From 793d01401c9718cce16328d424a9a10297da4ccd Mon Sep 17 00:00:00 2001
From: Aaron Schulz <aschulz@wikimedia.org>
Date: Wed, 29 Apr 2015 19:19:23 -0700
Subject: [PATCH] Added ObjectCache::getMainStashInstance() and $wgMainStash

* This provides factory/config more appropriate for stashes
* Cleaned up some neighboring config comments while at it

Bug: T88493
Bug: T97620
Change-Id: Id370ee50be6e493a4700c858df863a183abc05fd
---
 includes/DefaultSettings.php         | 22 +++++++++++++++++++--
 includes/objectcache/ObjectCache.php | 29 ++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+), 2 deletions(-)

diff --git a/includes/DefaultSettings.php b/includes/DefaultSettings.php
index 857d69ee51..2ea8b294b6 100644
--- a/includes/DefaultSettings.php
+++ b/includes/DefaultSettings.php
@@ -2160,7 +2160,7 @@ $wgObjectCaches = array(
 );
 
 /**
- * Main cache Wide-Area-Network cache type. This should be a cache with fast access,
+ * Main Wide-Area-Network cache type. This should be a cache with fast access,
  * but it may have limited space. By default, it is disabled, since the basic stock
  * cache is not fast enough to make it worthwhile. For single data-center setups, this can
  * simply be pointed to a cache in $wgWANObjectCaches that uses a local $wgObjectCaches
@@ -2171,7 +2171,8 @@ $wgObjectCaches = array(
  *                       a relayer (only matters if there are multiple data-centers)
  *   - CACHE_NONE:       Do not cache
  *   - (other):          A string may be used which identifies a cache
- *                       configuration in $wgWANObjectCaches.
+ *                       configuration in $wgWANObjectCaches
+ * @since 1.26
  */
 $wgMainWANCache = false;
 
@@ -2187,6 +2188,8 @@ $wgMainWANCache = false;
  * a cache identifier from $wgObjectCaches. The "relayerConfig" parameter is an
  * array used to construct an EventRelayer object. The "pool" parameter is a
  * string that is used as a PubSub channel prefix.
+ *
+ * @since 1.26
  */
 $wgWANObjectCaches = array(
 	CACHE_NONE => array(
@@ -2205,6 +2208,21 @@ $wgWANObjectCaches = array(
 	*/
 );
 
+/**
+ * Main object stash type. This should be a fast storage system for storing
+ * lightweight data like hit counters and user activity. Sites with multiple
+ * data-centers should have this use a store that replicates all writes. The
+ * store should have enough consistency for CAS operations to be usable.
+ *
+ * The options are:
+ *   - db:      Store cache objects in the DB
+ *   - (other): A string may be used which identifies a cache
+ *              configuration in $wgObjectCaches
+ *
+ * @since 1.26
+ */
+$wgMainStash = 'db';
+
 /**
  * The expiry time for the parser cache, in seconds.
  * The default is 86400 (one day).
diff --git a/includes/objectcache/ObjectCache.php b/includes/objectcache/ObjectCache.php
index 5da22f0fd2..7faf4bb6a8 100644
--- a/includes/objectcache/ObjectCache.php
+++ b/includes/objectcache/ObjectCache.php
@@ -26,6 +26,13 @@ use MediaWiki\Logger\LoggerFactory;
 /**
  * Functions to get cache objects
  *
+ * The word "cache" has two main dictionary meanings, and both
+ * are used in this factory class. They are:
+ *   - a) A place to store copies or computations on existing data
+ *     for higher access speeds (the computer science definition)
+ *   - b) A place to store lightweight data that is not canonically
+ *     stored anywhere else (e.g. a "hoard" of objects)
+ *
  * @ingroup Cache
  */
 class ObjectCache {
@@ -227,4 +234,26 @@ class ObjectCache {
 
 		return self::getWANInstance( $wgMainWANCache );
 	}
+
+	/**
+	 * Stash objects are BagOStuff instances suitable for storing light
+	 * weight data that is not canonically stored elsewhere (such as RDBMS).
+	 * Stashes should be configured to propagate changes to all data-centers.
+	 *
+	 * Callers should be prepared for:
+	 *   - a) Writes to be slower in non-"primary" (e.g. HTTP GET/HEAD only) DCs
+	 *   - b) Reads to be eventually consistent, e.g. for get()/getMulti()
+	 * In general, this means avoiding updates on idempotent HTTP requests and
+	 * avoiding an assumption of perfect serializability (or accepting anomalies).
+	 * Reads may be eventually consistent or data might rollback as nodes flap.
+	 *
+	 *
+	 * @return BagOStuff
+	 * @since 1.26
+	 */
+	static function getMainStashInstance() {
+		global $wgMainStash;
+
+		return self::getInstance( $wgMainStash );
+	}
 }
-- 
2.20.1