I am feeling stuck in my MPA ABP app development between three principles that I believe are DDD ones :
Application service has not to be tied to a controller. I mean a controller doesn't have to use always only one application service. Because application service conception has not presentation in mind.
DTO has to fit the view needs to minimize HTTP data transfer. So sometimes we have to design one DTO class per entity/concept and per view.
Application services get and return always DTO's.
Now I have a view which follows the Master-Detail principle: select one entity in the Master part from an entity list loads entity details in the Details part by Ajax call. But the entity selection in Master part is made by Ajax-synchronized cascade of dropdown lists: ParentEntities > Entities.
What choice respects better DDD ?
Put GetAllParent(), GetAllEntities(parentId) and GetEntity(id) all in MyViewApplicationService, then my application service can return optimized DTO's for my view needs, but violates DDD principle,
Put each of theses three methods in different application services, isolated more "domain" in mind, but DTO's are domain-oriented, somewhat generic. So DTO's are not optimized.
Let the controller the responsability to map into a DTO that fits view needs, but it should not do that.
Application service has not to be tied to a controller. I mean a controller doesn't have to use always only one application service.
Because application service conception has not presentation in mind.
Application services are not tied to the client type, but to the client needings. They return the data that client needs, so in that sense application services has the client (presentation) in mind.
Application services get and return always DTO's.
Not always. There are alternatives to DTOs, as Vaughn Vernon says in his book Implementing DDD (page 512):
Mediator
Domain Payload Object
State Representations
Use Case optimal repository queries (closed to CQRS)
Data Transformers
What choice respects better DDD ?
Put GetAllParent(), GetAllEntities(parentId) and GetEntity(id) all in MyViewApplicationService, then my application service can return
optimized DTO's for my view needs, but violates DDD principle,
You shouldn't name the application service refering the client technology (MyView), but according to the functionality it offers.
Put each of theses three methods in different application services, isolated more "domain" in mind, but DTO's are
domain-oriented, somewhat generic. So DTO's are not optimized.
It doesn't matter if you put the 3 methods in just one service or you have one service for each method. The controller should call them anyway.
Let the controller the responsability to map into a DTO that fits view needs, but it should not do that.
If you mean the application service returns domain objects and the controller translates them to DTOs, then no, you shouldn't do that as you are exposing the domain to clients.
I understand your problem I think, let's start at the beginning:
...Put GetAllParent(), GetAllEntities(parentId) and GetEntity(id) all in MyViewApplicationService...
Which of those words would a business person understand? Are any of those words part of the ubiquitous language?
They are of course all purely technical, therefore should be detail and should not influence the architecture. Basically they would be in the wrong place, they should not be visible at all.
...but DTO's are domain-oriented, somewhat generic. So DTO's are not optimized...
DTOs should not be part of anything remotely object-oriented. However, you did not say you want object-orientation so let's not dwell on that.
Still, if your object is supposed to be domain-oriented, then how come it's unfit (not optimized) for the application that is written specifically for that domain?
I think the problem is that your "object" is actually modeling something different than the domain. It's likely modeling the database table or its records.
I mean, if you are showing a profile for a product, then your "object" should be ProductProfile, not the generic Product. Or ProductDetails, or ProductHeroImage, and so on. Those things are of the domain and likely mentioned in the requirements document too.
Let the controller the responsability to map into a DTO that fits view needs, but it should not do that.
Why should it not do that? If the purpose of your feature is to show some data to the user, then why is that not considered a "business function". I mean it should be literally the other way around. The "view" is the business function you want, and the database/repository/controller/service or whatever is "just" technology that should be just a detail and not visible in the architecture.
Disclaimer: I must admit these views are not what most projects do under DDD, but maybe you find some sense in it to question those projects more in the future.:)
Related
In DDD, you're strongly encouraged to have all business logic within the domain entities instead of separate from it. Which makes sense.
You also have the idea of Domain Services to encapsulate certain pieces of logic.
What I can't work out is how to have the domain entities perform their business logic that itself depends on external services.
Take, for example, a user management system. In this there is a User domain entity, on which there are various business actions to perform. One of these is verify_email_address - which sends an email to the users email address in order to verify that it's valid.
Except that sending an email involves interactions with external services. So it seems reasonable for this logic to be encapsulated within a Domain Service that the User entity then makes use of. But how does it actually do this?
I've seen some things suggest that the User entity is constructed with references to every domain service that it needs. Which makes it quite big and bloated, and especially hard to test.
