From 41d6bf368bda2d6c86ae4aadffb3428460b3e03c Mon Sep 17 00:00:00 2001
From: Brion Vibber <brion@users.mediawiki.org>
Date: Fri, 16 Sep 2005 12:00:23 +0000
Subject: [PATCH] * Correct blob caching to reduce redundant blob loads on
 backups

---
 RELEASE-NOTES                |  1 +
 includes/ExternalStoreDB.php | 65 ++++++++++++++++++++++++++++++------
 2 files changed, 55 insertions(+), 11 deletions(-)

diff --git a/RELEASE-NOTES b/RELEASE-NOTES
index 5f15b0b92c..6541a093ed 100644
--- a/RELEASE-NOTES
+++ b/RELEASE-NOTES
@@ -109,6 +109,7 @@ fully support the editing toolbar, but was found to be too confusing.
 * (bug 1423) LanguageJa.php update
 * Clean up duplicate code for selection of changeslist style
 * (bug 3475) anon contrib links on Special:Newpages
+* Correct blob caching to reduce redundant blob loads on backups
 
 
 === Caveats ===
diff --git a/includes/ExternalStoreDB.php b/includes/ExternalStoreDB.php
index 3c61726523..d07e23b4d1 100644
--- a/includes/ExternalStoreDB.php
+++ b/includes/ExternalStoreDB.php
@@ -11,20 +11,38 @@ require_once( 'LoadBalancer.php' );
 
 /** @package MediaWiki */
 
+/**
+ * External database storage will use one (or more) separate connection pools
+ * from what the main wiki uses. If we load many revisions, such as when doing
+ * bulk backups or maintenance, we want to keep them around over the lifetime
+ * of the script.
+ *
+ * Associative array of LoadBalancer objects, indexed by cluster name.
+ */
+global $wgExternalLoadBalancers;
+$wgExternalLoadBalancers = array();
+
+/**
+ * One-step cache variable to hold base blobs; operations that
+ * pull multiple revisions may often pull multiple times from
+ * the same blob. By keeping the last-used one open, we avoid
+ * redundant unserialization and decompression overhead.
+ */
+global $wgExternalBlobCache;
+$wgExternalBlobCache = array();
+
 class ExternalStoreDB {
-	var $loadBalancers = array();
-	
 	/**
 	 * Fetch data from given URL
 	 * @param string $url An url
 	 */
 
 	function &getLoadBalancer( $cluster ) {
-		global $wgExternalServers;
-		if ( !array_key_exists( $cluster, $this->loadBalancers ) ) {
-			$this->loadBalancers[$cluster] = LoadBalancer::newFromParams( $wgExternalServers[$cluster] );
+		global $wgExternalServers, $wgExternalLoadBalancers;
+		if ( !array_key_exists( $cluster, $wgExternalLoadBalancers ) ) {
+			$wgExternalLoadBalancers[$cluster] = LoadBalancer::newFromParams( $wgExternalServers[$cluster] );
 		}
-		return $this->loadBalancers[$cluster];
+		return $wgExternalLoadBalancers[$cluster];
 	}
 	
 	function &getSlave( $cluster ) {
@@ -51,14 +69,39 @@ class ExternalStoreDB {
 			$itemID = false;
 		}
 
-		$dbr =& $this->getSlave( $cluster );
-		$ret = $dbr->selectField( 'blobs', 'blob_text', array( 'blob_id' => $id ) ); 
+		$ret =& $this->fetchBlob( $cluster, $id, $itemID );
 
 		if ( $itemID !== false ) {
-			# Unserialise object and get item
-			$obj = unserialize( $ret );
-			$ret = $obj->getItem( $itemID );
+			return $ret->getItem( $itemID );
+		}
+		return $ret;
+	}
+	
+	/**
+	 * Fetch a blob item out of the database; a cache of the last-loaded
+	 * blob will be kept so that multiple loads out of a multi-item blob
+	 * can avoid redundant database access and decompression.
+	 * @return mixed
+	 * @access private
+	 */
+	function &fetchBlob( $cluster, $id, $itemID ) {
+		global $wgExternalBlobCache;
+		$cacheID = ( $itemID === false ) ? "$cluster/$id" : "$cluster/$id/";
+		if( isset( $wgExternalBlobCache[$cacheID] ) ) {
+			wfDebug( "ExternalStoreDB::fetchBlob cache hit on $cacheID\n" );
+			return $wgExternalBlobCache[$cacheID];
+		}
+		
+		wfDebug( "ExternalStoreDB::fetchBlob cache miss on $cacheID\n" );
+		
+		$dbr =& $this->getSlave( $cluster );
+		$ret = $dbr->selectField( 'blobs', 'blob_text', array( 'blob_id' => $id ) );
+		if( $itemID !== false ) {
+			// Unserialise object; caller extracts item
+			$ret = unserialize( $ret );
 		}
+		
+		$wgExternalBlobCache = array( $cacheID => &$ret );
 		return $ret;
 	}
 
-- 
2.20.1