wiki.techinc.nl/includes/db/ChronologyProtector.php

<?php
/**
 * Generator of database load balancing objects.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup Database
 */

/**
 * Class for ensuring a consistent ordering of events as seen by the user, despite replication.
 * Kind of like Hawking's [[Chronology Protection Agency]].
 */
class ChronologyProtector {
	/** @var BagOStuff */
	protected $store;

	/** @var string Storage key name */
	protected $key;
	/** @var array Map of (ip: <IP>, agent: <user-agent>) */
	protected $client;
	/** @var bool Whether to no-op all method calls */
	protected $enabled = true;
	/** @var bool Whether to check and wait on positions */
	protected $wait = true;

	/** @var bool Whether the client data was loaded */
	protected $initialized = false;
	/** @var DBMasterPos[] Map of (DB master name => position) */
	protected $startupPositions = [];
	/** @var DBMasterPos[] Map of (DB master name => position) */
	protected $shutdownPositions = [];

	/**
	 * @param BagOStuff $store
	 * @param array $client Map of (ip: <IP>, agent: <user-agent>)
	 * @since 1.27
	 */
	public function __construct( BagOStuff $store, array $client ) {
		$this->store = $store;
		$this->client = $client;
		$this->key = $store->makeGlobalKey(
			'ChronologyProtector',
			md5( $client['ip'] . "\n" . $client['agent'] )
		);
	}

	/**
	 * @param bool $enabled Whether to no-op all method calls
	 * @since 1.27
	 */
	public function setEnabled( $enabled ) {
		$this->enabled = $enabled;
	}

	/**
	 * @param bool $enabled Whether to check and wait on positions
	 * @since 1.27
	 */
	public function setWaitEnabled( $enabled ) {
		$this->wait = $enabled;
	}

	/**
	 * Initialise a LoadBalancer to give it appropriate chronology protection.
	 *
	 * If the stash has a previous master position recorded, this will try to
	 * make sure that the next query to a slave of that master will see changes up
	 * to that position by delaying execution. The delay may timeout and allow stale
	 * data if no non-lagged slaves are available.
	 *
	 * @param LoadBalancer $lb
	 * @return void
	 */
	public function initLB( LoadBalancer $lb ) {
		if ( !$this->enabled || $lb->getServerCount() <= 1 ) {
			return; // non-replicated setup or disabled
		}

		$this->initPositions();

		$masterName = $lb->getServerName( $lb->getWriterIndex() );
		if ( !empty( $this->startupPositions[$masterName] ) ) {
			$info = $lb->parentInfo();
			$pos = $this->startupPositions[$masterName];
			wfDebugLog( 'replication', __METHOD__ .
				": LB '" . $info['id'] . "' waiting for master pos $pos\n" );
			$lb->waitFor( $pos );
		}
	}

	/**
	 * Notify the ChronologyProtector that the LoadBalancer is about to shut
	 * down. Saves replication positions.
	 *
	 * @param LoadBalancer $lb
	 * @return void
	 */
	public function shutdownLB( LoadBalancer $lb ) {
		if ( !$this->enabled || $lb->getServerCount() <= 1 ) {
			return; // non-replicated setup or disabled
		}

		$info = $lb->parentInfo();
		$masterName = $lb->getServerName( $lb->getWriterIndex() );

		// Only save the position if writes have been done on the connection
		$db = $lb->getAnyOpenConnection( $lb->getWriterIndex() );
		if ( !$db || !$db->doneWrites() ) {
			wfDebugLog( 'replication', __METHOD__ . ": LB {$info['id']}, no writes done\n" );

			return; // nothing to do
		}

		$pos = $db->getMasterPos();
		wfDebugLog( 'replication', __METHOD__ . ": LB {$info['id']} has master pos $pos\n" );
		$this->shutdownPositions[$masterName] = $pos;
	}

	/**
	 * Notify the ChronologyProtector that the LBFactory is done calling shutdownLB() for now.
	 * May commit chronology data to persistent storage.
	 *
	 * @return array Empty on success; returns the (db name => position) map on failure
	 */
	public function shutdown() {
		if ( !$this->enabled || !count( $this->shutdownPositions ) ) {
			return true; // nothing to save
		}

		wfDebugLog( 'replication',
			__METHOD__ . ": saving master pos for " .
			implode( ', ', array_keys( $this->shutdownPositions ) ) . "\n"
		);

		$shutdownPositions = $this->shutdownPositions;
		$ok = $this->store->merge(
			$this->key,
			function ( $store, $key, $curValue ) use ( $shutdownPositions ) {
				/** @var $curPositions DBMasterPos[] */
				if ( $curValue === false ) {
					$curPositions = $shutdownPositions;
				} else {
					$curPositions = $curValue['positions'];
					// Use the newest positions for each DB master
					foreach ( $shutdownPositions as $db => $pos ) {
						if ( !isset( $curPositions[$db] )
							|| $pos->asOfTime() > $curPositions[$db]->asOfTime()
						) {
							$curPositions[$db] = $pos;
						}
					}
				}

				return [ 'positions' => $curPositions ];
			},
			BagOStuff::TTL_MINUTE,
			10,
			BagOStuff::WRITE_SYNC // visible in all datacenters
		);

		if ( !$ok ) {
			// Raced out too many times or stash is down
			wfDebugLog( 'replication',
				__METHOD__ . ": failed to save master pos for " .
				implode( ', ', array_keys( $this->shutdownPositions ) ) . "\n"
			);

			return $this->shutdownPositions;
		}

		return [];
	}

	/**
	 * Load in previous master positions for the client
	 */
	protected function initPositions() {
		if ( $this->initialized ) {
			return;
		}

		$this->initialized = true;
		if ( $this->wait ) {
			$data = $this->store->get( $this->key );
			$this->startupPositions = $data ? $data['positions'] : [];

			wfDebugLog( 'replication', __METHOD__ . ": key is {$this->key} (read)\n" );
		} else {
			$this->startupPositions = [];

			wfDebugLog( 'replication', __METHOD__ . ": key is {$this->key} (unread)\n" );
		}
	}
}