CQRS aggregates

Question

I'm new to the CQRS/ES world and I have a question. I'm working on an invoicing web application which uses event sourcing and CQRS.

My question is this - to my understanding, a new command coming into the system (let's say ChangeLineItemPrice) should pass through the domain model so it can be validated as a legal command (for example, to check if this line item actually exists, the price doesn't violate any business rules, etc). If all goes well (the command is not rejected) - then the appropriate event is created and stored (for example LineItemPriceChanged)

The thing I didn't quite get is how do I keep this aggregate in memory to begin with, before trying to apply the command. If I have a million invoices in the system, should I playback the whole history every time I want to apply a command? Do I always save the event without any validations and do the validations when constructing the view models / projections?

If I misunderstood any part of the process I would appreciate your feedback.

Thanks for your help!

Answer 1

It is right that you have to replay the event to 'rehydrate' the domain aggregate. But you don't have to replay all events for all invoices. If you store the entity id of the root aggregate in the events, you can just select and replay the events that with the relevant id.

Then, how do you find the relevant aggregate root id? One of the read repositories should contain the relevant information to get the id, based on a set of search criteria.