DDD Quickly

Create entire Aggregates as a unit, enforcing their invariants

There are several design patterns used to implement Factories. The book Design Patterns by Gamma et all. describes them in detail

A Factory Method is an object method which contains and hides knowledge necessary to create another object

If an object cannot be created properly, an exception should be raised, making sure that an invalid value is not returned

If objects of an Aggregate are stored in a database, only the root should be obtainable through queries. The other objects should be obtained through traversal associations

How is the Aggregate ensuring data integrity and enforcing the invariants? Since other objects can hold references only to the root, it means that they cannot directly change the other objects in the aggregate. All they can do is to change the root, or ask the root to perform some actions

The root can hold references to any of the aggregate objects, and the other objects can hold references to each other, but an outside object can hold references only to the root object

Each Aggregate has one root. The root is an Entity, and it is the only object accessible from outside

An Aggregate is a group of associated objects which are considered as one unit with regard to data changes

It is recommended to have some flexibility, and allow the modules to evolve with the project, and should not be kept frozen. It is true that module refactoring may be more expensive than a class refactoring, but when a module design mistake is found, it is better to address it by changing the module then by finding ways around it