Merge "objectcache: add "staleTTL" into WANObjectCache::getWithSetCallback()"

[lhc/web/wiklou.git] / includes / libs / objectcache / WANObjectCache.php

diff --git a/includes/libs/objectcache/WANObjectCache.php b/includes/libs/objectcache/WANObjectCache.php
index 723ccc0..f5c561f 100644 (file)
--- a/includes/libs/objectcache/WANObjectCache.php
+++ b/includes/libs/objectcache/WANObjectCache.php
@@ -613,20 +613,22 @@ class WANObjectCache implements IExpiringStore, LoggerAwareInterface {
         * 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.
+        * The "check" key essentially represents a last-modified time of an entity.
+        * When the key is touched, the timestamp will be updated to the current time.
+        * Keys using the "check" key via get(), getMulti(), or getWithSetCallback() will
+        * be invalidated. The timestamp of "check" is treated as being HOLDOFF_TTL seconds
+        * in the future by get*() methods in order to avoid race conditions where keys are
+        * updated with stale values (e.g. from a DB replica DB).
+        *
+        * This method is typically useful for keys with hardcoded names or in some cases
+        * dynamically generated names, provided the number of such keys is modest. It sets a
+        * high TTL on the "check" key, making it possible to know the timestamp of the last
+        * change to the corresponding entities in most cases.
+        *
+        * 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.
         *
@@ -657,12 +659,9 @@ class WANObjectCache implements IExpiringStore, LoggerAwareInterface {
         *        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.
+        * The advantage here is that the "check" keys, which have high TTLs, will only
+        * be created when a get*() method actually uses that key. This is better when
+        * a large number of "check" keys are invalided in a short period of time.
         *
         * Note that "check" keys won't collide with other regular keys.
         *