I've seen some things that suggest that you should pass in the domain service to the method that you are calling - which just feels weird. Why would someone outside of the domain entity need to pass in the EmailClient service when calling verify_email_address?
I've also then seen the suggestion that you instead pass the User entity in to the domain service, except that this seems to imply that the logic is in the domain service and not the entity.
And finally, I've seen suggestions that the entity should raise a domain event to trigger the email, which the domain service then reacts to. That means that all of the logic for whether to send the email or not - no need if it's already verified - and which email address to send it to are in the right place, and it also means that the user only needs a way to raise events and nothing more. So it feels less tightly coupled. But it also means you need a whole eventing mechanism, which is itself quite complicated.
So, am I missing something here? How can I achieve this?
(I'm working in Rust if that matters, but I don't see that it should)
What I can't work out is the best way to have the domain entities perform their business logic that itself depends on external services.
That's not your fault; the literature is a mess.
In DDD, you're strongly encouraged to have all business logic within the domain entities instead of separate from it.
That's not quite right; the common idea is that all business logic belongs within the domain layer (somewhere). But that doesn't necessarily mean that the logic must be within a domain entity.
Evans, in Chapter 5, writes:
In some cases, the clearest and most pragmatic design includes operations that do not belong to any object. Rather than force the issue, we can follow the natural contours of the problems space and include SERVICES explicitly in the model.
There are important domain operations that can't find a natural home in an ENTITY or VALUE OBJECT....
It's a very Kingdom of Nouns idea; we have code that actually does something useful, so there must be an object it can belong to.
Having a module (in the Parnas sense) in the domain layer that is responsible for the coordination of an email client and a domain entity (or for that matter, a repository) to achieve some goal is a perfectly reasonable option.
Could that module be the domain entity itself? It certainly could.
You might find, however, that coupling the management of the in-memory representation of domain information and the orchestration of a domain process that interacts with "the real world", as it were, complicates your maintenance responsibilities by introducing a heavy coupling between two distinct concepts that should instead be lightly coupled.
Clean Architecture (and the predecessors in that lineage) suggests to separate "entities" from "use cases". Ivar Jacobson's Objectory Process distinguished "entities" from "controls". So the notion of a service that is decoupled from the entity shouldn't be too alien.
Ruth Malan writes:
Design is what we do when we want to get more of what we want than we'd get by just doing it.
Finding the right design depends a lot on finding the right "what we want" for our local context (including our best guess at how this local context is going to evolve over the time window we care about).
VoiceOfUnReason has a perfectly valid answer.
I just want to boil down your question to the grits.
What I can't work out is how to have the domain entities perform their business logic that itself depends on external services.
I've also then seen the suggestion that you instead pass the User entity in to the domain service, except that this seems to imply that the logic is in the domain service and not the entity.
That's the key. All logic that belongs to domain entities should be done on domain entities. But at the same time, domain entities MUST be independent of the outside world (even other domain entities).
That's why we have domain services and application services.
Domain services are used to coordinate things between multiple entities, like transferring money between two accounts:
public class TransferService
{
IAccountRepos _repos;
public void Transfer(string fromAccountNumber, string toAccountNumber, decimal amount)
{
var account1 = _repos.Get(fromAccountNumber);
var account2 = _repos.Get(fromAccountNumber);
var money = account1.Withdraw(amount);
account2.Deposit(money);
_repos.Update(account1);
_repos.Update(account2);
}
}
That's a domain service since it's still only using the domain only.
Application services on the other hand are used to communicate over boundaries and with external services.
And it's an external service that you should create in this case. It looks similar to domain services but are at a layer over it (it can use domain services to solve its purpose).
To summarize:
Entities must be used to perform actions on themself (the easiest way is to make all setters private which forces you to add methods).
Domains services should be used as soon as two or more entities must be used to solve a problem (can even be two entities of the same type as in the example above)
Application services are used to interact with things outside the domain and can use entities and/or domain services to solve the problem.
Recently I've been trying to make my web application use separated layers.
If I understand the concept correctly I've managed to extract:
Domain layer
This is where my core domain entities, aggregate roots, value objects reside in. I'm forcing myself to have pure domain model, meaning i do not have any service definitions here. The only thing i define here is the repositories, which is actually hidden because axon framework implements that for me automatically.
