From: Tim Starling <tstarling@wikimedia.org>
Date: Fri, 4 Jan 2013 09:23:26 +0000 (+1100)
Subject: Use consistent hashing for SqlBagOStuff servers
X-Git-Tag: 1.31.0-rc.0~21145
X-Git-Url: http://git.cyclocoop.org/%7B%24admin_url%7Dcompta/comptes/journal.php?a=commitdiff_plain;h=7c4198805ab93facdd1f2c4e1d647ecf4c1fa21b;p=lhc%2Fweb%2Fwiklou.git

Use consistent hashing for SqlBagOStuff servers

Also factor out consistent hash code from Redis and the new application
into a class called ArrayUtils. The name "ArrayUtils" is from
I0f4e7d7c, I expect that change will be rebased on top of this one at
some point.

Change-Id: I9375087f4d7a6e8e629d97bfb6b117d9cb7d1bcf
---

diff --git a/includes/ArrayUtils.php b/includes/ArrayUtils.php
new file mode 100644
index 0000000000..4ff31b863d
--- /dev/null
+++ b/includes/ArrayUtils.php
@@ -0,0 +1,34 @@
+<?php
+
+class ArrayUtils {
+	/**
+	 * Sort the given array in a pseudo-random order which depends only on the
+	 * given key and each element value. This is typically used for load
+	 * balancing between servers each with a local cache.
+	 *
+	 * Keys are preserved. The input array is modified in place.
+	 *
+	 * Note: Benchmarking on PHP 5.3 and 5.4 indicates that for small
+	 * strings, md5() is only 10% slower than hash('joaat',...) etc.,
+	 * since the function call overhead dominates. So there's not much
+	 * justification for breaking compatibility with installations
+	 * compiled with ./configure --disable-hash.
+	 * 
+	 * @param $array The array to sort
+	 * @param $key The string key
+	 * @param $separator A separator used to delimit the array elements and the
+	 *     key. This can be chosen to provide backwards compatibility with 
+	 *     various consistent hash implementations that existed before this
+	 *     function was introduced.
+	 */
+	static function consistentHashSort( &$array, $key, $separator = "\000" ) {
+		$hashes = array();
+		foreach ( $array as $elt ) {
+			$hashes[$elt] = md5( $elt . $separator . $key );
+		}
+		uasort( $array, function ( $a, $b ) use ( $hashes ) {
+			return strcmp( $hashes[$a], $hashes[$b] );
+		} );
+	}
+}
+
diff --git a/includes/AutoLoader.php b/includes/AutoLoader.php
index ecfe20801a..fd3459063a 100644
--- a/includes/AutoLoader.php
+++ b/includes/AutoLoader.php
@@ -33,6 +33,7 @@ $wgAutoloadLocalClasses = array(
 	'AjaxDispatcher' => 'includes/AjaxDispatcher.php',
 	'AjaxResponse' => 'includes/AjaxResponse.php',
 	'AlphabeticPager' => 'includes/Pager.php',
+	'ArrayUtils' => 'includes/ArrayUtils.php',
 	'Article' => 'includes/Article.php',
 	'AtomFeed' => 'includes/Feed.php',
 	'AuthPlugin' => 'includes/AuthPlugin.php',
diff --git a/includes/objectcache/RedisBagOStuff.php b/includes/objectcache/RedisBagOStuff.php
index 40784f547c..4715859119 100644
--- a/includes/objectcache/RedisBagOStuff.php
+++ b/includes/objectcache/RedisBagOStuff.php
@@ -288,23 +288,10 @@ class RedisBagOStuff extends BagOStuff {
 		if ( count( $this->servers ) === 1 ) {
 			$candidates = $this->servers;
 		} else {
-			// Use consistent hashing
-			//
-			// Note: Benchmarking on PHP 5.3 and 5.4 indicates that for small
-			// strings, md5() is only 10% slower than hash('joaat',...) etc.,
-			// since the function call overhead dominates. So there's not much
-			// justification for breaking compatibility with installations
-			// compiled with ./configure --disable-hash.
-			$hashes = array();
-			foreach ( $this->servers as $server ) {
-				$hashes[$server] = md5( $server . '/' . $key );
-			}
-			asort( $hashes );
+			$candidates = $this->servers;
+			ArrayUtils::consistentHashSort( $candidates, $key, '/' );
 			if ( !$this->automaticFailover ) {
-				reset( $hashes );
-				$candidates = array( key( $hashes ) );
-			} else {
-				$candidates = array_keys( $hashes );
+				$candidates = array_slice( $candidates, 0, 1 );
 			}
 		}
 
diff --git a/includes/objectcache/SqlBagOStuff.php b/includes/objectcache/SqlBagOStuff.php
index 653fb15e73..eccfe00521 100644
--- a/includes/objectcache/SqlBagOStuff.php
+++ b/includes/objectcache/SqlBagOStuff.php
@@ -33,6 +33,7 @@ class SqlBagOStuff extends BagOStuff {
 	var $lb;
 
 	var $serverInfos;
+	var $serverNames;
 	var $numServers;
 	var $conns;
 	var $lastExpireAll = 0;
@@ -75,6 +76,10 @@ class SqlBagOStuff extends BagOStuff {
 		if ( isset( $params['servers'] ) ) {
 			$this->serverInfos = $params['servers'];
 			$this->numServers = count( $this->serverInfos );
+			$this->serverNames = array();
+			foreach ( $this->serverInfos as $i => $info ) {
+				$this->serverNames[$i] = isset( $info['host'] ) ? $info['host'] : "#$i";
+			}
 		} elseif ( isset( $params['server'] ) ) {
 			$this->serverInfos = array( $params['server'] );
 			$this->numServers = count( $this->serverInfos );
@@ -154,16 +159,21 @@ class SqlBagOStuff extends BagOStuff {
 	 * @return Array: server index and table name
 	 */
 	protected function getTableByKey( $key ) {
-		$numTables = $this->shards * $this->numServers ;
-		if ( $numTables > 1 ) {
-			$hash = ( hexdec( substr( md5( $key ), 0, 8 ) ) & 0x7fffffff ) % $numTables;
+		if ( $this->shards > 1 ) {
+			$hash = hexdec( substr( md5( $key ), 0, 8 ) ) & 0x7fffffff;
 			$tableIndex = $hash % $this->shards;
-			$serverIndex = intval( round( ( $hash - $tableIndex ) / $this->shards ) );
-			$tableName = $this->getTableNameByShard( $tableIndex );
-			return array( $serverIndex, $tableName );
 		} else {
-			return array( 0, $this->tableName );
+			$tableIndex = 0;
+		}
+		if ( $this->numServers > 1 ) {
+			$sortedServers = $this->serverNames;
+			ArrayUtils::consistentHashSort( $sortedServers, $key );
+			reset( $sortedServers );
+			$serverIndex = key( $sortedServers );
+		} else {
+			$serverIndex = 0;
 		}
+		return array( $serverIndex, $this->getTableNameByShard( $tableIndex ) );
 	}
 
 	/**