I think I can keep calling by etherscan to find new transactions and I the best way is that I can receive a url callback once an event created on erc20 token event log. Any API I can use for this? Or I must keep calling etherscan API for doing this? Thanks
parity support Log filters. You can easily run a node and subscribe to the Transfer event for tokens you care about (or all Transfer events if you wish), and then pass that information onto whichever system needs it.
Cindercloud has an API which allows you to do this. Internally,it uses geth and parity filters, but provides you with readable data.