The site is being down frequently in recent two months. At first I was too lazy to fix it, so I received tons of alert emails from NewRelic which is quite annoying. From NewRelic analysis page, the site was having too much traffic with POST request to xml-rpc.php file and then my database eat up all memory. As a result, site went down. This is a typical wordpress xml-rpc attack.
With the help of this article, I’ve done the modification to my configuration files. Hopefully the site is more robust from now on.