jenkins-bot has submitted this change and it was merged.
Change subject: Small cleanups to WANObjectCache
......................................................................
Small cleanups to WANObjectCache
* Added a few comments
* Renamed $locked => $lockAcquired for clarity
Change-Id: I45710974971731205d072a1f4b0f9cb37e2cb2a2
---
M includes/libs/objectcache/WANObjectCache.php
1 file changed, 14 insertions(+), 10 deletions(-)
Approvals:
Legoktm: Looks good to me, approved
jenkins-bot: Verified
diff --git a/includes/libs/objectcache/WANObjectCache.php
b/includes/libs/objectcache/WANObjectCache.php
index 56c544f..130caeb 100644
--- a/includes/libs/objectcache/WANObjectCache.php
+++ b/includes/libs/objectcache/WANObjectCache.php
@@ -260,9 +260,10 @@
/**
* Purge a key from all clusters
*
- * This deletes the key and instantiates a hold-off period where the key
- * cannot be written to for the next few seconds (HOLDOFF_TTL). This is
to
- * avoid the following race condition:
+ * 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
@@ -274,9 +275,10 @@
* 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
*
- * This should only be called when the underlying data (being cached)
- * changes in a significant way. If called twice on the same key, then
- * the last TTL takes precedence.
+ * 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]
@@ -469,6 +471,7 @@
* 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.
* - tempTTL : TTL of the temp key used to cache values while a key
is tombstoned.
* This avoids excessive regeneration of hot keys on
delete() but may
* result in stale values.
@@ -491,23 +494,24 @@
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 );
- $locked = false;
+ $lockAcquired = false;
if ( $isHot ) {
// Acquire a cluster-local non-blocking lock
if ( $this->cache->lock( $key, 0, self::LOCK_TTL ) ) {
// Lock acquired; this thread should update the
key
- $locked = true;
+ $lockAcquired = true;
} elseif ( $value !== false ) {
// If it cannot be acquired; then the stale
value can be used
return $value;
}
}
- if ( !$locked && ( $isTombstone || $isHot ) ) {
+ if ( !$lockAcquired && ( $isTombstone || $isHot ) ) {
// 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.
@@ -529,7 +533,7 @@
$this->cache->set( self::STASH_KEY_PREFIX . $key,
$value, $tempTTL );
}
- if ( $locked ) {
+ if ( $lockAcquired ) {
$this->cache->unlock( $key );
}
--
To view, visit https://gerrit.wikimedia.org/r/239150
To unsubscribe, visit https://gerrit.wikimedia.org/r/settings
Gerrit-MessageType: merged
Gerrit-Change-Id: I45710974971731205d072a1f4b0f9cb37e2cb2a2
Gerrit-PatchSet: 1
Gerrit-Project: mediawiki/core
Gerrit-Branch: master
Gerrit-Owner: Aaron Schulz <[email protected]>
Gerrit-Reviewer: Legoktm <[email protected]>
Gerrit-Reviewer: jenkins-bot <>
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