649 lines
17 KiB
PHP
649 lines
17 KiB
PHP
<?php
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/**
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*
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*/
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/**
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* Database load balancing object
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*
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* @todo document
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*/
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class LoadBalancer {
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/* private */ var $mServers, $mConnections, $mLoads, $mGroupLoads;
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/* private */ var $mFailFunction, $mErrorConnection;
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/* private */ var $mForce, $mReadIndex, $mLastIndex, $mAllowLagged;
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/* private */ var $mWaitForFile, $mWaitForPos, $mWaitTimeout;
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/* private */ var $mLaggedSlaveMode, $mLastError = 'Unknown error';
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/**
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* Scale polling time so that under overload conditions, the database server
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* receives a SHOW STATUS query at an average interval of this many microseconds
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*/
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const AVG_STATUS_POLL = 2000;
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function __construct( $servers, $failFunction = false, $waitTimeout = 10, $waitForMasterNow = false )
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{
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$this->mServers = $servers;
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$this->mFailFunction = $failFunction;
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$this->mReadIndex = -1;
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$this->mWriteIndex = -1;
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$this->mForce = -1;
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$this->mConnections = array();
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$this->mLastIndex = -1;
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$this->mLoads = array();
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$this->mWaitForFile = false;
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$this->mWaitForPos = false;
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$this->mWaitTimeout = $waitTimeout;
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$this->mLaggedSlaveMode = false;
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$this->mErrorConnection = false;
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$this->mAllowLag = false;
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foreach( $servers as $i => $server ) {
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$this->mLoads[$i] = $server['load'];
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if ( isset( $server['groupLoads'] ) ) {
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foreach ( $server['groupLoads'] as $group => $ratio ) {
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if ( !isset( $this->mGroupLoads[$group] ) ) {
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$this->mGroupLoads[$group] = array();
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}
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$this->mGroupLoads[$group][$i] = $ratio;
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}
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}
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}
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if ( $waitForMasterNow ) {
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$this->loadMasterPos();
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}
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}
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static function newFromParams( $servers, $failFunction = false, $waitTimeout = 10 )
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{
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return new LoadBalancer( $servers, $failFunction, $waitTimeout );
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}
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/**
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* Given an array of non-normalised probabilities, this function will select
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* an element and return the appropriate key
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*/
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function pickRandom( $weights )
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{
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if ( !is_array( $weights ) || count( $weights ) == 0 ) {
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return false;
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}
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$sum = array_sum( $weights );
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if ( $sum == 0 ) {
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# No loads on any of them
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# In previous versions, this triggered an unweighted random selection,
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# but this feature has been removed as of April 2006 to allow for strict
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# separation of query groups.
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return false;
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}
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$max = mt_getrandmax();
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$rand = mt_rand(0, $max) / $max * $sum;
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$sum = 0;
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foreach ( $weights as $i => $w ) {
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$sum += $w;
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if ( $sum >= $rand ) {
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break;
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}
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}
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return $i;
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}
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function getRandomNonLagged( $loads ) {
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# Unset excessively lagged servers
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$lags = $this->getLagTimes();
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foreach ( $lags as $i => $lag ) {
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if ( $i != 0 && isset( $this->mServers[$i]['max lag'] ) &&
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( $lag === false || $lag > $this->mServers[$i]['max lag'] ) )
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{
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unset( $loads[$i] );
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}
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}
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# Find out if all the slaves with non-zero load are lagged
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$sum = 0;
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foreach ( $loads as $load ) {
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$sum += $load;
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}
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if ( $sum == 0 ) {
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# No appropriate DB servers except maybe the master and some slaves with zero load
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# Do NOT use the master
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# Instead, this function will return false, triggering read-only mode,
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# and a lagged slave will be used instead.
