$cache->set( "key$i", 1 );
$this->assertEquals( 1, $cache->get( "key$i" ) );
$this->assertEquals( 1, $backend->get( "key$i" ) );
$cache->set( "key$i", 1 );
$this->assertEquals( 1, $cache->get( "key$i" ) );
$this->assertEquals( 1, $backend->get( "key$i" ) );