Domain-Driven Design – Identifying Aggregate Root

I feel like you were really close, but just missed it

order = orderRepository.Get(123)
OrderService.AddOrderLine(order: order, product: "Chocolate Cake", amount: 3))
orderRepository.Save(order)

What you were looking for is more like this:

order = orderRepository.Get(123)
order.AddOrderLine(orderService: orderService, product: "Chocolate Cake", amount: 3))
orderRepository.Save(order)

Domain services support queries within the domain model - they don't write changes to the model; the aggregates change and protect the domain state. So as a rule, you want to pass the domain service to the aggregate, then let the aggregate pass current state to the service, as needed. For instance

// Order.AddOrderLine()
orderLine = orderService.createOrderLine(this.id, product, amount);
this.lines.add(orderLine);

Note the (implied) separation of responsibilities

• The domain service creates an instance of the orderLine, using only the context provided to it. But there's no persistence here, it's just transient data in memory at this point.
• The aggregate then evaluates the orderLine to determine whether or not it satisfies the invariant. If it does, the orderLine will become part of the graph of entities reachable via the aggregate root.

However, having a Save method on repositories that concern non-root aggregates, allows for saving changes made to them directly

That's not a good thing?

orderLine = orderLineRepository.Get(31923)
orderLine.Amount = 5
OrderLineRepository.Save(orderLine)

But if this orderLine is part of an order aggregate, then presumably the Order is expected to check that the state of the order is consistent. It can't do that if you insist on being able to mutate the subordinate entities directly.

Part of the point to aggregates is that all changes to the domain model must go through paths that force the model to remain consistent. Are all of the OrderLine methods going to walk back up the graph to the Order to ensure that the invariant is still satisfied?

Of course, this might actually be a hint that the aggregate boundaries are in the wrong place. If you should be able to modify OrderLine without the entire Order, then perhaps OrderLines are aggregate roots, and not merely subordinate entities. Your domain experts might tell you that discrepancies between the Order and OrderLines aren't actually particularly expensive; ensuring that they are rare (rather than eliminating them entirely) may suffice. Horses for courses.

We have the same issue.

And I se no way to solve this problem but with transactions or with consistency check at the DB to get an exception in the worst case.

You can also use pessimistic lock, blocking aggregate root or only its parts for other clients until business transaction is completed what is to some extent equivalent to serializable transaction.

The way you go heavily depends on your system and business logic.
Concurrency is not an easy task at all.
Even if you can detect a problem how will you resolve it? Just cancel the operation or allow user to 'merge' changes?

We use Entity Framework and EF6 uses read_commited_snapshot by default, so two consecutive reads from repository can give us inconsistent data. We just keep that in mind for the future when business processes will be more clearly outlined and we can make infromed decision. And yes we still check consistency at model level as you do. This at least allows to test BL separately from DB.

I also suggest you to think over repository consistency in case you have 'long' business transactions. It is quite tricky as it turned out.