Aaron Schulz has uploaded a new change for review. (
https://gerrit.wikimedia.org/r/348320 )
Change subject: Split up LoadBalancer::getReaderIndex() and simplify the code a
bit
......................................................................
Split up LoadBalancer::getReaderIndex() and simplify the code a bit
Change-Id: I4e0b5318ad2c987b2a059c4ef2bda3de14486687
---
M includes/libs/rdbms/loadbalancer/LoadBalancer.php
1 file changed, 69 insertions(+), 53 deletions(-)
git pull ssh://gerrit.wikimedia.org:29418/mediawiki/core
refs/changes/20/348320/1
diff --git a/includes/libs/rdbms/loadbalancer/LoadBalancer.php
b/includes/libs/rdbms/loadbalancer/LoadBalancer.php
index e8069c0..56ef241 100644
--- a/includes/libs/rdbms/loadbalancer/LoadBalancer.php
+++ b/includes/libs/rdbms/loadbalancer/LoadBalancer.php
@@ -311,60 +311,99 @@
public function getReaderIndex( $group = false, $domain = false ) {
if ( count( $this->mServers ) == 1 ) {
- # Skip the load balancing if there's only one server
+ // Skip the load balancing if there's only one server
return $this->getWriterIndex();
} elseif ( $group === false && $this->mReadIndex >= 0 ) {
- # Shortcut if generic reader exists already
+ // Shortcut if the generic reader index was already
cached
return $this->mReadIndex;
}
- # Find the relevant load array
if ( $group !== false ) {
+ // Use the server weight array for this load group
if ( isset( $this->mGroupLoads[$group] ) ) {
- $nonErrorLoads = $this->mGroupLoads[$group];
+ $loads = $this->mGroupLoads[$group];
} else {
- # No loads for this group, return false and the
caller can use some other group
+ // No loads for this group, return false and
the caller can use some other group
$this->connLogger->info( __METHOD__ . ": no
loads for group $group" );
return false;
}
} else {
- $nonErrorLoads = $this->mLoads;
+ // Use the generic load group
+ $loads = $this->mLoads;
}
- if ( !count( $nonErrorLoads ) ) {
+ // Scale the configured load ratios according to each server's
load and state
+ $this->getLoadMonitor()->scaleLoads( $loads, $domain );
+
+ // Pick a server to use, accounting for weights, load, lag, and
mWaitForPos
+ list( $i, $laggedReplicaMode ) = $this->pickReaderIndex(
$loads, $domain );
+ if ( $i === false ) {
+ // Replica DB connection unsuccessful
+ return false;
+ }
+
+ if ( $this->mWaitForPos && $i != $this->getWriterIndex() ) {
+ // Before any data queries are run, wait for the server
to catch up to the
+ // specified position. This is used to improve session
consistency. Note that
+ // when LoadBalancer::waitFor() sets mWaitForPos, the
waiting triggers here,
+ // so update laggedReplicaMode as needed for
consistency.
+ if ( !$this->doWait( $i ) ) {
+ $laggedReplicaMode = true;
+ }
+ }
+
+ if ( $this->mReadIndex <= 0 && $this->mLoads[$i] > 0 && $group
=== false ) {
+ // Cache the generic reader index for future ungrouped
DB_REPLICA handles
+ $this->mReadIndex = $i;
+ // Record if the generic reader index is in "lagged
replica DB" mode
+ if ( $laggedReplicaMode ) {
+ $this->laggedReplicaMode = true;
+ }
+ }
+
+ $serverName = $this->getServerName( $i );
+ $this->connLogger->debug( __METHOD__ . ": using server
$serverName for group '$group'" );
+
+ return $i;
+ }
+
+ /**
+ * @param array $loads List of server weights
+ * @param string|bool $domain
+ * @return array|bool (reader index, lagged replica mode) or false on
failure
+ */
+ private function pickReaderIndex( array $loads, $domain = false ) {
+ if ( !count( $loads ) ) {
throw new InvalidArgumentException( "Empty server array
given to LoadBalancer" );
}
- # Scale the configured load ratios according to the dynamic
load if supported
- $this->getLoadMonitor()->scaleLoads( $nonErrorLoads, $domain );
-
+ /** @var $i int|bool Index of selected server */
+ $i = false;
+ /** @var $laggedReplicaMode bool Whether server is considered
lagged */
$laggedReplicaMode = false;
- # No server found yet
- $i = false;
- # First try quickly looking through the available servers for a
server that
- # meets our criteria
- $currentLoads = $nonErrorLoads;
+ // Quickly looking through the available servers for a server
that meets criteria...