Infrastructure layer
This is where the axon implements the repository definitions for my aggregates in the domain layer
Projection layer
This is where the event handlers are implemented to project the data for the read model using MongoDB to persist it. It does not know anything other than event model (plain data classes in kotlin)
Application layer
This is where the confusion starts.
Controller layer
This is where I'm implementing the GraphQL/REST controllers, this controller layer is using the command and query model, meaning it has knowledge about the Domain Layer commands as well as the Projection Layer query model.
As I've mentioned the confusion starts with the application layer, let me explain it a bit with simplified example.
Considering I want a domain model to implement Pokemon fighting logic. I need to use PokemonAPI that would provide me data of the Pokemon names stats etc, this would be an external API i would use to get some data.
Let's say that i would have domain implemented like this:
(Keep in mind that I've stretched this implementation so it forces some issues that i have in my own domain)
Pokemon {
id: ID
}
PokemonFight {
id: ID
pokemon_1: ID
pokemon_2: ID
handle(cmd: Create) {
publish(PokemonFightCreated)
}
handle(cmd: ProvidePokemonStats) {
//providing the stats for the pokemons
publish(PokemonStatsProvided)
}
handle(cmd: Start) {
//fights only when the both pokemon stats were provided
publish(PokemonsFought)
}
The flow of data between layers would be like this.
User -> [HTTP] -> Controller -> [CommandGateway] -> (Application | Domain) -> [EventGateway] -> (Application | Domain)
Let's assume that two of pokemons are created and the use case of pokemon fight is basically that when it gets created the stats are provided and then when the stats are provided the fight automatically starts.
This use case logic can be solved by using event processor or even saga.
However as you see in the PokemonFight aggregate, there is [ProvidePokemonStats] command, which basically provides their stats, however my domain do not know how to get such data, this data is provided with the PokemonAPI.
This confuses me a bit because the use case would need to be implemented on both layers, the application (so it provides the stats using the external api) and also in the domain? the domain use case would just use purely domain concepts. But shouldn't i have one place for the use cases?
If i think about it, the only purpose saga/event processor that lives in the application layer is to provide proper data to my domain, so it can continue with it's use cases. So when external API fails, i send command to the domain and then it can decide what to do.
For example i could just put every saga / event processor in the application, so when i decide to change some automation flow i exactly know what module i need to edit and where to find it.
The other confusion is where i have multiple domains, and i want to create use case that uses many of them and connects the data between them, it immediately rings in my brain that this should be application layer that would use domain APIs to control the use case, because I don't think that i should add dependency of different domain in the core one.
TL;DR
What layer should be responsible of implementing the automated process between aggregates (can be single but you know what i mean) if the process requires some external API data.
What layer should be responsible of implementing the automated process between aggregates that live in different domains / micro services.
Thank you in advance, and I'm also sorry if what I've wrote sounds confusing or it's too much of text, however any answers about layering the DDD applications and proper locations of the components i would highly appreciate.
I will try to put it clear. If you use CQRS:
In the Write Side (commands): The application services are the command handlers. A cmd handler accesses the domain (repositories, aggreagates, etc) in order to implement a use case.
If the use case needs to access data from another bounded context (microservice), it uses an infraestructure service (via dependency injection). You define the infraestructure service interface in the application service layer, and the implementation in the infra layer. The infra then access the remote microservice via http rest for example. Or integration through events.
In the Read Side (queries): The application service is the query method (I think you call it projection), which access the database directly. There's no domain here.
Hope it helps.
I do agree your wording might be a bit vague, but a couple of things do pop up in my mind which might steer you in the right direction.
Mind you, the wording makes it so that I am not 100% sure whether this is what you're looking for. If it isn't, please comment and correct my on the answer I'll provide, so I can update it accordingly.
Now, before your actual question, I'd firstly like to point out the following.
What I am guessing you're mixing is the notion of the Messages and your Domain Model belonging to the same layer. To me personally, the Messages (aka your Commands, Events and Queries) are your public API. They are the language your application speaks, so should be freely sharable with any component and/or service within your Bounded Context.
As such, any component in your 'application layer' contained in the same Bounded Context should be allowed to be aware of this public API. The one in charge of the API will be your Domain Model, that's true, but these concepts have to be shared to be able to communicate with one another.
That said, the component which will provide the states to your aggregate can be viewed from two directions I think.
It's a component that handles a specific 'Start Pokemon Match' Command. This component has the smarts to know to firstly retrieve the states prior to being able to dispatch a Create and ProvidePokemonStats command, thus ensuring it'll consistently create a working match with the stats in it by not dispatching any of both of the external stats-retrieval API fails.
