Cache key
382 * @param integer $ttl How long to block writes to the key [seconds]
383 * @return bool True if the item was purged or not found, false on failure
385 final public function delete( $key, $ttl = self
::HOLDOFF_TTL
) {
386 $key = self
::VALUE_KEY_PREFIX
. $key;
387 // Avoid indefinite key salting for sanity
388 $ttl = max( $ttl, 1 );
389 // Update the local datacenter immediately
390 $ok = $this->cache
->set( $key, self
::PURGE_VAL_PREFIX
. microtime( true ), $ttl );
391 // Publish the purge to all datacenters
392 return $this->relayPurge( $key, $ttl ) && $ok;
396 * Fetch the value of a timestamp "check" key
398 * The key will be *initialized* to the current time if not set,
399 * so only call this method if this behavior is actually desired
401 * The timestamp can be used to check whether a cached value is valid.
402 * Callers should not assume that this returns the same timestamp in
403 * all datacenters due to relay delays.
405 * The level of staleness can roughly be estimated from this key, but
406 * if the key was evicted from cache, such calculations may show the
407 * time since expiry as ~0 seconds.
409 * Note that "check" keys won't collide with other regular keys
412 * @return float UNIX timestamp of the key
414 final public function getCheckKeyTime( $key ) {
415 $key = self
::TIME_KEY_PREFIX
. $key;
417 $time = self
::parsePurgeValue( $this->cache
->get( $key ) );
418 if ( $time === false ) {
419 // Casting assures identical floats for the next getCheckKeyTime() calls
420 $time = (string)microtime( true );
421 $this->cache
->add( $key, self
::PURGE_VAL_PREFIX
. $time, self
::CHECK_KEY_TTL
);
422 $time = (float)$time;
429 * Purge a "check" key from all datacenters, invalidating keys that use it
431 * This should only be called when the underlying data (being cached)
432 * changes in a significant way, and it is impractical to call delete()
433 * on all keys that should be changed. When get() is called on those
434 * keys, the relevant "check" keys must be supplied for this to work.
436 * The "check" key essentially represents a last-modified field.
437 * It is set in the future a few seconds when this is called, to
438 * avoid race conditions where dependent keys get updated with a
439 * stale value (e.g. from a DB slave).
441 * This is typically useful for keys with static names or some cases
442 * dynamically generated names where a low number of combinations exist.
443 * When a few important keys get a large number of hits, a high cache
444 * time is usually desired as well as lockTSE logic. The resetCheckKey()
445 * method is less appropriate in such cases since the "time since expiry"
446 * cannot be inferred.
448 * Note that "check" keys won't collide with other regular keys
450 * @see WANObjectCache::get()
452 * @param string $key Cache key
453 * @return bool True if the item was purged or not found, false on failure
455 final public function touchCheckKey( $key ) {
456 $key = self
::TIME_KEY_PREFIX
. $key;
457 // Update the local datacenter immediately
458 $ok = $this->cache
->set( $key,
459 self
::PURGE_VAL_PREFIX
. microtime( true ), self
::CHECK_KEY_TTL
);
460 // Publish the purge to all datacenters
461 return $this->relayPurge( $key, self
::CHECK_KEY_TTL
) && $ok;
465 * Delete a "check" key from all datacenters, invalidating keys that use it
467 * This is similar to touchCheckKey() in that keys using it via
468 * getWithSetCallback() will be invalidated. The differences are:
469 * - a) The timestamp will be deleted from all caches and lazily
470 * re-initialized when accessed (rather than set everywhere)
471 * - b) Thus, dependent keys will be known to be invalid, but not
472 * for how long (they are treated as "just" purged), which
473 * effects any lockTSE logic in getWithSetCallback()
475 * The advantage is that this does not place high TTL keys on every cache
476 * server, making it better for code that will cache many different keys
477 * and either does not use lockTSE or uses a low enough TTL anyway.
479 * This is typically useful for keys with dynamically generated names
480 * where a high number of combinations exist.
482 * Note that "check" keys won't collide with other regular keys
484 * @see WANObjectCache::touchCheckKey()
485 * @see WANObjectCache::get()
487 * @param string $key Cache key
488 * @return bool True if the item was purged or not found, false on failure
490 final public function resetCheckKey( $key ) {
491 $key = self
::TIME_KEY_PREFIX
. $key;
492 // Update the local datacenter immediately
493 $ok = $this->cache
->delete( $key );
494 // Publish the purge to all datacenters
495 return $this->relayDelete( $key ) && $ok;
499 * Method to fetch/regenerate cache keys
501 * On cache miss, the key will be set to the callback result via set()
502 * unless the callback returns false. The arguments supplied to it are:
503 * (current value or false, &$ttl, &$setOpts)
504 * The callback function returns the new value given the current
505 * value (false if not present). Preemptive re-caching and $checkKeys
506 * can result in a non-false current value. The TTL of the new value
507 * can be set dynamically by altering $ttl in the callback (by reference).