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return false;
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}
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if ( count( $loads ) == 0 ) {
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return false;
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}
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#wfDebugLog( 'connect', var_export( $loads, true ) );
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# Return a random representative of the remainder
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return $this->pickRandom( $loads );
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}
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/**
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* Get the index of the reader connection, which may be a slave
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* This takes into account load ratios and lag times. It should
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* always return a consistent index during a given invocation
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*
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* Side effect: opens connections to databases
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*/
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function getReaderIndex() {
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global $wgReadOnly, $wgDBClusterTimeout;
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$fname = 'LoadBalancer::getReaderIndex';
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wfProfileIn( $fname );
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$i = false;
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if ( $this->mForce >= 0 ) {
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$i = $this->mForce;
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} elseif ( count( $this->mServers ) == 1 ) {
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# Skip the load balancing if there's only one server
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$i = 0;
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} else {
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if ( $this->mReadIndex >= 0 ) {
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$i = $this->mReadIndex;
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} else {
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# $loads is $this->mLoads except with elements knocked out if they
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# don't work
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$loads = $this->mLoads;
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$done = false;
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$totalElapsed = 0;
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do {
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if ( $wgReadOnly or $this->mAllowLagged ) {
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$i = $this->pickRandom( $loads );
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} else {
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$i = $this->getRandomNonLagged( $loads );
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if ( $i === false && count( $loads ) != 0 ) {
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# All slaves lagged. Switch to read-only mode
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$wgReadOnly = wfMsgNoDBForContent( 'readonly_lag' );
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$i = $this->pickRandom( $loads );
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}
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}
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$serverIndex = $i;
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if ( $i !== false ) {
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wfDebugLog( 'connect', "$fname: Using reader #$i: {$this->mServers[$i]['host']}...\n" );
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$this->openConnection( $i );
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if ( !$this->isOpen( $i ) ) {
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wfDebug( "$fname: Failed\n" );
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unset( $loads[$i] );
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$sleepTime = 0;
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} else {
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if ( isset( $this->mServers[$i]['max threads'] ) ) {
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$status = $this->mConnections[$i]->getStatus("Thread%");
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if ( $status['Threads_running'] > $this->mServers[$i]['max threads'] ) {
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# Too much load, back off and wait for a while.
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# The sleep time is scaled by the number of threads connected,
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# to produce a roughly constant global poll rate.
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$sleepTime = self::AVG_STATUS_POLL * $status['Threads_connected'];
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# If we reach the timeout and exit the loop, don't use it
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$i = false;
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} else {
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$done = true;
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$sleepTime = 0;
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}
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} else {
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$done = true;
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$sleepTime = 0;
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}
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}
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} else {
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$sleepTime = 500000;
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}
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if ( $sleepTime ) {
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$totalElapsed += $sleepTime;
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$x = "{$this->mServers[$serverIndex]['host']} [$serverIndex]";
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wfProfileIn( "$fname-sleep $x" );
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usleep( $sleepTime );
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wfProfileOut( "$fname-sleep $x" );
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}
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} while ( count( $loads ) && !$done && $totalElapsed / 1e6 < $wgDBClusterTimeout );
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if ( $totalElapsed / 1e6 >= $wgDBClusterTimeout ) {
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$this->mErrorConnection = false;
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$this->mLastError = 'All servers busy';
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}
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if ( $i !== false && $this->isOpen( $i ) ) {
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# Wait for the session master pos for a short time
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if ( $this->mWaitForFile ) {
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if ( !$this->doWait( $i ) ) {
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$this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
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}
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}
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if ( $i !== false ) {
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$this->mReadIndex = $i;
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}
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} else {
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$i = false;
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}
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}
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}
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wfProfileOut( $fname );
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return $i;
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}
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/**
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* Get a random server to use in a query group
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*/
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function getGroupIndex( $group ) {
