How to delete multiple aggregates in DDD?

Question

I know in DDD that deleting the Aggregate Root must remove everything within the Aggregate boundary all at once.

But in the Agile example that vaughn vernon gave it here https://vaughnvernon.co/?p=838 the BackLogItem and Product Aggregates are exist in separate Aggregates and the BackLogItem Aggregate Root is referencing Product Aggregate Root by Id.So, If I want to delete the Product Aggregate Root wouldn't mean that I should delete its BackLogItems?

So, my question how to delete multiple Aggregates in DDD and if so would be that possible using Domain Services , Domain Event or whatever?

P.S

Depending on vaughn vernon that we should not modify more than one aggregate in the same transaction(in some cases we are forced to use eventual consistency).

Answer 1

The usual mechanism for behavior distributed across multiple aggregates is to use a process manager.

I recommend starting from Rinat's writeup, because it really does get to the core of the matter; a process manager is just a stand in for a human being that reacts to events by sending commands to other aggregates.

• Oh look, the Product was removed
• I should load a list of the BackLogItems that reference that Product
• And remove each of them in turn

If your modeling of the back log items as belonging to a distinct aggregate from the product is correct, then it follows that the changes to the back log items can be separated in time from the changes to the product.

Also, see Udi Dahan: Don't Delete -- Just Don't.

Answer 2

Use Saga Pattern. It executes a scenario in the transaction and imitates clients work. Saga sends multiple commands just as client does.

Answer 3

I would argue that your problem is not only how to delete multiple aggregates, but also which aggregates you should delete. In your question, you mention that when you delete the Product Aggregate you should delete its Backlog Items, but is that all? What about the Kanban Board and its custom columns and workflow, and N other features associated with that Product Aggregate?

The point of having the BacklogItems knowing about the Product, but not the other way around is to avoid having a God object, which knows about everything because dozens of features can be attached to the Product Aggregate over time.

When deleting the Product Aggregate, there's not a single place that knows everything that needs to be deleted, but everything that needs to be deleted will know. Therefore, leave the responsibility to delete (or something else) to every other aggregate: From the ProductAggregate, publish a ProductDeletedEvent and subscribe to it from all the other aggregates that care about it.