Question on concurrent RecipeInstances about baker

[jacum]

Recipe instances are independent and run along each other.
Interaction instances are just stateless functions and could indeed be used by any recipe instance.
Each recipe instance performs its own call to the interaction, and has no knowledge of underlying implementation.
Each call to an interaction is performed independently of any other calls to the same interaction. You can think of an interaction as an HTTP server, which multiple clients can call, simultaneously, each in its own context, each getting its own result. Hope this answers your question.

It is however up to the actual interaction implementation to serialise the calls and guarantee the integrity of the data behind that interaction.
In your example, if you have two orders running concurrently, and you need to reserve same SKU for each order independently, you do need to use some sort of locking/transaction on your stock data, or a queue, to guarantee that you won't reserve the same stock twice for two orders.