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Make WANObjectCache sets account for slave lag

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* This gets lag information that is useful when
  the calling code is about to run queries that
  will have their results cached.
* This is now used in place of trxTimestamp() for
  WANObjectCache set() and getWithSetCallback().
* The WAN cache will use a low TTL if the lag is
  too high to avoid caching stale data for weeks.
* Bumped MAX_COMMIT_DELAY as nothing enforces it.

Bug: T113204
Change-Id: I2a95b4088cff42d6e980351555f81a4b13519e24
This commit is contained in:
Aaron Schulz 2015-09-30 19:40:09 -07:00
parent 93ced1b350
commit db0b9ef264
12 changed files with 190 additions and 56 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

2
includes/User.php
View file

@ -458,7 +458,7 @@ class User implements IDBAccessObject {
$data['mVersion'] = self::VERSION;
$key = wfMemcKey( 'user', 'id', $this->mId );
$opts = array( 'since' => wfGetDB( DB_SLAVE )->trxTimestamp() );
$opts = DatabaseBase::getCacheSetOptions( wfGetDB( DB_SLAVE ) );
ObjectCache::getMainWANInstance()->set( $key, $data, 3600, $opts );
}

7
includes/actions/InfoAction.php
View file

@ -674,7 +674,11 @@ class InfoAction extends FormlessAction {
$title = $page->getTitle();
$id = $title->getArticleID();
$dbr = wfGetDB( DB_SLAVE );
$dbrWatchlist = wfGetDB( DB_SLAVE, 'watchlist' );
$setOpts += DatabaseBase::getCacheSetOptions( $dbr, $dbrWatchlist );
$result = array();
// Number of page watchers
@ -709,7 +713,6 @@ class InfoAction extends FormlessAction {
$result['visitingWatchers'] = $visitingWatchers;
}
$dbr = wfGetDB( DB_SLAVE );
// Total number of edits
$edits = (int)$dbr->selectField(
'revision',
@ -808,8 +811,6 @@ class InfoAction extends FormlessAction {
$fname
);
$setOpts = array( 'since' => $dbr->trxTimestamp() );
return $result;
},
86400 * 7

24
includes/changetags/ChangeTags.php
View file

@ -1090,7 +1090,9 @@ class ChangeTags {
public static function listExtensionActivatedTags() {
return ObjectCache::getMainWANInstance()->getWithSetCallback(
wfMemcKey( 'active-tags' ),
function() {
function ( $oldValue, &$ttl, array &$setOpts ) {
$setOpts += DatabaseBase::getCacheSetOptions( wfGetDB( DB_SLAVE ) );
// Ask extensions which tags they consider active
$extensionActive = array();
Hooks::run( 'ChangeTagsListActive', array( &$extensionActive ) );
@ -1130,10 +1132,12 @@ class ChangeTags {
return ObjectCache::getMainWANInstance()->getWithSetCallback(
wfMemcKey( 'valid-tags-db' ),
function() use ( $fname ) {
function ( $oldValue, &$ttl, array &$setOpts ) use ( $fname ) {
$dbr = wfGetDB( DB_SLAVE );
$tags = $dbr->selectFieldValues(
'valid_tag', 'vt_tag', array(), $fname );
$setOpts += DatabaseBase::getCacheSetOptions( $dbr );
$tags = $dbr->selectFieldValues( 'valid_tag', 'vt_tag', array(), $fname );
return array_filter( array_unique( $tags ) );
},
@ -1155,7 +1159,9 @@ class ChangeTags {
public static function listExtensionDefinedTags() {
return ObjectCache::getMainWANInstance()->getWithSetCallback(
wfMemcKey( 'valid-tags-hook' ),
function() {
function ( $oldValue, &$ttl, array &$setOpts ) {
$setOpts += DatabaseBase::getCacheSetOptions( wfGetDB( DB_SLAVE ) );
$tags = array();
Hooks::run( 'ListDefinedTags', array( &$tags ) );
return array_filter( array_unique( $tags ) );
@ -1212,10 +1218,11 @@ class ChangeTags {
$fname = __METHOD__;
$cachedStats = ObjectCache::getMainWANInstance()->getWithSetCallback(
wfMemcKey( 'change-tag-statistics' ),
function() use ( $fname ) {
$out = array();
function ( $oldValue, &$ttl, array &$setOpts ) use ( $fname ) {
$dbr = wfGetDB( DB_SLAVE, 'vslow' );
$setOpts += DatabaseBase::getCacheSetOptions( $dbr );
$res = $dbr->select(
'change_tag',
array( 'ct_tag', 'hitcount' => 'count(*)' ),
@ -1224,6 +1231,7 @@ class ChangeTags {
array( 'GROUP BY' => 'ct_tag', 'ORDER BY' => 'hitcount DESC' )
);
$out = array();
foreach ( $res as $row ) {
$out[$row->ct_tag] = $row->hitcount;
}