+ $currentLoads = $loads;
while ( count( $currentLoads ) ) {
if ( $this->mAllowLagged || $laggedReplicaMode ) {
$i = ArrayUtils::pickRandom( $currentLoads );
} else {
$i = false;
if ( $this->mWaitForPos &&
$this->mWaitForPos->asOfTime() ) {
- # ChronologyProtecter causes
mWaitForPos to be set via sessions.
- # This triggers doWait() after connect,
so it's especially good to
- # avoid lagged servers so as to avoid
just blocking in that method.
+ // ChronologyProtecter sets mWaitForPos
for session consistency.
+ // This triggers doWait() after
connect, so it's especially good to
+ // avoid lagged servers so as to avoid
excessive delay in that method.
$ago = microtime( true ) -
$this->mWaitForPos->asOfTime();
- # Aim for <= 1 second of waiting (being
too picky can backfire)
+ // Aim for <= 1 second of waiting
(being too picky can backfire)
$i = $this->getRandomNonLagged(
$currentLoads, $domain, $ago + 1 );
}
if ( $i === false ) {
- # Any server with less lag than it's
'max lag' param is preferable
+ // Any server with less lag than it's
'max lag' param is preferable
$i = $this->getRandomNonLagged(
$currentLoads, $domain );
}
if ( $i === false && count( $currentLoads ) !=
0 ) {
- # All replica DBs lagged. Switch to
read-only mode
+ // All replica DBs lagged. Switch to
read-only mode
$this->replLogger->error( "All replica
DBs lagged. Switch to read-only mode" );
$i = ArrayUtils::pickRandom(
$currentLoads );
$laggedReplicaMode = true;
@@ -372,12 +411,12 @@
}
if ( $i === false ) {
- # pickRandom() returned false
- # This is permanent and means the configuration
or the load monitor
- # wants us to return false.
+ // pickRandom() returned false.
+ // This is permanent and means the
configuration or the load monitor
+ // wants us to return false.
$this->connLogger->debug( __METHOD__ . ":
pickRandom() returned false" );
- return false;
+ return [ false, false ];
}
$serverName = $this->getServerName( $i );
@@ -386,8 +425,7 @@
$conn = $this->openConnection( $i, $domain );
if ( !$conn ) {
$this->connLogger->warning( __METHOD__ . ":
Failed connecting to $i/$domain" );
- unset( $nonErrorLoads[$i] );
- unset( $currentLoads[$i] );
+ unset( $currentLoads[$i] ); // avoid this
server next iteration
$i = false;
continue;
}
@@ -398,38 +436,16 @@
$this->reuseConnection( $conn );
}
- # Return this server
+ // Return this server
break;
}
- # If all servers were down, quit now
- if ( !count( $nonErrorLoads ) ) {
+ // If all servers were down, quit now
+ if ( !count( $currentLoads ) ) {
$this->connLogger->error( "All servers down" );
}
- if ( $i !== false ) {
- # Replica DB connection successful.
- # Wait for the session master pos for a short time.
- if ( $this->mWaitForPos && $i > 0 ) {
- # When LoadBalancer::waitFor() set mWaitForPos,
the wait will happen here.
- # Be sure to update laggedReplicaMode
accordingly for consistency.
- if ( !$this->doWait( $i ) ) {
- $laggedReplicaMode = true;
- }
- }
- if ( $this->mReadIndex <= 0 && $this->mLoads[$i] > 0 &&
$group === false ) {
- $this->mReadIndex = $i;
- # Record if the generic reader index is in
"lagged replica DB" mode
- if ( $laggedReplicaMode ) {
- $this->laggedReplicaMode = true;
- }
- }
- $serverName = $this->getServerName( $i );
- $this->connLogger->debug(
- __METHOD__ . ": using server $serverName for
group '$group'" );
- }
-
- return $i;
+ return [ $i, $laggedReplicaMode ];
}
public function waitFor( $pos ) {
--
To view, visit https://gerrit.wikimedia.org/r/348320
To unsubscribe, visit https://gerrit.wikimedia.org/r/settings
Gerrit-MessageType: newchange
Gerrit-Change-Id: I4e0b5318ad2c987b2a059c4ef2bda3de14486687
Gerrit-PatchSet: 1
Gerrit-Project: mediawiki/core
Gerrit-Branch: master
Gerrit-Owner: Aaron Schulz <asch...@wikimedia.org>
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