Whether it’s called a Settlement Engine or a Ledger Adapter, we’re talking about the API from the connector to this external system. The main sticking points are:
- Whether the API is a stream of balance updates or calls to send money
- Whether the logic for tracking account balances and triggering settlements lives in the connector or this external component
As far as I can see, the stream of balance updates is strictly worse than explicit send money calls on a number of dimensions and I am having trouble seeing any benefits to it:
- @kincaid brought up an important security consideration about the balance stream API: before the settlement engine would trigger a settlement, it would need to first make an API call to the connector to “claim” or set aside whatever portion of the balance it was going to try to settle, and wait until that API call was resolved before settling. If you don’t, the account holder could send ILP packets that would be deducted from the balance at the same time that the SE is sending an outgoing settlement, thus getting paid out twice what they owe. It seems more straightforward to have the connector check whether it needs to settle, deduct the balance, and then send a send money call to the external system.
- Too many and too few messages. Using a debounced stream would mean that the connector sends many completely unnecessary messages to the external system (i.e. when it does not need to settle) and too few messages if the account experiences a sudden burst of activity. If the connector instead send messages only when the account actually needs to settle, it would send no messages when the external system doesn’t need to take any action and it would send more in quick succession if the account has a lot of activity and needs to settle more than once.
- If the balance logic lives in the external shared component, that means that we will have a drawn out debate like this one every time we need to change it. If we keep the logic out of this shared component, we can change the logic the connector (or business logic thing that’s attached to the connector) uses to determine when it should call send money whenever and however we want.
- The balance stream API and logic behind it is more different from what’s already implemented in the plugins and so would take more work to convert the existing integrations.