何时使用node.js以及何时使用ajax？

I had a website , which was coded entirely in php and basic jquery ajax . However after i learned about node.js/socket.io i recoded almost all of the real time stuff that involved ajax using nodejs and socket.io . I am a bit confused as to whether i did the right thing . So my question is when is ajax a more optimal solution than using node/socket ?I had a website , which was coded entirely in p