Implementing Domain-Driven Design

Rule: Use Eventual Consistency Outside the Boundary

There is a frequently overlooked statement found in the [Evans] Aggregate pattern definition. It bears heavily on what we must do to achieve model consistency when multiple Aggregates must be affected by a single client request:

Any rule that spans AGGREGATES will not be expected to be up-to-date at all times. Through event processing, batch processing, or other update mechanisms, other dependencies can be resolved within some specific time. [Evans, p. 128]

Thus, if executing a command on one Aggregate instance requires that additional business rules execute on one or more other Aggregates, use eventual consistency. Accepting that all Aggregate instances in a large-scale, high-traffic enterprise ...