CQRS Command how to store and query entities that are not persisted to data store immediately

Question

In CQRS, we separate Commands and Queries. As I understand it, Commands raise Domain Events that may modify Entity states while Queries return View specific DTO's directly from a data store. According to this article, the UI makes commands through a Command Bus which creates Commands that are handled by their respective CommandHandlers who then orchestrate the Domain Logic to determine the occurrence of Domain Events and persist/publish any state changes to a Repository (optionally using Event Sourcing). After being persisted, state changes are available through Queries.

Now, what if a Command creates an Entity that is not persisted/published immediately? Firstly, where is that not-yet-persisted Entity held? Is it in the Command Bus, the Command Handler, the Repository, or should a new thin application layer hold it? How should a Query gain access to it?

The problem here is that it seems like any Queries for unpersisted Entities differ significantly from those of persisted Entities, unless CQRS demands that ALL Entities be persisted upon creation, which IMO is not necessarily compatible with all Domains.

Specifically, I'm trying to build software to record training information for various Training Sessions. However, I would like it if Training Sessions were persisted manually by a Save Session button as opposed to always upon creation. I don't know where a StartNewTrainingSessionCommand would store the new Training Session so that it can be Queried, if not in the data store.

Answer 1

I think you understood things a bit wrong: A command is sent via a service bus to a command handler which uses the business objects to do the work. Domain events should be generated by the business (domain) objects, but sometimes the command handler does that too.

I don't see a reason for a created entity not to be saved. In your particular case, if the domain allows it, you can have a default, empty TrainingSession saved automatically then updated when the user press the Save button.

If this approach is not feasible, then simply store the input data, pretty much the view models in a temporary place (session, db) and issue the command only when the user clicks the button.