4
includes/db/DBConnRef.php
View file

@ -457,6 +457,10 @@ class DBConnRef implements IDatabase {
return $this->__call( __FUNCTION__, func_get_args() );
}
public function getSessionLagStatus() {
return $this->__call( __FUNCTION__, func_get_args() );
}
public function maxListLen() {
return $this->__call( __FUNCTION__, func_get_args() );
}

88
includes/db/Database.php
View file

@ -45,6 +45,9 @@ abstract class DatabaseBase implements IDatabase {
protected $mServer, $mUser, $mPassword, $mDBname;
/** @var BagOStuff APC cache */
protected $srvCache;
/** @var resource Database connection */
protected $mConn = null;
protected $mOpened = false;
@ -96,6 +99,9 @@ abstract class DatabaseBase implements IDatabase {
*/
private $mTrxTimestamp = null;
/** @var float Lag estimate at the time of BEGIN */
private $mTrxSlaveLag = null;
/**
* Remembers the function name given for starting the most recent transaction via begin().
* Used to provide additional context for error reporting.
@ -601,6 +607,9 @@ abstract class DatabaseBase implements IDatabase {
function __construct( array $params ) {
global $wgDBprefix, $wgDBmwschema, $wgCommandLineMode;
$this->mTrxAtomicLevels = new SplStack;
$this->srvCache = ObjectCache::newAccelerator( 'hash' );
$server = $params['host'];
$user = $params['user'];
$password = $params['password'];
@ -3466,6 +3475,11 @@ abstract class DatabaseBase implements IDatabase {
$this->mTrxPreCommitCallbacks = array();
$this->mTrxShortId = wfRandomString( 12 );
$this->mTrxWriteDuration = 0.0;
// First SELECT after BEGIN will establish the snapshot in REPEATABLE-READ.
// Get an estimate of the slave lag before then, treating estimate staleness
// as lag itself just to be safe
$status = $this->getApproximateLagStatus();
$this->mTrxSlaveLag = $status['lag'] + ( microtime( true ) - $status['since'] );
}
/**
@ -3742,6 +3756,80 @@ abstract class DatabaseBase implements IDatabase {
return true;
}
/**
* Get the slave lag when the current transaction started
* or a general lag estimate if not transaction is active
*
* This is useful when transactions might use snapshot isolation
* (e.g. REPEATABLE-READ in innodb), so the "real" lag of that data
* is this lag plus transaction duration. If they don't, it is still
* safe to be pessimistic. In AUTO-COMMIT mode, this still gives an
* indication of the staleness of subsequent reads.
*
* @return array ('lag': seconds, 'since': UNIX timestamp of BEGIN)
* @since 1.27
*/
public function getSessionLagStatus() {
return $this->getTransactionLagStatus() ?: $this->getApproximateLagStatus();
}
/**
* Get the slave lag when the current transaction started
*
* This is useful when transactions might use snapshot isolation
* (e.g. REPEATABLE-READ in innodb), so the "real" lag of that data
* is this lag plus transaction duration. If they don't, it is still
* safe to be pessimistic. This returns null if there is no transaction.
*
* @return array|null ('lag': seconds, 'since': UNIX timestamp of BEGIN)
* @since 1.27
*/
public function getTransactionLagStatus() {
return $this->mTrxLevel
? array( 'lag' => $this->mTrxSlaveLag, 'since' => $this->trxTimestamp() )
: null;
}
/**
* Get a slave lag estimate for this server
*
* @return array ('lag': seconds, 'since': UNIX timestamp of estimate)
* @since 1.27
*/
public function getApproximateLagStatus() {
return array(
'lag' => $this->getLBInfo( 'slave' ) ? $this->getLag() : 0,
'since' => microtime( true )
);
}
/**
* Merge the result of getSessionLagStatus() for several DBs
* using the most pessimistic values to estimate the lag of
* any data derived from them in combination
*
* This is information is useful for caching modules
*
* @see WANObjectCache::set()
* @see WANObjectCache::getWithSetCallback()
*
* @param IDatabase $db1
* @param IDatabase ...
* @return array ('lag': highest lag, 'since': lowest estimate UNIX timestamp)
* @since 1.27
*/
public static function getCacheSetOptions( IDatabase $db1 ) {
$res = array( 'lag' => 0, 'since' => INF );
foreach ( func_get_args() as $db ) {
/** @var IDatabase $db */
$status = $db->getSessionLagStatus();
$res['lag'] = max( $res['lag'], $status['lag'] );
$res['since'] = min( $res['since'], $status['since'] );
}
return $res;
}
/**
* Get slave lag. Currently supported only by MySQL.
*

