def incr_site_stats engine, stats
    # Increment the site:stats counters (in redis). We use 'created_at'
    # to checkpoint the time we created these stats
    redis.multi do
        redis.hsetnx 'site:stats', 'created_at', Time.now.utc.to_string
        stats.each_pair do |k, v|
            redis.hincrby 'site:stats', k, v.to_i
        end
    end
        
    # Start the transaction by doing optimistic locking with watch
    redis.watch 'site:stats'
    site_stats = redis.hgetall 'site:stats'
    created_at = Time.parse_utc site_stats['created_at']
    elapsed = Time.now.utc.to_i - created_at.utc.to_i

    # Save the stats in CouchDB every 5 minutes
    if elapsed > 300
        ok = redis.multi { redis.del 'site:stats' }
            
        # If the del operation fails, then there was a conflict and someone else
        # updated the site:stats hash. Otherwise, increment the counters in CouchDB
        if ok
            db.update_doc 'stats' do |_doc|
                site_stats.each_pair do |k, v|
                    next if k == 'created_at'
                    _doc[k] ||= 0
                    _doc[k] += v.to_i
                end
                _doc
            end
        end
    else
        redis.unwatch
    end
end