Aaron Schulz has uploaded a new change for review.
https://gerrit.wikimedia.org/r/235100
Change subject: Clarified WANObjectCache::delete() comments
......................................................................
Clarified WANObjectCache::delete() comments
Change-Id: I8c1cffa284cf92461045f9163ff1d1e9bf8c2244
---
M includes/libs/objectcache/WANObjectCache.php
1 file changed, 13 insertions(+), 5 deletions(-)
git pull ssh://gerrit.wikimedia.org:29418/mediawiki/core
refs/changes/00/235100/1
diff --git a/includes/libs/objectcache/WANObjectCache.php
b/includes/libs/objectcache/WANObjectCache.php
index a3bd531..59ed58c 100644
--- a/includes/libs/objectcache/WANObjectCache.php
+++ b/includes/libs/objectcache/WANObjectCache.php
@@ -256,11 +256,19 @@
/**
* Purge a key from all clusters
*
- * This instantiates a hold-off period where the key cannot be
- * written to avoid race conditions where dependent keys get updated
- * with a stale value (e.g. from a DB slave). 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.
+ * 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:
+ * a) Some DB data changes and delete() is called on a corresponding
key
+ * b) A request refills the key with a stale value from a lagged DB
+ * c) The stale value is stuck there until the key is expired/evicted
+ *
+ * This is implemented by storing a special "tombstone" value at the
cache
+ * key that this class recognizes; get() calls will return false for
the key
+ * and any set() calls will refuse to replace tombstone values at the
key.
+ * For this to always avoid writing stale values, the following must
hold:
+ * a) Replication lag is bounded to being less than HOLDOFF_TTL; or
+ * b) If lag is higher, the DB will have gone into read-only mode
already
*
* This should only be called when the underlying data (being cached)
* changes in a significant way. If called twice on the same key, then
--
To view, visit https://gerrit.wikimedia.org/r/235100
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Gerrit-MessageType: newchange
Gerrit-Change-Id: I8c1cffa284cf92461045f9163ff1d1e9bf8c2244
Gerrit-PatchSet: 1
Gerrit-Project: mediawiki/core
Gerrit-Branch: master
Gerrit-Owner: Aaron Schulz <[email protected]>
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