508 * The $setOpts array can be altered and is given to set() when called;
509 * it is recommended to set the 'since' field to avoid race conditions.
510 * Setting 'lag' helps avoids keys getting stuck in long-term stale states.
512 * Usually, callbacks ignore the current value, but it can be used
513 * to maintain "most recent X" values that come from time or sequence
514 * based source data, provided that the "as of" id/time is tracked.
516 * Usage of $checkKeys is similar to get() and getMulti(). However,
517 * rather than the caller having to inspect a "current time left"
518 * variable (e.g. $curTTL, $curTTLs), a cache regeneration will be
519 * triggered using the callback.
521 * The simplest way to avoid stampedes for hot keys is to use
522 * the 'lockTSE' option in $opts. If cache purges are needed, also:
523 * - a) Pass $key into $checkKeys
524 * - b) Use touchCheckKey( $key ) instead of delete( $key )
526 * Example usage (typical key):
528 * $catInfo = $cache->getWithSetCallback(
529 * // Key to store the cached value under
530 * wfMemcKey( 'cat-attributes', $catId ),
531 * // Time-to-live (seconds)
533 * // Function that derives the new key value
534 * function ( $oldValue, &$ttl, array &$setOpts ) {
535 * $dbr = wfGetDB( DB_SLAVE );
536 * // Account for any snapshot/slave lag
537 * $setOpts += Database::getCacheSetOptions( $dbr );
539 * return $dbr->selectRow( ... );
544 * Example usage (key that is expensive and hot):
546 * $catConfig = $cache->getWithSetCallback(
547 * // Key to store the cached value under
548 * wfMemcKey( 'site-cat-config' ),
549 * // Time-to-live (seconds)
551 * // Function that derives the new key value
552 * function ( $oldValue, &$ttl, array &$setOpts ) {
553 * $dbr = wfGetDB( DB_SLAVE );
554 * // Account for any snapshot/slave lag
555 * $setOpts += Database::getCacheSetOptions( $dbr );
557 * return CatConfig::newFromRow( $dbr->selectRow( ... ) );
559 * // Calling touchCheckKey() on this key invalidates the cache
561 * 'checkKeys' => array( wfMemcKey( 'site-cat-config' ) ),
562 * // Try to only let one datacenter thread manage cache updates at a time
568 * Example usage (key with dynamic dependencies):
570 * $catState = $cache->getWithSetCallback(
571 * // Key to store the cached value under
572 * wfMemcKey( 'cat-state', $cat->getId() ),
573 * // Time-to-live (seconds)
575 * // Function that derives the new key value
576 * function ( $oldValue, &$ttl, array &$setOpts ) {
577 * // Determine new value from the DB
578 * $dbr = wfGetDB( DB_SLAVE );
579 * // Account for any snapshot/slave lag
580 * $setOpts += Database::getCacheSetOptions( $dbr );
582 * return CatState::newFromResults( $dbr->select( ... ) );
584 * // The "check" keys that represent things the value depends on;
585 * // Calling touchCheckKey() on any of them invalidates the cache
587 * 'checkKeys' => array(
588 * wfMemcKey( 'sustenance-bowls', $cat->getRoomId() ),
589 * wfMemcKey( 'people-present', $cat->getHouseId() ),
590 * wfMemcKey( 'cat-laws', $cat->getCityId() ),
596 * Example usage (hot key holding most recent 100 events):
598 * $lastCatActions = $cache->getWithSetCallback(
599 * // Key to store the cached value under
600 * wfMemcKey( 'cat-last-actions', 100 ),
601 * // Time-to-live (seconds)
603 * // Function that derives the new key value
604 * function ( $oldValue, &$ttl, array &$setOpts ) {
605 * $dbr = wfGetDB( DB_SLAVE );
606 * // Account for any snapshot/slave lag
607 * $setOpts += Database::getCacheSetOptions( $dbr );
609 * // Start off with the last cached list
610 * $list = $oldValue ?: array();
611 * // Fetch the last 100 relevant rows in descending order;
612 * // only fetch rows newer than $list[0] to reduce scanning
613 * $rows = iterator_to_array( $dbr->select( ... ) );
614 * // Merge them and get the new "last 100" rows
615 * return array_slice( array_merge( $new, $list ), 0, 100 );
617 * // Try to only let one datacenter thread manage cache updates at a time
618 * array( 'lockTSE' => 30 )
622 * @see WANObjectCache::get()
623 * @see WANObjectCache::set()
625 * @param string $key Cache key
626 * @param integer $ttl Seconds to live for key updates. Special values are:
627 * - WANObjectCache::TTL_NONE : Cache forever
628 * - WANObjectCache::TTL_UNCACHEABLE: Do not cache at all
629 * @param callable $callback Value generation function
630 * @param array $opts Options map:
631 * - checkKeys: List of "check" keys.
