title: CQRS aggregates
body: <p>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.</p>
  
  <p>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)</p>
  
  <p>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?</p>
  
  <p>If I misunderstood any part of the process I would appreciate your feedback.</p>
  
  <p>Thanks for your help!</p>