We have an important update to the HTTP postback (webhook) APIs that will go live on Monday, 13th Nov, 2017. In addition to the Postbacks you receive currently (COMPLETE, CANCELLED, REJECTED), you will start receiving a new UPDATE Postback for every update that happens to an order including parameter modifications and partial fills and trades.
What changes? Refer to the Postback JSON structure here. Starting Monday (13 Nov), you will get Postbacks that come with "status": "UPDATE" in addition to the three existing statuses (COMPLETE, CANCELLED, REJECTED). There is no change to the rest of the body.
You'll want to keep an eye on the "pending_quantity" and "filled_quantity" fields to track trade updates to an order.
EDIT: What's in the next release (#4)? - Updated WebSocket (binary) protocol with new fields (trade timestamp, Open Interest, circuit values) - Realtime order and trade postbacks over WebSockets - Realtime position updates over WebSockets