632 * - lowTTL: Consider pre-emptive updates when the current TTL (sec) of the key is less than
633 * this. It becomes more likely over time, becoming a certainty once the key is expired.
634 * Default: WANObjectCache::LOW_TTL seconds.
635 * - lockTSE: If the key is tombstoned or expired (by checkKeys) less than this many seconds
636 * ago, then try to have a single thread handle cache regeneration at any given time.
637 * Other threads will try to use stale values if possible. If, on miss, the time since
638 * expiration is low, the assumption is that the key is hot and that a stampede is worth
639 * avoiding. Setting this above WANObjectCache::HOLDOFF_TTL makes no difference. The
640 * higher this is set, the higher the worst-case staleness can be.
641 * Use WANObjectCache::TSE_NONE to disable this logic. Default: WANObjectCache::TSE_NONE.
642 * @return mixed Value to use for the key
644 final public function getWithSetCallback(
645 $key, $ttl, $callback, array $opts = array(), $oldOpts = array()
647 // Back-compat with 1.26: Swap $ttl and $callback
648 if ( is_int( $callback ) ) {
653 // Back-compat with 1.26: $checkKeys as separate parameter
654 if ( $oldOpts ||
( is_array( $opts ) && isset( $opts[0] ) ) ) {
658 $checkKeys = isset( $opts['checkKeys'] ) ?
$opts['checkKeys'] : array();
661 $lowTTL = isset( $opts['lowTTL'] ) ?
$opts['lowTTL'] : min( self
::LOW_TTL
, $ttl );
662 $lockTSE = isset( $opts['lockTSE'] ) ?
$opts['lockTSE'] : self
::TSE_NONE
;
664 // Get the current key value
666 $cValue = $this->get( $key, $curTTL, $checkKeys ); // current value
667 $value = $cValue; // return value
669 // Determine if a regeneration is desired
670 if ( $value !== false && $curTTL > 0 && !$this->worthRefresh( $curTTL, $lowTTL ) ) {
674 // A deleted key with a negative TTL left must be tombstoned
675 $isTombstone = ( $curTTL !== null && $value === false );
676 // Assume a key is hot if requested soon after invalidation
677 $isHot = ( $curTTL !== null && $curTTL <= 0 && abs( $curTTL ) <= $lockTSE );
678 // Decide whether a single thread should handle regenerations.
679 // This avoids stampedes when $checkKeys are bumped and when preemptive
680 // renegerations take too long. It also reduces regenerations while $key
681 // is tombstoned. This balances cache freshness with avoiding DB load.
682 $useMutex = ( $isHot ||
( $isTombstone && $lockTSE > 0 ) );
684 $lockAcquired = false;
686 // Acquire a datacenter-local non-blocking lock
687 if ( $this->cache
->lock( $key, 0, self
::LOCK_TTL
) ) {
688 // Lock acquired; this thread should update the key
689 $lockAcquired = true;
690 } elseif ( $value !== false ) {
691 // If it cannot be acquired; then the stale value can be used
694 // Use the stash value for tombstoned keys to reduce regeneration load.
695 // For hot keys, either another thread has the lock or the lock failed;
696 // use the stash value from the last thread that regenerated it.
697 $value = $this->cache
->get( self
::STASH_KEY_PREFIX
. $key );
698 if ( $value !== false ) {
704 if ( !is_callable( $callback ) ) {
705 throw new InvalidArgumentException( "Invalid cache miss callback provided." );
708 // Generate the new value from the callback...
710 $value = call_user_func_array( $callback, array( $cValue, &$ttl, &$setOpts ) );
711 // When delete() is called, writes are write-holed by the tombstone,
712 // so use a special stash key to pass the new value around threads.
713 if ( $useMutex && $value !== false && $ttl >= 0 ) {
714 $tempTTL = max( 1, (int)$lockTSE ); // set() expects seconds
715 $this->cache
->set( self
::STASH_KEY_PREFIX
. $key, $value, $tempTTL );
718 if ( $lockAcquired ) {
719 $this->cache
->unlock( $key );
722 if ( $value !== false && $ttl >= 0 ) {
723 // Update the cache; this will fail if the key is tombstoned
724 $setOpts['lockTSE'] = $lockTSE;
725 $this->set( $key, $value, $ttl, $setOpts );
732 * Get the "last error" registered; clearLastError() should be called manually
733 * @return int ERR_* constant for the "last error" registry
735 final public function getLastError() {
736 if ( $this->lastRelayError
) {
737 // If the cache and the relayer failed, focus on the later.
738 // An update not making it to the relayer means it won't show up
739 // in other DCs (nor will consistent re-hashing see up-to-date values).
