Re: Repository-interfaces: Domain Layer or Application Layer?

Long, 

 It's possible that you may need to use a repository in your Domain Layer. For example, a domain service may need to be injected with a repository to perform some specialized query, perhaps for performance considerations that you can't avoid. 
 

 On the other hand, you're less likely to need a repository in the domain layer when implementing CQRS because you should constrain yourself to only fetching aggregates by ID. Instead, you would fetch all aggregates involved in a business process at the application layer or use event-chaining or a saga to schedule read model lookups before sending another command.
 

 With that said, I actually create a common infrastructure project that contains mostly interfaces and some base classes. This common infrastructure project is where i maintain the interface(s) for my repositories. The common infrastructure project is referenced by the domain layer.
 

 App.Infrastructure - AggregateBase<TKey>, ValueObject<T>, IAggregateRepository<TAggregate>, IPublishEvents, etc
 

 App.Infrastructure.Persistence.Sql - Sql Implementation of AggregateRepository<TAggregate>
 

 App.Domain - Aggregates (inherit from AggregateBase<TKey>), Value Objects (inherit from ValueObject<T> which overrides equality operators)