loadbalancer.php

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<?php
/**
 *
 * @package MediaWiki
 */

/**
 * Depends on the database object
 */
require_once( 'Database.php' );

# Valid database indexes
# Operation-based indexes
define( 'DB_SLAVE', -1 );     # Read from the slave (or only server)
define( 'DB_MASTER', -2 );    # Write to master (or only server)
define( 'DB_LAST', -3 );     # Whatever database was used last

# Obsolete aliases
define( 'DB_READ', -1 );
define( 'DB_WRITE', -2 );


# Scale polling time so that under overload conditions, the database server
# receives a SHOW STATUS query at an average interval of this many microseconds
define( 'AVG_STATUS_POLL', 2000 );


/**
 * Database load balancing object
 *
 * @todo document
 * @package MediaWiki
 */
class LoadBalancer {
	/* private */ var $mServers, $mConnections, $mLoads, $mGroupLoads;
	/* private */ var $mFailFunction, $mErrorConnection;
	/* private */ var $mForce, $mReadIndex, $mLastIndex, $mAllowLagged;
	/* private */ var $mWaitForFile, $mWaitForPos, $mWaitTimeout;
	/* private */ var $mLaggedSlaveMode, $mLastError = 'Unknown error';

	function LoadBalancer()
	{
		$this->mServers = array();
		$this->mConnections = array();
		$this->mFailFunction = false;
		$this->mReadIndex = -1;
		$this->mForce = -1;
		$this->mLastIndex = -1;
		$this->mErrorConnection = false;
		$this->mAllowLag = false;
	}

	function newFromParams( $servers, $failFunction = false, $waitTimeout = 10 )
	{
		$lb = new LoadBalancer;
		$lb->initialise( $servers, $failFunction, $waitTimeout );
		return $lb;
	}

	function initialise( $servers, $failFunction = false, $waitTimeout = 10 )
	{
		$this->mServers = $servers;
		$this->mFailFunction = $failFunction;
		$this->mReadIndex = -1;
		$this->mWriteIndex = -1;
		$this->mForce = -1;
		$this->mConnections = array();
		$this->mLastIndex = 1;
		$this->mLoads = array();
		$this->mWaitForFile = false;
		$this->mWaitForPos = false;
		$this->mWaitTimeout = $waitTimeout;
		$this->mLaggedSlaveMode = false;

		foreach( $servers as $i => $server ) {
			$this->mLoads[$i] = $server['load'];
			if ( isset( $server['groupLoads'] ) ) {
				foreach ( $server['groupLoads'] as $group => $ratio ) {
					if ( !isset( $this->mGroupLoads[$group] ) ) {
						$this->mGroupLoads[$group] = array();
					}
					$this->mGroupLoads[$group][$i] = $ratio;
				}
			}
		}
	}

	/**
	 * Given an array of non-normalised probabilities, this function will select
	 * an element and return the appropriate key
	 */
	function pickRandom( $weights )
	{
		if ( !is_array( $weights ) || count( $weights ) == 0 ) {
			return false;
		}

		$sum = array_sum( $weights );
		if ( $sum == 0 ) {
			# No loads on any of them
			# In previous versions, this triggered an unweighted random selection,
			# but this feature has been removed as of April 2006 to allow for strict 
			# separation of query groups. 
			return false;
		}
		$max = mt_getrandmax();
		$rand = mt_rand(0, $max) / $max * $sum;

		$sum = 0;
		foreach ( $weights as $i => $w ) {
			$sum += $w;
			if ( $sum >= $rand ) {
				break;
			}
		}
		return $i;
	}

	function getRandomNonLagged( $loads ) {
		# Unset excessively lagged servers
		$lags = $this->getLagTimes();
		foreach ( $lags as $i => $lag ) {
			if ( isset( $this->mServers[$i]['max lag'] ) && $lag > $this->mServers[$i]['max lag'] ) {
				unset( $loads[$i] );
			}
		}

		# Find out if all the slaves with non-zero load are lagged
		$sum = 0;
		foreach ( $loads as $load ) {
			$sum += $load;
		}
		if ( $sum == 0 ) {
			# No appropriate DB servers except maybe the master and some slaves with zero load
			# Do NOT use the master
			# Instead, this function will return false, triggering read-only mode,
			# and a lagged slave will be used instead.
			return false;
		}

		if ( count( $loads ) == 0 ) {
			return false;
		}

		#wfDebugLog( 'connect', var_export( $loads, true ) );

		# Return a random representative of the remainder
		return $this->pickRandom( $loads );
	}

	/**
	 * Get the index of the reader connection, which may be a slave
	 * This takes into account load ratios and lag times. It should
	 * always return a consistent index during a given invocation
	 *
	 * Side effect: opens connections to databases
	 */
	function getReaderIndex() {
		global $wgReadOnly, $wgDBClusterTimeout;

		$fname = 'LoadBalancer::getReaderIndex';
		wfProfileIn( $fname );

