Identifying flows
Since stateful flows may handle the same type of event multiple times, you need to identify the flow instance based on a unique id. This id will usually be derived from a received event. For that, add a function to the identity object that maps the received event to the flow's identity.
If a flow only handles events from a single aggregate instance, its identity is probably just the aggregate id, but in more complex scenarios you need to find a common denominator.
Provide identity
You must provide an identity function for each event that is to be handled by a stateful flow, even if the identity is the event's aggregate id.
E.g., to identify a flow based on an order's submitted event and an invoice's issued event, where the common denominator is the order id shared by both events, use the following code:
const identity = {
'accounting.invoice.issued': event => event.data.orderId,
'delivery.order.submitted': event => event.aggregate.id
};