[PallarelCoedr]

The stream is the consistency (locking) boundary. Your first step it to get your model aligned with such boundaries. For example, your amazon shopping basket is independent of my basket. Then you chose your concurrency model - append to a stream with an expected version (pessimistic) or just append anyway (no expected version). Your amazon basket may be the later, your amazon payment and shipping checkout may be the former.

Locking across streams is an anti-pattern / smell. It can be done (as can anything) but it usually points to a modelling problem. Example: cancelling an amazon order is a _request_ that is in a race with the fulfillment system (boundary); it may or may not be successful.