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The benifit of having repositories when using DDD is that they allows one to design a domain model without worrying about how objects will be persisted. It also allows the final product to be more flexible, as different implementations of repositories can be swapped in and out easily. So it's possible for the implementation of repositories to be based on SQL databases, REST web services, XML files, or any other method of storing and retrieving data. From the model's perspective the expectation is that there are just these magic collections that can be use to store and retrieve aggregate roots objects.

Now if I have two normal in-memory collections, say an IList<Order> and an IList<Customer>, I would never expect that modifying one collection would affect the other. So should the same logic apply to repositories? Should the actual implementation of repositories be totally isolated from one another, even if they in reality access the same database?

For example a cascade-on-delete relationship may be setup in a SQL database between a Customers table and an Orders table so that corresponding orders are deleted when a customer is deleted. Yet this functionality would break if later the SQLCustomerRepository is replaced by a RESTCustomerRepository.

So am I correct in thinking that the model should always be under the assumption that repositories are totally isolated from one another, and correspondingly the actual implementation of repositories should be isolated as well?

So if Orders should be deleted when a Customer is deleted should this be defined explicitly in the domain model, rather then relying on the database? Say through a CustomerService.DeleteCustomer() method which accesses the current ICustomerRepository and IOrderRepository.

I think I am just having a hard time getting my head out of the relational world and into the DDD world. I keep wanting to think of things in terms of tables and PK/FK relationships, where I should just ignore that a database is involved at all.

Eric Anastas
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I believe that point you miss is that aggregate roots draws context boundaries.
In simple words - stuff underneath makes sense only together w/ aggregate root itself.

As I see it - Order is not an aggregate root but an entity which lives in Customer aggregate root context. That means - there is no need for Order repository because repositories are supposed to be per aggregate root. So there should be only CustomerRepository which is supposed to know how to persist Customer.Orders too.

I myself don't worry that much and omit repository pattern altogether and just rely on NHibernate ORM. Rich domain model that correctly tracks and monitors state changes is much more important than way how you actually send update/select sql statements.

Also - think twice before deleting stuff.

Arnis Lapsa
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    Order is not part of the customer aggregate, you are not saving/modifying a customer when you add an order – Nexus Oct 25 '11 at 07:24
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Your domain model should model the transactional operations of your domain. By putting Orders on Customer, in your Customer entity, you are saying that when a Customer is deleted, so should his Orders.

If you have OrderIds on your Customer, that's different. Than you have an association between Customer and Orders. In this case, you are saying that by adding or removing from the list of OrderIds on Customers, you are adding or removing associations, not adding or deleting Orders.

Should the actual implementation of repositories be totally isolated from one another, even if they in reality access the same database?

Yes, for the most part. If you decide to make both Order and Customer Aggregate Roots, you are saying they are independant of one another, and should be allowed to change independently and simultaneously. That is, you don't need the changes to be transactional between the two. If you only make Customer an Aggregate Root, and have it have a list of Orders, now you are saying that the Customer entity dictates what happens to the Orders, and changing a Customer will cascade changes to it's Orders.

Now in your example, it seems you'd have Customers as aggregate roots. And Orders as aggregate roots. Each with their own repo. Customers would have a list of OrderIds to model the one to many association. If you deleted a Customer, you could publish a customer deleted event, and have everything related to this customer clean itself up.

Didier A.
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Never delete a customer, a customer is not deleted, it is made inactive or something. Also please don't cascade delete orders it will get you into strange places, orders should always be preserved when they are processed. Think of reports for your application, so 1.1 Million revenue just went away because you decided to cascade delete.

Nexus
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You have a repository per aggregate root not per entity, thus even cascading deletion of childs of aggregate root is applicable in the aggregate root repository as it is still isolated.

Dont cascade deletion or have any side effects to other aggregate roots, co-ordinate this logic in the application layer.

Mohamed Abed
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