Your angle in the question is to have an Event Handling Component that reacts on the creation of a Match. From here, I'd state a short-lived saga would be in place, as you'd need to deal with the fault scenario of not being able to retrieve the stats. A regular Event Handler is likely to lean to deal with this correctly.
Regardless of the two options you select, this service will deal with messages, a.k.a. your public API. As such it's within your application and not a component others will deal with directly, ever.
When it comes to your second question, I feel the some notion still holds. Two distinct applications/microservices only more so suggests your talking about two different Bounded Contexts. Certainly then a Saga would be in place to coordinate the operations between both contexts. Note that between Bounded Contexts, you want to share consciously when it comes to the public API, as you'd ideally not expose everything to the outside world.
Hope this helps you out and if not, like I said, please comment and provide me guidance how to answer your question properly.
I am learning about DDD so apologies if my question is naive. I think I need to use Local Data Transfer Object in order to display data to the users as a lot of properties are not part of any of Entity / Value Objects.
However, I am not sure where this DTO should be implemented - in a Domain Layer or in an Application Service Layer. The DTO implementation seems part of the Domain, but it means that when I create a collection of DTOs in the Service Layer and pass it to a Presentation Layer, I have to reference Domain Layer in Presentation Layer, which seems wrong.
What is the correct way to implement DTO using DDD principles?
Define the DTO to the layer where the source of the values comes from.
Relative to OP's question: place the DTO in the Application Service Layer. DTO is an output of that layer, it makes sense if you define it there. Don't put your DTO in the Domain Layer. The Domain Layer does not care about mapping things to serve external layers (the domain does not know there is a world outside of its own).
Presentation Layer (closes to consumers)
This could be your API
Has Models or Dto definitions of its own with attributes relative to its layer. If this is an API then Models/DTO have attributes for formatting or data type validations
This is the "Application Root" (meaning that it must reference both Domain Service layer, Data/Infrastructure Layer to be able to inject services)
Maps data between ApplicationService.Dto and Presentation.Dto
Application Service Layer
Has Dto definitions of its own to be able to return data without exposing the domain entities.
Bridge between Presentation Layer and Domain Layer.
Contains Application Services. See answer https://stackoverflow.com/a/3840552/1027250 for detailed definition of Application Services.
Domain Layer
Domain entities
May contain Interfaces bridging the infrastructure layer, defined in words that can be understood by the business, free from technical terms (IE: IExcelReport, IGoogleSheetReport, IRepository)
May contain "Domain Services"
Data / Infrastructure Layer (closest to your database or external services)
Database infrastructure (mapping).
Excel libraries if you define this layer as infrastructure code.
Mail or notification services.
PDF output files
Such DTOs that are exposed to the outside world become part of a contract. Depending on their form, a good place for them is either the Application Layer or the Presentation Layer.
If the DTOs are only for presentation purposes, then the Presentation Layer is a good choice.
If they are part of an API, be it for input or output, that is an Application Layer concern. The Application Layer is what connects your domain model to the outside world.
As an interesting observation, it follows that the Presentation Layer should access the domain model only through the Application Layer. Otherwise, we lose our single point of access - we'd have multiple layers invoking the domain model. The Application Layer exposes all of our use cases. Whether they are invoked by a call from another service or by the Presentation Layer makes little difference.
Sources
The core of these concepts I learned from The Red Book by Vaughn Vernon. (I would quote from it, but I don't have it handy.) The chapters about the Application Layer and the Presentation Layer are relevant.
Primarily, my conclusions come from being strict with the concepts as presented by Eric Evans and Vaughn Vernon, and prioritizing freedom in the domain model, as this is Domain-Driven Design:
The domain model should be easy to change. That means not exposing domain objects externally, as having external dependants would make them hard to change (without breaking things).
The Application Layer is the external point of access. It is what defines the use cases on the domain model. That means not operating on the domain model from elsewhere. The Presentation Layer can only go through the Application Layer. Nobody likes having to deal with many different points of access!
Yorro is right about where to place DTO but I encourage you to avoid "DTO mindset". This way of thinking collides with DDD way of thinking.
Thinking about "I need a DTO here" is thinking about technical representation (as plalx says); it is a level of abstraction too low. Try a higer level of abtraction and think about your domain, user's tasks and your UI.
Do you need get view data to the user? Bring it to UI through a View Service that return a specific YourViewInfo class.