23
includes/db/DatabaseMysqlBase.php
View file

@ -35,9 +35,6 @@ abstract class DatabaseMysqlBase extends DatabaseBase {
/** @var string Method to detect slave lag */
protected $lagDetectionMethod;
/** @var BagOStuff APC cache */
protected $srvCache;
/** @var string|null */
private $serverVersion = null;
@ -55,8 +52,6 @@ abstract class DatabaseMysqlBase extends DatabaseBase {
$this->lagDetectionMethod = isset( $params['lagDetectionMethod'] )
? $params['lagDetectionMethod']
: 'Seconds_Behind_Master';
$this->srvCache = ObjectCache::newAccelerator( 'hash' );
}
/**
@ -684,6 +679,24 @@ abstract class DatabaseMysqlBase extends DatabaseBase {
return false;
}
public function getApproximateLagStatus() {
if ( $this->lagDetectionMethod === 'pt-heartbeat' ) {
// Disable caching since this is fast enough and we don't wan't
// to be *too* pessimistic by having both the cache TTL and the
// pt-heartbeat interval count as lag in getSessionLagStatus()
return parent::getApproximateLagStatus();
}
$key = wfGlobalCacheKey( 'mysql-lag', $this->getServer() );
$approxLag = $this->srvCache->get( $key );
if ( !$approxLag ) {
$approxLag = parent::getApproximateLagStatus();
$this->srvCache->set( $key, $approxLag, 1 );
}
return $approxLag;
}
/**
* Wait for the slave to catch up to a given master position.
* @todo Return values for this and base class are rubbish

15
includes/db/IDatabase.php
View file

@ -1365,6 +1365,21 @@ interface IDatabase {
*/
public function getLag();
/**
* Get the slave lag when the current transaction started
* or a general lag estimate if not transaction is active
*
* This is useful when transactions might use snapshot isolation
* (e.g. REPEATABLE-READ in innodb), so the "real" lag of that data
* is this lag plus transaction duration. If they don't, it is still
* safe to be pessimistic. In AUTO-COMMIT mode, this still gives an
* indication of the staleness of subsequent reads.
*
* @return array ('lag': seconds, 'since': UNIX timestamp of BEGIN)
* @since 1.27
*/
public function getSessionLagStatus();
/**
* Return the maximum number of items allowed in a list, or 0 for unlimited.
*

2
includes/filerepo/LocalRepo.php
View file

@ -205,7 +205,7 @@ class LocalRepo extends FileRepo {
function ( $oldValue, &$ttl, array &$setOpts ) use ( $that, $title ) {
$dbr = $that->getSlaveDB(); // possibly remote DB
$setOpts = array( 'since' => $dbr->trxTimestamp() );
$setOpts += DatabaseBase::getCacheSetOptions( $dbr );
if ( $title instanceof Title ) {
$row = $dbr->selectRow(

2
includes/filerepo/file/LocalFile.php
View file

@ -309,7 +309,7 @@ class LocalFile extends File {
// Cache presence for 1 week and negatives for 1 day
$ttl = $this->fileExists ? 86400 * 7 : 86400;
$opts = array( 'since' => wfGetDB( DB_SLAVE )->trxTimestamp() );
$opts = DatabaseBase::getCacheSetOptions( $this->repo->getSlaveDB() );
ObjectCache::getMainWANInstance()->set( $key, $cacheVal, $ttl, $opts );
}