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if ( isset( $this->mGroupLoads[$group] ) ) {
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$i = $this->pickRandom( $this->mGroupLoads[$group] );
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} else {
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$i = false;
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}
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wfDebug( "Query group $group => $i\n" );
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return $i;
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}
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/**
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* Set the master wait position
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* If a DB_SLAVE connection has been opened already, waits
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* Otherwise sets a variable telling it to wait if such a connection is opened
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*/
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function waitFor( $file, $pos ) {
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$fname = 'LoadBalancer::waitFor';
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wfProfileIn( $fname );
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wfDebug( "User master pos: $file $pos\n" );
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$this->mWaitForFile = false;
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$this->mWaitForPos = false;
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if ( count( $this->mServers ) > 1 ) {
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$this->mWaitForFile = $file;
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$this->mWaitForPos = $pos;
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$i = $this->mReadIndex;
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if ( $i > 0 ) {
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if ( !$this->doWait( $i ) ) {
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$this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
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$this->mLaggedSlaveMode = true;
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}
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}
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}
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wfProfileOut( $fname );
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}
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/**
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* Wait for a given slave to catch up to the master pos stored in $this
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*/
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function doWait( $index ) {
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global $wgMemc;
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$retVal = false;
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# Debugging hacks
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if ( isset( $this->mServers[$index]['lagged slave'] ) ) {
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return false;
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} elseif ( isset( $this->mServers[$index]['fake slave'] ) ) {
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return true;
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}
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$key = 'masterpos:' . $index;
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$memcPos = $wgMemc->get( $key );
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if ( $memcPos ) {
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list( $file, $pos ) = explode( ' ', $memcPos );
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# If the saved position is later than the requested position, return now
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if ( $file == $this->mWaitForFile && $this->mWaitForPos <= $pos ) {
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$retVal = true;
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}
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}
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if ( !$retVal && $this->isOpen( $index ) ) {
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$conn =& $this->mConnections[$index];
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wfDebug( "Waiting for slave #$index to catch up...\n" );
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$result = $conn->masterPosWait( $this->mWaitForFile, $this->mWaitForPos, $this->mWaitTimeout );
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if ( $result == -1 || is_null( $result ) ) {
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# Timed out waiting for slave, use master instead
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wfDebug( "Timed out waiting for slave #$index pos {$this->mWaitForFile} {$this->mWaitForPos}\n" );
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$retVal = false;
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} else {
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$retVal = true;
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wfDebug( "Done\n" );
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}
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}
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return $retVal;
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}
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/**
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* Get a connection by index
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*/
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function &getConnection( $i, $fail = true, $groups = array() )
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{
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global $wgDBtype;
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$fname = 'LoadBalancer::getConnection';
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wfProfileIn( $fname );
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# Query groups
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if ( !is_array( $groups ) ) {
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$groupIndex = $this->getGroupIndex( $groups, $i );
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if ( $groupIndex !== false ) {
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$i = $groupIndex;
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}
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} else {
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foreach ( $groups as $group ) {
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$groupIndex = $this->getGroupIndex( $group, $i );
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if ( $groupIndex !== false ) {
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$i = $groupIndex;
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break;
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}
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}
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}
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# For now, only go through all this for mysql databases
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if ($wgDBtype != 'mysql') {
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$i = $this->getWriterIndex();
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}
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# Operation-based index
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elseif ( $i == DB_SLAVE ) {
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$i = $this->getReaderIndex();
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} elseif ( $i == DB_MASTER ) {
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$i = $this->getWriterIndex();
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} elseif ( $i == DB_LAST ) {
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# Just use $this->mLastIndex, which should already be set
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$i = $this->mLastIndex;
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if ( $i === -1 ) {
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# Oh dear, not set, best to use the writer for safety
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wfDebug( "Warning: DB_LAST used when there was no previous index\n" );
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$i = $this->getWriterIndex();
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}
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}
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# Couldn't find a working server in getReaderIndex()?