		$i = false;
		if ( $this->mForce >= 0 ) {
			$i = $this->mForce;
		} else {
			if ( $this->mReadIndex >= 0 ) {
				$i = $this->mReadIndex;
			} else {
				# $loads is $this->mLoads except with elements knocked out if they
				# don't work
				$loads = $this->mLoads;
				$done = false;
				$totalElapsed = 0;
				do {
					if ( $wgReadOnly or $this->mAllowLagged ) {
						$i = $this->pickRandom( $loads );
					} else {
						$i = $this->getRandomNonLagged( $loads );
						if ( $i === false && count( $loads ) != 0 )  {
							# All slaves lagged. Switch to read-only mode
							$wgReadOnly = wfMsgNoDB( 'readonly_lag' );
							$i = $this->pickRandom( $loads );
						}
					}
					$serverIndex = $i;
					if ( $i !== false ) {
						wfDebugLog( 'connect', "$fname: Using reader #$i: {$this->mServers[$i]['host']}...\n" );
						$this->openConnection( $i );

						if ( !$this->isOpen( $i ) ) {
							wfDebug( "$fname: Failed\n" );
							unset( $loads[$i] );
							$sleepTime = 0;
						} else {
							$status = $this->mConnections[$i]->getStatus("Thread%");
							if ( isset( $this->mServers[$i]['max threads'] ) &&
							  $status['Threads_running'] > $this->mServers[$i]['max threads'] )
							{
								# Too much load, back off and wait for a while.
								# The sleep time is scaled by the number of threads connected,
								# to produce a roughly constant global poll rate.
								$sleepTime = AVG_STATUS_POLL * $status['Threads_connected'];

								# If we reach the timeout and exit the loop, don't use it
								$i = false;
							} else {
								$done = true;
								$sleepTime = 0;
							}
						}
					} else {
						$sleepTime = 500000;
					}
					if ( $sleepTime ) {
							$totalElapsed += $sleepTime;
							$x = "{$this->mServers[$serverIndex]['host']} [$serverIndex]";
							wfProfileIn( "$fname-sleep $x" );
							usleep( $sleepTime );
							wfProfileOut( "$fname-sleep $x" );
					}
				} while ( count( $loads ) && !$done && $totalElapsed / 1e6 < $wgDBClusterTimeout );

				if ( $totalElapsed / 1e6 >= $wgDBClusterTimeout ) {
					$this->mErrorConnection = false;
					$this->mLastError = 'All servers busy';
				}

				if ( $i !== false && $this->isOpen( $i ) ) {
					# Wait for the session master pos for a short time
					if ( $this->mWaitForFile ) {
						if ( !$this->doWait( $i ) ) {
							$this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
						}
					}
					if ( $i !== false ) {
						$this->mReadIndex = $i;
					}
				} else {
					$i = false;
				}
			}
		}
		wfProfileOut( $fname );
		return $i;
	}

	/**
	 * Get a random server to use in a query group
	 */
	function getGroupIndex( $group ) {
		if ( isset( $this->mGroupLoads[$group] ) ) {
			$i = $this->pickRandom( $this->mGroupLoads[$group] );
		} else {
			$i = false;
		}
		wfDebug( "Query group $group => $i\n" );
		return $i;
	}

	/**
	 * Set the master wait position
	 * If a DB_SLAVE connection has been opened already, waits
	 * Otherwise sets a variable telling it to wait if such a connection is opened
	 */
	function waitFor( $file, $pos ) {
		$fname = 'LoadBalancer::waitFor';
		wfProfileIn( $fname );

		wfDebug( "User master pos: $file $pos\n" );
		$this->mWaitForFile = false;
		$this->mWaitForPos = false;

		if ( count( $this->mServers ) > 1 ) {
			$this->mWaitForFile = $file;
			$this->mWaitForPos = $pos;
			$i = $this->mReadIndex;

			if ( $i > 0 ) {
				if ( !$this->doWait( $i ) ) {
					$this->mServers[$i]['slave pos'] = $this->mConnections[$i]->getSlavePos();
					$this->mLaggedSlaveMode = true;
				}
			}
		}
		wfProfileOut( $fname );
	}

	/**
	 * Wait for a given slave to catch up to the master pos stored in $this
	 */
	function doWait( $index ) {
		global $wgMemc;

		$retVal = false;

		# Debugging hacks
		if ( isset( $this->mServers[$index]['lagged slave'] ) ) {
			return false;
		} elseif ( isset( $this->mServers[$index]['fake slave'] ) ) {
			return true;
		}

		$key = 'masterpos:' . $index;
		$memcPos = $wgMemc->get( $key );
		if ( $memcPos ) {
			list( $file, $pos ) = explode( ' ', $memcPos );
			# If the saved position is later than the requested position, return now
			if ( $file == $this->mWaitForFile && $this->mWaitForPos <= $pos ) {
				$retVal = true;
			}
		}

		if ( !$retVal && $this->isOpen( $index ) ) {
			$conn =& $this->mConnections[$index];
			wfDebug( "Waiting for slave #$index to catch up...\n" );
			$result = $conn->masterPosWait( $this->mWaitForFile, $this->mWaitForPos, $this->mWaitTimeout );

			if ( $result == -1 || is_null( $result ) ) {
				# Timed out waiting for slave, use master instead
				wfDebug( "Timed out waiting for slave #$index pos {$this->mWaitForFile} {$this->mWaitForPos}\n" );
				$retVal = false;
			} else {
				$retVal = true;
				wfDebug( "Done\n" );
			}
		}
		return $retVal;
	}

	/**
	 * Get a connection by index
	 */