4
includes/interwiki/Interwiki.php
View file

@ -221,6 +221,8 @@ class Interwiki {
function ( $oldValue, &$ttl, array &$setOpts ) use ( $prefix ) {
$dbr = wfGetDB( DB_SLAVE );
$setOpts += DatabaseBase::getCacheSetOptions( $dbr );
$row = $dbr->selectRow(
'interwiki',
Interwiki::selectFields(),
@ -228,8 +230,6 @@ class Interwiki {
__METHOD__
);
$setOpts = array( 'since' => $dbr->trxTimestamp() );
return $row ? (array)$row : '!NONEXISTENT';
},
$wgInterwikiExpiry

71
includes/libs/objectcache/WANObjectCache.php
View file

@ -69,13 +69,13 @@ class WANObjectCache {
protected $lastRelayError = self::ERR_NONE;
/** Max time expected to pass between delete() and DB commit finishing */
const MAX_COMMIT_DELAY = 1;
/** Max expected replication lag for a reasonable storage setup */
const MAX_REPLICA_LAG = 7;
const MAX_COMMIT_DELAY = 3;
/** Max replication lag before applying TTL_LAGGED to set() */
const MAX_REPLICA_LAG = 5;
/** Max time since snapshot transaction start to avoid no-op of set() */
const MAX_SNAPSHOT_LAG = 6;
const MAX_SNAPSHOT_LAG = 5;
/** Seconds to tombstone keys on delete() */
const HOLDOFF_TTL = 14; // MAX_COMMIT_DELAY + MAX_REPLICA_LAG + MAX_SNAPSHOT_LAG
const HOLDOFF_TTL = 14; // MAX_COMMIT_DELAY + MAX_REPLICA_LAG + MAX_SNAPSHOT_LAG + 1
/** Seconds to keep dependency purge keys around */
const CHECK_KEY_TTL = 31536000; // 1 year
@ -92,6 +92,8 @@ class WANObjectCache {
const TTL_UNCACHEABLE = -1;
/** Idiom for getWithSetCallback() callbacks to 'lockTSE' logic */
const TSE_NONE = -1;
/** Max TTL to store keys when a data sourced is lagged */
const TTL_LAGGED = 30;
/** Cache format version number */
const VERSION = 1;
@ -270,18 +272,21 @@ class WANObjectCache {
* Example usage:
* @code
* $dbr = wfGetDB( DB_SLAVE );
* $setOpts = DatabaseBase::getCacheSetOptions( $dbr );
* // Fetch the row from the DB
* $row = $dbr->selectRow( ... );
* $key = wfMemcKey( 'building', $buildingId );
* // Give the age of the transaction snapshot the data came from
* $opts = array( 'since' => $dbr->trxTimestamp() );
* $cache->set( $key, $row, 86400, $opts );
* $cache->set( $key, $row, 86400, $setOpts );
* @endcode
*
* @param string $key Cache key
* @param mixed $value
* @param integer $ttl Seconds to live [0=forever]
* @param array $opts Options map:
* - lag : Seconds of slave lag. Typically, this is either the slave lag
* before the data was read or, if applicable, the slave lag before
* the snapshot-isolated transaction the data was read from started.
* [Default: 0 seconds]
* - since : UNIX timestamp of the data in $value. Typically, this is either
* the current time the data was read or (if applicable) the time when
* the snapshot-isolated transaction the data was read from started.
@ -296,6 +301,12 @@ class WANObjectCache {
final public function set( $key, $value, $ttl = 0, array $opts = array() ) {
$lockTSE = isset( $opts['lockTSE'] ) ? $opts['lockTSE'] : self::TSE_NONE;
$age = isset( $opts['since'] ) ? max( 0, microtime( true ) - $opts['since'] ) : 0;
$lag = isset( $opts['lag'] ) ? $opts['lag'] : 0;
if ( $lag > self::MAX_REPLICA_LAG ) {
// Too much lag detected; lower TTL so it converges faster
$ttl = $ttl ? min( $ttl, self::TTL_LAGGED ) : self::TTL_LAGGED;
}
if ( $age > self::MAX_SNAPSHOT_LAG ) {
if ( $lockTSE >= 0 ) {
@ -515,15 +526,12 @@ class WANObjectCache {
* // Key to store the cached value under
* wfMemcKey( 'cat-attributes', $catId ),
* // Function that derives the new key value
* function( $oldValue, &$ttl, array &$setOpts ) {
* // Fetch row from the DB