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if ( $i === false ) {
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$this->reportConnectionError( $this->mErrorConnection );
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}
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# Now we have an explicit index into the servers array
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$this->openConnection( $i, $fail );
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wfProfileOut( $fname );
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return $this->mConnections[$i];
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}
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/**
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* Open a connection to the server given by the specified index
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* Index must be an actual index into the array
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* Returns success
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* @access private
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*/
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function openConnection( $i, $fail = false ) {
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$fname = 'LoadBalancer::openConnection';
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wfProfileIn( $fname );
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$success = true;
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if ( !$this->isOpen( $i ) ) {
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$this->mConnections[$i] = $this->reallyOpenConnection( $this->mServers[$i] );
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}
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if ( !$this->isOpen( $i ) ) {
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wfDebug( "Failed to connect to database $i at {$this->mServers[$i]['host']}\n" );
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if ( $fail ) {
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$this->reportConnectionError( $this->mConnections[$i] );
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}
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$this->mErrorConnection = $this->mConnections[$i];
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$this->mConnections[$i] = false;
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$success = false;
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}
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$this->mLastIndex = $i;
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wfProfileOut( $fname );
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return $success;
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}
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/**
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* Test if the specified index represents an open connection
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* @access private
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*/
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function isOpen( $index ) {
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if( !is_integer( $index ) ) {
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return false;
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}
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if ( array_key_exists( $index, $this->mConnections ) && is_object( $this->mConnections[$index] ) &&
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$this->mConnections[$index]->isOpen() )
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{
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return true;
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} else {
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return false;
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}
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}
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/**
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* Really opens a connection
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* @access private
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*/
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function reallyOpenConnection( &$server ) {
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if( !is_array( $server ) ) {
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throw new MWException( 'You must update your load-balancing configuration. See DefaultSettings.php entry for $wgDBservers.' );
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}
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extract( $server );
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# Get class for this database type
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$class = 'Database' . ucfirst( $type );
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# Create object
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$db = new $class( $host, $user, $password, $dbname, 1, $flags );
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$db->setLBInfo( $server );
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return $db;
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}
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function reportConnectionError( &$conn )
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{
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$fname = 'LoadBalancer::reportConnectionError';
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wfProfileIn( $fname );
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# Prevent infinite recursion
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static $reporting = false;
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if ( !$reporting ) {
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$reporting = true;
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if ( !is_object( $conn ) ) {
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// No last connection, probably due to all servers being too busy
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$conn = new Database;
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if ( $this->mFailFunction ) {
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$conn->failFunction( $this->mFailFunction );
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$conn->reportConnectionError( $this->mLastError );
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} else {
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// If all servers were busy, mLastError will contain something sensible
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throw new DBConnectionError( $conn, $this->mLastError );
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}
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} else {
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if ( $this->mFailFunction ) {
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$conn->failFunction( $this->mFailFunction );
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} else {
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$conn->failFunction( false );
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}
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$server = $conn->getProperty( 'mServer' );
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$conn->reportConnectionError( "{$this->mLastError} ({$server})" );
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}
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$reporting = false;
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}
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wfProfileOut( $fname );
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}
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function getWriterIndex() {
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return 0;
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}
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/**
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* Force subsequent calls to getConnection(DB_SLAVE) to return the
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* given index. Set to -1 to restore the original load balancing
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* behaviour. I thought this was a good idea when I originally
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* wrote this class, but it has never been used.
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*/
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function force( $i ) {
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$this->mForce = $i;
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}
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/**
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* Returns true if the specified index is a valid server index
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*/
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function haveIndex( $i ) {
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return array_key_exists( $i, $this->mServers );
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}
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/**
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* Returns true if the specified index is valid and has non-zero load
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*/
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function isNonZeroLoad( $i ) {
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return array_key_exists( $i, $this->mServers ) && $this->mLoads[$i] != 0;
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}
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/**
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* Get the number of defined servers (not the number of open connections)
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*/
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function getServerCount() {
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return count( $this->mServers );
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}
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/**
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* Save master pos to the session and to memcached, if the session exists
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*/
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function saveMasterPos() {
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if ( session_id() != '' && count( $this->mServers ) > 1 ) {
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# If this entire request was served from a slave without opening a connection to the
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# master (however unlikely that may be), then we can fetch the position from the slave.
