Current File : //www/varak.net/wiki.varak.net/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"
);
// CP-protected writes should overwhemingly go to the master datacenter, so get DC-local
// lock to merge the values. Use a DC-local get() and a synchronous all-DC set(). This
// makes it possible for the BagOStuff class to write in parallel to all DCs with one RTT.
if ( $this->store->lock( $this->key, 3 ) ) {
$ok = $this->store->set(
$this->key,
self::mergePositions( $this->store->get( $this->key ), $this->shutdownPositions ),
BagOStuff::TTL_MINUTE,
BagOStuff::WRITE_SYNC
);
$this->store->unlock( $this->key );
} else {
$ok = false;
}
if ( !$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" );
}
}
/**
* @param array|bool $curValue
* @param DBMasterPos[] $shutdownPositions
* @return array
*/
private static function mergePositions( $curValue, array $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 ];
}
}
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