How can I get the source code of 'bookinfo'

such as the title.
I want to get the source code to analyze the micro-service which supported by Istio.
I can’t understand how the service reviews get the IP address of the service ratings.

source here:

@dnd_shantaram – reviews service relies on DNS to access ratings via http://ratings:9080/ratings


Just want to add that DNS is pretty central to all the samples and the functioning of the service mesh.