Command Query Responsibility Segregation (CQRS) pattern
In its general form, the usage of this pattern is quite easy: use different structures to store and query data. Here, the requirements regarding how to store and update data differ from the requirements of queries. In the case of DDD, the unit of storage is the aggregate, so additions, deletions, and updates involve aggregates, while queries usually involve more or less complicated transformations of properties that have been taken from several aggregates.
Moreover, usually, we don't perform business operations on query results. We just use them to compute other data (averages, sums, and so on). Therefore, while updates require entities with full object-oriented semantics (methods, validation rules, encapsulated information, and so on), query results just need sets of property/value pairs, so Data Transfer Objects (DTOs) with only public properties and no methods work well.
In its common form, the pattern can be...
