Added $wgDataCenterId/$wgDataCenterRoles

* This is used to set sticky DC cookies to avoid
  session replication lag (which also makes sure
  ChronologyProtector works)

Bug: T91816
Change-Id: I7bc2f8185a3c05cb3ca5ccc42d300eccffae48e1

RELEASE-NOTES-1.27

@@ -16,6 +16,9 @@ production.
   1000 for the latter) are now hard-coded.
 
 === New features in 1.27 ===
+* $wgDataCenterId and $wgDataCenterRoles where added, which will serve as
+  basic configuration settings needed for multi-datacenter setups.
+  $wgDataCenterUpdateStickTTL was also added.
 
 ==== External libraries ====
 

includes/DefaultSettings.php

@@ -1868,6 +1868,32 @@ $wgDBservers = false;
  */
 $wgLBFactoryConf = array( 'class' => 'LBFactorySimple' );
 
+/**
+ * The ID of the current data center
+ * @since 1.27
+ */
+$wgDataCenterId = 'default';
+
+/**
+ * Map of data center IDs to their role ("master" or "slave")
+ *
+ * Multiple data centers can be setup to handle MediaWiki, with HTTP
+ * POSTs routed to the master data center and GET/HEAD/OPTION routed to
+ * any data center (usually the closest to the end user). In such setups,
+ * this setting should be set to the appropriate value in the site
+ * config for each data center.
+ * @since 1.27
+ */
+$wgDataCenterRoles = array( 'default' => 'master' );
+
+/**
+ * After a state-changing request is done by a client, this determines
+ * how many seconds that client should keep using the master datacenter.
+ * This avoids unexpected stale or 404 responses due to replication lag.
+ * @since 1.27
+ */
+$wgDataCenterUpdateStickTTL = 10;
+
 /**
  * File to log database errors to
  */

includes/GlobalFunctions.php

@@ -3605,6 +3605,28 @@ function wfSplitWikiID( $wiki ) {
 	return $bits;
 }
 
+/**
+ * @see $wgDataCenterRoles
+ * @return string The current cluster ID
+ * @since 1.27
+ */
+function wfDataCenter() {
+	global $wgDataCenterId;
+
+	return $wgDataCenterId;
+}
+
+/**
+ * @see $wgDataCenterRoles
+ * @return string The current cluster role; one of (master/slave)
+ * @since 1.27
+ */
+function wfDataCenterRole() {
+	global $wgDataCenterId, $wgDataCenterRoles;
+
+	return $wgDataCenterRoles[$wgDataCenterId];
+}
+
 /**
  * Get a Database object.
  *

includes/MediaWiki.php

@@ -504,6 +504,14 @@ class MediaWiki {
 		$factory->shutdown();
 
 		wfDebug( __METHOD__ . ' completed; all transactions committed' );
+
+		// Set a cookie to tell all CDN edge nodes to "stick" the user to the
+		// DC that handles this POST request (e.g. the "master" data center)
+		$request = $this->context->getRequest();
+		if ( $request->wasPosted() && $factory->hasOrMadeRecentMasterChanges() ) {
+			$expires = time() + $this->config->get( 'DataCenterUpdateStickTTL' );
+			$request->response()->setCookie( 'UseDC', 'master', $expires );
+		}
 	}
 
 	/**

includes/db/loadbalancer/LBFactory.php

@@ -202,9 +202,9 @@ abstract class LBFactory {
 	}
 
 	/**
-	 * Detemine if any master connection has pending changes.
-	 * @since 1.23
+	 * Determine if any master connection has pending changes
 	 * @return bool
+	 * @since 1.23
 	 */
 	public function hasMasterChanges() {
 		$ret = false;
@@ -213,6 +213,19 @@ abstract class LBFactory {
 		} );
 		return $ret;
 	}
+
+	/**
+	 * Determine if any master connection has pending/written changes from this request
+	 * @return bool
+	 * @since 1.27
+	 */
+	public function hasOrMadeRecentMasterChanges() {
+		$ret = false;
+		$this->forEachLB( function ( LoadBalancer $lb ) use ( &$ret ) {
+			$ret = $ret || $lb->hasOrMadeRecentMasterChanges();
+		} );
+		return $ret;
+	}
 }
 
 /**