Web Server Listen For When ERC20 Token Transferred

Question

Is it possible to have a web server listen or know when a transfer has been made on a particular ERC20 Token Contract?

From what I researched, we are not able to make a HTTP request inside the contract's transfer method.

Is there some way for an external web server to "watch" the blockchain for when those custom tokens are transferred?

Thanks

Answer 1

As you can see in the token tutorial, at the end of the transfer method the following code is executed:

/* Notify anyone listening that this transfer took place */
Transfer(msg.sender, _to, _value);

This emits an Event.

You can 'listen' for emitted Events using web3.js for example.