740 // On the other hand, if just the cache update failed, then it should
741 // eventually be applied by the relayer.
742 return $this->lastRelayError
;
745 $code = $this->cache
->getLastError();
747 case BagOStuff
::ERR_NONE
:
748 return self
::ERR_NONE
;
749 case BagOStuff
::ERR_NO_RESPONSE
:
750 return self
::ERR_NO_RESPONSE
;
751 case BagOStuff
::ERR_UNREACHABLE
:
752 return self
::ERR_UNREACHABLE
;
754 return self
::ERR_UNEXPECTED
;
759 * Clear the "last error" registry
761 final public function clearLastError() {
762 $this->cache
->clearLastError();
763 $this->lastRelayError
= self
::ERR_NONE
;
767 * Do the actual async bus purge of a key
769 * This must set the key to "PURGED:<UNIX timestamp>"
771 * @param string $key Cache key
772 * @param integer $ttl How long to keep the tombstone [seconds]
773 * @return bool Success
775 protected function relayPurge( $key, $ttl ) {
776 $event = $this->cache
->modifySimpleRelayEvent( array(
779 'val' => 'PURGED:$UNIXTIME$',
780 'ttl' => max( $ttl, 1 ),
781 'sbt' => true, // substitute $UNIXTIME$ with actual microtime
784 $ok = $this->relayer
->notify( "{$this->pool}:purge", $event );
786 $this->lastRelayError
= self
::ERR_RELAY
;
793 * Do the actual async bus delete of a key
795 * @param string $key Cache key
796 * @return bool Success
798 protected function relayDelete( $key ) {
799 $event = $this->cache
->modifySimpleRelayEvent( array(
804 $ok = $this->relayer
->notify( "{$this->pool}:purge", $event );
806 $this->lastRelayError
= self
::ERR_RELAY
;
813 * Check if a key should be regenerated (using random probability)
815 * This returns false if $curTTL >= $lowTTL. Otherwise, the chance
816 * of returning true increases steadily from 0% to 100% as the $curTTL
817 * moves from $lowTTL to 0 seconds. This handles widely varying
818 * levels of cache access traffic.
820 * @param float $curTTL Approximate TTL left on the key if present
821 * @param float $lowTTL Consider a refresh when $curTTL is less than this
824 protected function worthRefresh( $curTTL, $lowTTL ) {
825 if ( $curTTL >= $lowTTL ) {
827 } elseif ( $curTTL <= 0 ) {
831 $chance = ( 1 - $curTTL / $lowTTL );
833 return mt_rand( 1, 1e9
) <= 1e9
* $chance;
837 * Do not use this method outside WANObjectCache
839 * @param mixed $value
840 * @param integer $ttl [0=forever]
843 protected function wrap( $value, $ttl ) {
845 self
::FLD_VERSION
=> self
::VERSION
,
846 self
::FLD_VALUE
=> $value,
847 self
::FLD_TTL
=> $ttl,
848 self
::FLD_TIME
=> microtime( true )
853 * Do not use this method outside WANObjectCache
855 * @param array|string|bool $wrapped
856 * @param float $now Unix Current timestamp (preferrable pre-query)
857 * @return array (mixed; false if absent/invalid, current time left)
859 protected function unwrap( $wrapped, $now ) {
860 // Check if the value is a tombstone
861 $purgeTimestamp = self
::parsePurgeValue( $wrapped );
862 if ( is_float( $purgeTimestamp ) ) {
863 // Purged values should always have a negative current $ttl
864 $curTTL = min( -0.000001, $purgeTimestamp - $now );
865 return array( false, $curTTL );
868 if ( !is_array( $wrapped ) // not found
869 ||
!isset( $wrapped[self
::FLD_VERSION
] ) // wrong format
870 ||
$wrapped[self
::FLD_VERSION
] !== self
::VERSION
// wrong version
872 return array( false, null );
875 if ( $wrapped[self
::FLD_TTL
] > 0 ) {
876 // Get the approximate time left on the key
877 $age = $now - $wrapped[self
::FLD_TIME
];
878 $curTTL = max( $wrapped[self
::FLD_TTL
] - $age, 0.0 );
880 // Key had no TTL, so the time left is unbounded
884 return array( $wrapped[self
::FLD_VALUE
], $curTTL );
889 * @param string $prefix
892 protected static function prefixCacheKeys( array $keys, $prefix ) {
894 foreach ( $keys as $key ) {
895 $res[] = $prefix . $key;
902 * @param string $value String like "PURGED:<timestamp>"
903 * @return float|bool UNIX timestamp or false on failure
905 protected static function parsePurgeValue( $value ) {
907 if ( is_string( $value ) &&
908 preg_match( '/^' . self
::PURGE_VAL_PREFIX
. '([^:]+)$/', $value, $m )