* function ( $oldValue, &$ttl, array &$setOpts ) {
* $dbr = wfGetDB( DB_SLAVE );
* $row = $dbr->selectRow( ... );
* // Account for any snapshot/slave lag
* $setOpts += DatabaseBase::getCacheSetOptions( $dbr );
*
* // Set age of the transaction snapshot the data came from
* $setOpts = array( 'since' => $dbr->trxTimestamp() );
*
* return $row;
* return $dbr->selectRow( ... );
* },
* // Time-to-live (seconds)
* 60
@ -536,15 +544,12 @@ class WANObjectCache {
* // Key to store the cached value under
* wfMemcKey( 'site-cat-config' ),
* // Function that derives the new key value
* function( $oldValue, &$ttl, array &$setOpts ) {
* // Fetch row from the DB
* function ( $oldValue, &$ttl, array &$setOpts ) {
* $dbr = wfGetDB( DB_SLAVE );
* $config = CatConfig::newFromRow( $dbr->selectRow( ... ) );
* // Account for any snapshot/slave lag
* $setOpts += DatabaseBase::getCacheSetOptions( $dbr );
*
* // Set age of the transaction snapshot the data came from
* $setOpts = array( 'since' => $dbr->trxTimestamp() );
*
* return $config;
* return CatConfig::newFromRow( $dbr->selectRow( ... ) );
* },
* // Time-to-live (seconds)
* 86400,
@ -561,15 +566,13 @@ class WANObjectCache {
* // Key to store the cached value under
* wfMemcKey( 'cat-state', $cat->getId() ),
* // Function that derives the new key value
* function( $oldValue, &$ttl, array &$setOpts ) {
* function ( $oldValue, &$ttl, array &$setOpts ) {
* // Determine new value from the DB
* $dbr = wfGetDB( DB_SLAVE );
* $state = CatState::newFromResults( $dbr->select( ... ) );
* // Account for any snapshot/slave lag
* $setOpts += DatabaseBase::getCacheSetOptions( $dbr );
*
* // Set age of the transaction snapshot the data came from
* $setOpts = array( 'since' => $dbr->trxTimestamp() );
*
* return $state;
* return CatState::newFromResults( $dbr->select( ... ) );
* },
* // Time-to-live (seconds)
* 900,
@ -589,20 +592,18 @@ class WANObjectCache {
* // Key to store the cached value under
* wfMemcKey( 'cat-last-actions', 100 ),
* // Function that derives the new key value
* function( $oldValue, &$ttl, array &$setOpts ) {
* function ( $oldValue, &$ttl, array &$setOpts ) {
* $dbr = wfGetDB( DB_SLAVE );
* // Account for any snapshot/slave lag
* $setOpts += DatabaseBase::getCacheSetOptions( $dbr );
*
* // Start off with the last cached list
* $list = $oldValue ?: array();
* // Fetch the last 100 relevant rows in descending order;
* // only fetch rows newer than $list[0] to reduce scanning
* $rows = iterator_to_array( $dbr->select( ... ) );
* // Merge them and get the new "last 100" rows
* $list = array_slice( array_merge( $new, $list ), 0, 100 );
*
* // Set age of the transaction snapshot the data came from
* $setOpts = array( 'since' => $dbr->trxTimestamp() );
*
* return $list;
* return array_slice( array_merge( $new, $list ), 0, 100 );
* },
* // Time-to-live (seconds)
* 10,

4
tests/phpunit/includes/utils/BatchRowUpdateTest.php
View file

@ -238,12 +238,16 @@ class BatchRowUpdateTest extends MediaWikiTestCase {
protected function mockDb() {
// Cant mock from DatabaseType or DatabaseBase, they dont
// have the full gamut of methods
// FIXME: the constructor normally sets mAtomicLevels and mSrvCache
$databaseMysql = $this->getMockBuilder( 'DatabaseMysql' )
->disableOriginalConstructor()
->getMock();
$databaseMysql->expects( $this->any() )
->method( 'isOpen' )
->will( $this->returnValue( true ) );
$databaseMysql->expects( $this->any() )
->method( 'getApproximateLagStatus' )
->will( $this->returnValue( array( 'lag' => 0, 'since' => 0 ) ) );
return $databaseMysql;
}
}