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if ( empty( $this->mConnections[0] ) ) {
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$conn =& $this->getConnection( DB_SLAVE );
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list( $file, $pos ) = $conn->getSlavePos();
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wfDebug( "Saving master pos fetched from slave: $file $pos\n" );
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} else {
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$conn =& $this->getConnection( 0 );
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list( $file, $pos ) = $conn->getMasterPos();
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wfDebug( "Saving master pos: $file $pos\n" );
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}
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if ( $file !== false ) {
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$_SESSION['master_log_file'] = $file;
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$_SESSION['master_pos'] = $pos;
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}
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}
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}
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|
|
|
/**
|
|
* Loads the master pos from the session, waits for it if necessary
|
|
*/
|
|
function loadMasterPos() {
|
|
if ( isset( $_SESSION['master_log_file'] ) && isset( $_SESSION['master_pos'] ) ) {
|
|
$this->waitFor( $_SESSION['master_log_file'], $_SESSION['master_pos'] );
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Close all open connections
|
|
*/
|
|
function closeAll() {
|
|
foreach( $this->mConnections as $i => $conn ) {
|
|
if ( $this->isOpen( $i ) ) {
|
|
// Need to use this syntax because $conn is a copy not a reference
|
|
$this->mConnections[$i]->close();
|
|
}
|
|
}
|
|
}
|
|
|
|
function commitAll() {
|
|
foreach( $this->mConnections as $i => $conn ) {
|
|
if ( $this->isOpen( $i ) ) {
|
|
// Need to use this syntax because $conn is a copy not a reference
|
|
$this->mConnections[$i]->immediateCommit();
|
|
}
|
|
}
|
|
}
|
|
|
|
function waitTimeout( $value = NULL ) {
|
|
return wfSetVar( $this->mWaitTimeout, $value );
|
|
}
|
|
|
|
function getLaggedSlaveMode() {
|
|
return $this->mLaggedSlaveMode;
|
|
}
|
|
|
|
/* Disables/enables lag checks */
|
|
function allowLagged($mode=null) {
|
|
if ($mode===null)
|
|
return $this->mAllowLagged;
|
|
$this->mAllowLagged=$mode;
|
|
}
|
|
|
|
function pingAll() {
|
|
$success = true;
|
|
foreach ( $this->mConnections as $i => $conn ) {
|
|
if ( $this->isOpen( $i ) ) {
|
|
if ( !$this->mConnections[$i]->ping() ) {
|
|
$success = false;
|
|
}
|
|
}
|
|
}
|
|
return $success;
|
|
}
|
|
|
|
/**
|
|
* Get the hostname and lag time of the most-lagged slave
|
|
* This is useful for maintenance scripts that need to throttle their updates
|
|
*/
|
|
function getMaxLag() {
|
|
$maxLag = -1;
|
|
$host = '';
|
|
foreach ( $this->mServers as $i => $conn ) {
|
|
if ( $this->openConnection( $i ) ) {
|
|
$lag = $this->mConnections[$i]->getLag();
|
|
if ( $lag > $maxLag ) {
|
|
$maxLag = $lag;
|
|
$host = $this->mServers[$i]['host'];
|
|
}
|
|
}
|
|
}
|
|
return array( $host, $maxLag );
|
|
}
|
|
|
|
/**
|
|
* Get lag time for each DB
|
|
* Results are cached for a short time in memcached
|
|
*/
|
|
function getLagTimes() {
|
|
wfProfileIn( __METHOD__ );
|
|
$expiry = 5;
|
|
$requestRate = 10;
|
|
|
|
global $wgMemc;
|
|
$times = $wgMemc->get( wfMemcKey( 'lag_times' ) );
|
|
if ( $times ) {
|
|
# Randomly recache with probability rising over $expiry
|
|
$elapsed = time() - $times['timestamp'];
|
|
$chance = max( 0, ( $expiry - $elapsed ) * $requestRate );
|
|
if ( mt_rand( 0, $chance ) != 0 ) {
|
|
unset( $times['timestamp'] );
|
|
wfProfileOut( __METHOD__ );
|
|
return $times;
|
|
}
|
|
wfIncrStats( 'lag_cache_miss_expired' );
|
|
} else {
|
|
wfIncrStats( 'lag_cache_miss_absent' );
|
|
}
|
|
|
|
# Cache key missing or expired
|
|
|
|
$times = array();
|
|
foreach ( $this->mServers as $i => $conn ) {
|
|
if ($i==0) { # Master
|
|
$times[$i] = 0;
|
|
} elseif ( $this->openConnection( $i ) ) {
|
|
$times[$i] = $this->mConnections[$i]->getLag();
|
|
}
|
|
}
|
|
|
|
# Add a timestamp key so we know when it was cached
|
|
$times['timestamp'] = time();
|
|
$wgMemc->set( wfMemcKey( 'lag_times' ), $times, $expiry );
|
|
|
|
# But don't give the timestamp to the caller
|
|
unset($times['timestamp']);
|
|
wfProfileOut( __METHOD__ );
|
|
return $times;
|
|
}
|
|
}
|
|
|
|
|