Order

The order in which a Ferry.perKey sends a key's waiting inputs: ascending rank, ties in arrival order.

Inputs that wait together are sent in rank order, so each batch is in rank order and, under load, so is everything that arrived during the previous round trip. An input that arrives after inputs of a higher rank were already sent is not held back or rejected: it goes in the key's next batch, and the downstream, which holds the key's state, decides where it belongs. To give out-of-order inputs more time to meet, set a positive maxBatchDelay.

rank is evaluated once per input, when it is submitted. It must be pure, total and cheap.

Implement it to rank by anything an input carries, e.g. an effective date as date.toEpochDay, a timestamp as epoch milliseconds, or a sequence number.