objectcache: improve comments for ATTR_/QOS_ constants

Change-Id: Iaa5361a09e1dc559fee8eb034c9c05bc999c776e

diff --git a/includes/libs/objectcache/IExpiringStore.php b/includes/libs/objectcache/IExpiringStore.php
index c1edff7..61a4c61 100644 (file)
--- a/includes/libs/objectcache/IExpiringStore.php
+++ b/includes/libs/objectcache/IExpiringStore.php
@@ -21,7 +21,7 @@
  */
 
 /**
- * Generic base class for storage interfaces.
+ * Generic interface for lightweight expiring object stores.
  *
  * Provides convenient TTL constants.
  *
@@ -44,16 +44,19 @@ interface IExpiringStore {
 
        const TTL_INDEFINITE = 0;
 
-       // Attribute and QoS constants; higher QOS values with the same prefix rank higher...
-       // Medium attributes constants related to emulation or media type
+       // Emulation/fallback medium attribute (e.g. SQLBagOStuff)
        const ATTR_EMULATION = 1;
+       // Quality of service constants for ATTR_EMULATION (higher means faster)
        const QOS_EMULATION_SQL = 1;
-       // Medium attributes constants related to replica consistency
-       const ATTR_SYNCWRITES = 2; // SYNC_WRITES flag support
+
+       // Replica synchronization/consistency attribute of medium when SYNC_WRITES is used
+       const ATTR_SYNCWRITES = 2;
+       // Quality of service constants for ATTR_SYNCWRITES (higher means more consistent)
        const QOS_SYNCWRITES_NONE = 1; // replication only supports eventual consistency or less
        const QOS_SYNCWRITES_BE = 2; // best effort synchronous with limited retries
        const QOS_SYNCWRITES_QC = 3; // write quorum applied directly to state machines where R+W > N
        const QOS_SYNCWRITES_SS = 4; // strict-serializable, nodes refuse reads if possible stale
+
        // Generic "unknown" value that is useful for comparisons (e.g. always good enough)
        const QOS_UNKNOWN = INF;