I’m leading the team responsible for the ICP Ledger and I can explain to you why there is a notification mechanism. Before I start though, I want to emphasise that most standards on the IC don’t have a notification mechanism. That is for a very good reason.
Let me explain why this is the case. There are several ways to notify a Principal about a transaction on the IC. The approach you propose is the more automatic one, which requires the Ledger to notify when a transaction has been successful. With this approach, the Ledger must be sure that the notification arrives at the receiver canister. On the IC, this requires the receiver to answer to the on_ft_received call with either a simple ack in case the handler is implemented on the receiver side, or with an error in case the handler is not implemented. What is the problem then? Well, there are two problems with this: 1) the Ledger could not get any answer from the receiver canister when it calls on_ft_received and 2) the Ledger receives a different error than the one saying the receiver canister misses the on_ft_received endpoint. Let’s talk about each one of these.
If on_ft_received doesn’t return then the Ledger canister is effectively stuck. Remember that the workflow relies on the fact that the message is received. The Ledger theoretically could not even stop the call to the receiver canister because then the payment would be lost. Note that it’s very problematic to upgrade a canister when there is an in-flight message and there is no way for the Ledger to do anything about it.
The situation is not better if the ledger receives a different error from the call, e.g. if the canister doesn’t exist or the canister exists but its queue is full. The Ledger doesn’t know and cannot know whether the canister will exist in future and even if it did, there would be no way to tell when the canister will be available and when it will be able to receive messages. The Ledger could retry indefinitely but then we have another issue: the more notification pending, the more space and time the Ledger would need.
Consequently, I believe that a notification mechanism that involves the ledger (directly), inevitably leads to issues at some point.
Needless to say, no safe standard should support notify without a well defined and tested approach.
The Ledger notify method is another approach. It is more secure than the one you proposed because it gives the responsibility to deliver the notification to the sender. Despite this, the notify method is still problematic and less convenient and we are deprecating it.
A final approach to the issues pertaining to notify is the so called fire and forget mechanism where the ledger sends the event to the canister and doesn’t wait for a response. This is possible and theoretically safer but then you end up with the same issue you have without notify. The caller must be able to call this mechanism multiple times in case the first notify doesn’t reach the receiver canister and that means that the client will need to track double-spending related information.
Note that most of the canisters on the IC work with the ICP Ledger. For instance, the CMC canister and the governance canister work just fine.