Aaron Schulz has uploaded a new change for review. (
https://gerrit.wikimedia.org/r/394525 )
Change subject: objectcache: add example use case to
WANObjectCache::getMultiCheckKeyTime()
......................................................................
objectcache: add example use case to WANObjectCache::getMultiCheckKeyTime()
Change-Id: I874070d2f770be7f9e06c381b2413f6e133418b8
---
M includes/libs/objectcache/WANObjectCache.php
1 file changed, 45 insertions(+), 0 deletions(-)
git pull ssh://gerrit.wikimedia.org:29418/mediawiki/core
refs/changes/25/394525/1
diff --git a/includes/libs/objectcache/WANObjectCache.php
b/includes/libs/objectcache/WANObjectCache.php
index 0bdfed3..f157cc9 100644
--- a/includes/libs/objectcache/WANObjectCache.php
+++ b/includes/libs/objectcache/WANObjectCache.php
@@ -598,7 +598,52 @@
* This works like getCheckKeyTime() except it takes a list of keys
* and returns a list of timestamps instead of just that of one key
*
+ * This might be useful when an entity depends on hundreds of other
other entities,
+ * which themselves are used by millions of entities. The later can use
"check" keys
+ * to invalidate their many dependee entities. However, it might be too
expensive for the
+ * former to fetch all the "check" keys of the entities it depends on
within every single
+ * getWithSetCallback() call. In this case the "check" keys can be
fetched and compared to
+ * only after a "time-till-verify" has passed. The verifications are
middle ground between
+ * a blind cache use and a full value computation.
+ *
+ * Example usage:
+ * @code
+ * $value = $cache->getWithSetCallback(
+ * $cache->makeGlobalKey( 'wikibase-item', $id ),
+ * self::INITIAL_TTV, // initial time-till-verify
+ * function ( $oldValue, &$ttv, &$setOpts, $oldAsOf ) use (
$checkKeys, $cache ) {
+ * $now = microtime( true );
+ * // Use $oldValue if it passes max ultimate age and
"check" key comparisons
+ * if ( $oldValue &&
+ * $oldAsOf > max( $cache->getCheckKeyTimeMulti(
$checkKeys ) ) &&
+ * ( $now - $oldValue['ctime'] ) <= self::MAX_CACHE_AGE
+ * ) {
+ * // Increase time-till-verify by 50% of last time to
reduce overhead
+ * $ttv = $cache->adaptiveTTL( $oldAsOf, self::MAX_TTV,
self::MIN_TTV, 1.5 );
+ * // Unlike $oldAsOf, "ctime" is the ultimate age of
the cached data
+ * return $oldValue;
+ * }
+ *
+ * $mtimes = []; // mtimes of dependencies; passed by
reference
+ * $value = [ 'data' => $this->computeValue( $mtimes ),
'ctime' => $now ];
+ * // Guess time-till-change among the dependencies, e.g.
1/(total change rate)
+ * $ttc = 1 / array_sum( array_map(
+ * function ( $mtime ) use ( $now ) {
+ * return 1 / ( $mtime ? ( $now - $mtime ) : 900 );
+ * },
+ * $mtimes
+ * ) );
+ * // The time-to-verify should not be overly pessimistic
nor optimistic
+ * $ttv = min( max( $ttc, self::MIN_TTV ), self::MAX_TTV );
+ *
+ * return $value;
+ * },
+ * [ 'staleTTL' => $cache::TTL_DAY ] // keep around to verify
and re-save
+ * );
+ * @endcode
+ *
* @see WANObjectCache::getCheckKeyTime()
+ * @see WANObjectCache::getWithSetCallback()
*
* @param array $keys
* @return float[] Map of (key => UNIX timestamps)
--
To view, visit https://gerrit.wikimedia.org/r/394525
To unsubscribe, visit https://gerrit.wikimedia.org/r/settings
Gerrit-MessageType: newchange
Gerrit-Change-Id: I874070d2f770be7f9e06c381b2413f6e133418b8
Gerrit-PatchSet: 1
Gerrit-Project: mediawiki/core
Gerrit-Branch: master
Gerrit-Owner: Aaron Schulz <asch...@wikimedia.org>
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