Make WAN cache HOLDOFF_TTL smaller by combining db/snapshot lag

* In the common ~0 second lag case, transactions up to ~7 seconds
  long will have normal set() behavior (instead of just ~5 seconds).
* Like wise with ~0 second transactions tolerating ~7 seconds of
  lag (instead of just ~5).
* The lower hold-off time lets caching resume 3 seconds sooner.

Change-Id: I21e2a0a6915492cec422b6a6af5541937745c15b

diff --git a/includes/libs/objectcache/WANObjectCache.php b/includes/libs/objectcache/WANObjectCache.php
index 0c7b7ed..d9a8d71 100644 (file)
--- a/includes/libs/objectcache/WANObjectCache.php
+++ b/includes/libs/objectcache/WANObjectCache.php
@@ -81,12 +81,10 @@ class WANObjectCache implements LoggerAwareInterface {
 
        /** Max time expected to pass between delete() and DB commit finishing */
        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 = 5;
+       /** Max replication+snapshot lag before applying TTL_LAGGED or disallowing set() */
+       const MAX_READ_LAG = 7;
        /** Seconds to tombstone keys on delete() */
-       const HOLDOFF_TTL = 14; // MAX_COMMIT_DELAY + MAX_REPLICA_LAG + MAX_SNAPSHOT_LAG + 1
+       const HOLDOFF_TTL = 11; // MAX_COMMIT_DELAY + MAX_READ_LAG + 1
 
        /** Seconds to keep dependency purge keys around */
        const CHECK_KEY_TTL = 31536000; // 1 year
@@ -327,27 +325,34 @@ class WANObjectCache implements LoggerAwareInterface {
                $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;
-
+               // Disallow set() if the source data is uncommitted as it might get rolled back
                if ( !empty( $opts['pending'] ) ) {
                        $this->logger->info( "Rejected set() for $key due to pending writes." );
 
                        return true; // no-op the write for being unsafe
                }
-
-               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;
-                       $this->logger->warning( "Lowered set() TTL for $key due to replication lag." );
-               }
-
-               if ( $age > self::MAX_SNAPSHOT_LAG ) {
+               // Check if there's a risk of writing stale data after the purge tombstone expired
+               if ( ( $lag + $age ) > self::MAX_READ_LAG ) {
                        if ( $lockTSE >= 0 ) {
+                               // Focus on avoiding stampedes; stash the value with a low TTL
                                $tempTTL = max( 1, (int)$lockTSE ); // set() expects seconds
                                $this->cache->set( self::STASH_KEY_PREFIX . $key, $value, $tempTTL );
                        }
-                       $this->logger->warning( "Rejected set() for $key due to snapshot lag." );
+                       // Case A: any long-running transaction; ignore this set()
+                       if ( $age > self::MAX_READ_LAG ) {
+                               $this->logger->warning( "Rejected set() for $key due to snapshot lag." );
+
+                               return true; // no-op the write for being unsafe
+                       // Case B: replication lag is high; lower TTL instead of ignoring all set()s
+                       } elseif ( $lag > self::MAX_READ_LAG ) {
+                               $ttl = $ttl ? min( $ttl, self::TTL_LAGGED ) : self::TTL_LAGGED;
+                               $this->logger->warning( "Lowered set() TTL for $key due to replication lag." );
+                       // Case C: medium length request during medium lag; ignore this set()
+                       } else {
+                               $this->logger->warning( "Rejected set() for $key due to high read lag." );
 
-                       return true; // no-op the write for being unsafe
+                               return true; // no-op the write for being unsafe
+                       }
                }
 
                $wrapped = $this->wrap( $value, $ttl );

diff --git a/tests/phpunit/includes/libs/objectcache/WANObjectCacheTest.php b/tests/phpunit/includes/libs/objectcache/WANObjectCacheTest.php
index c3702c5..54393a7 100644 (file)
--- a/tests/phpunit/includes/libs/objectcache/WANObjectCacheTest.php
+++ b/tests/phpunit/includes/libs/objectcache/WANObjectCacheTest.php
@@ -313,4 +313,32 @@ class WANObjectCacheTest extends MediaWikiTestCase {
                $t6 = $this->cache->getCheckKeyTime( $key );
                $this->assertEquals( $t5, $t6, 'Check key time did not change' );
        }
+
+       public function testSetWithLag() {
+               $value = 1;
+
+               $key = wfRandomString();
+               $opts = array( 'lag' => 300, 'since' => microtime( true ) );
+               $this->cache->set( $key, $value, 30, $opts );
+               $this->assertEquals( $value, $this->cache->get( $key ), "Rep-lagged value written." );
+
+               $key = wfRandomString();
+               $opts = array( 'lag' => 0, 'since' => microtime( true ) - 300 );
+               $this->cache->set( $key, $value, 30, $opts );
+               $this->assertEquals( false, $this->cache->get( $key ), "Trx-lagged value not written." );
+
+               $key = wfRandomString();
+               $opts = array( 'lag' => 5, 'since' => microtime( true ) - 5 );
+               $this->cache->set( $key, $value, 30, $opts );
+               $this->assertEquals( false, $this->cache->get( $key ), "Lagged value not written." );
+       }
+
+       public function testWritePending() {
+               $value = 1;
+
+               $key = wfRandomString();
+               $opts = array( 'pending' => true );
+               $this->cache->set( $key, $value, 30, $opts );
+               $this->assertEquals( false, $this->cache->get( $key ), "Pending value not written." );
+       }
 }