Do you need to send data to some Service to perform a task? Send it a specific TaskMessageInfo class or a specific Command class.
When you begin to modeling the internals of these classes is when you should start to thinking about its technical representation; then you could reach to the conclusion that could be, i.e., DTO classes for convenience.
Thinking this way helps you to model the system and doesn't trigger questions like
Where to put or belongs this thing?
DTO and Domain are different layers.
So it requires mapping from one to another and usually it is done in what is called Application Services layer.
Take a look at the following articles to go deeper with DTO and layering:
Is Layering Worth the Mapping? by Mark Seemann
DTO vs Value Object vs POCO: definitions
Hexagonal (Ports/Adapters) Architecture
What has to be mentioned here is so-called Hexagonal (Ports/Adapters) Architecture [Vernon, the red book p. 125]. It is very convenient to place objects that represent data for the external (outside the domain & the application) comsumers. The architecture is the great addition to Layered Architecture generally implied by DDD.
Here is the example.
Along with ports/adapters for a DBs, email services etc.
We may define a ports/adapters/http/ui/myestore/ShoppingCartResponse.valueobject.ext (assume we use some imaginary programming language EXT) that contains the data your UI application MyEStore will use to show the end user his shopping cart state.
ShoppingCartResponse.valueobject.ext is created by ports/adapters/http/ui/myestore/EStoreHTTP.adapter.ext (for brevity it could be a synonim for a very slim HTTP REST API controller from REST world in our example).
The adapter asks the domain service ShoppingCart.service.ext for aggregates, entities, other value objects. Then it creates the desired ShoppingCartResponse.valueobject.ext from them (by itself or with creators - factories, builders etc.). Then sends the value object as an HTTP response to the consumer.
DTO or Value Object?
Whether ShoppingCartResponse would be a value object or a DTO (.dto.ext) you should decide based on
The particular structure of your Ports/Adapters object hierarchy;
If it is justified to have another type of object, a DTO, in the system, or it is better to keep only value objects only;
A semantic meaning of DTO vs value objects in object hierarchy;
The responsibilities differentiation between them: say your value object would do some invariants logic-keeping, whereas DTO could be just a dumb object with no logic.
I would prefer starting with the simplest approach, allowing only for value objects, and add DTOs only as the clear architectural need in them appears.
This approach gives much flexibilty yet it keeps code design clean and simple. Ports/Adapters section accommodates objects that belong together (the adapter, VO or DTO, their creators) and keeps Application Layer clean keeping the space for more relevant application layer objects.
I need an advice on DDD (Domain Driven Design) and implementation of Repository pattern and encapsulation.
To my understanding is that Repository pattern enables me to put all the database access logic in one place and abstract that logic from other parts of application.
On the other side there is orm (Nhibernate, EntityFramework...) with its support for Linq and IQueryable.
My toughts are:
1. If I am using repository then I should not use IQueryable as my return type bust instead use IEnumerable. Because if I use IQueryable then this would allow leaking database code to other application layers (IE would allow other devs to do queries in mvc controller where they don't belong).
But every controls use IQueryable to access data and does this because is easier.
If I use IQueryable as return type of my repository methods then:
- I allow other developers to do database querying in other layers of application (and I think this should not be possible)
- It will leak my domain entities (domain model) to other layers of applications (ie. User interface) but should not, instead DTO should be used.
My question is is IQueryable a good practice in DDD?
I would say it's not a good practice.
Ideally you should have some sort of application layer on top of your domain. If you expose your domain objects directly or through Query object, someone might actually modify it outside of your control.
Personally i like to think of IQueryable as an implementation detail, ideally my domain would not depend on it (in case i want to switch my storage technology it could be a problem).
Most often i'll end up using IQueryable internally inside my repositories implementation. What i usually end up with is implementing generic repository that has a FindBySpecification method, and then have specialized repositories for every Aggregate root that inherits from it. For example:
public interface IRepository<TEntity>
{
TEntity Get(Guid ID);
void Add(TEntity entity);
void Remove(TEntity entity);
void Detach(TEntity entity);
IEnumerable<TEntity> WithSpecification(ISpecification<TEntity> specification);
}
public interface IOrdersRepository : IRepository<Order>
{
IEnumerable<Order> GetCompletedOrdersForAllPreferedCustomers(DateTime orderCompletedAfter);
Order GetOrderBySomeOtherComplicatedMeans();
}
Another aproach is to design your application to follow the CQRS principle.
Then you can have your DomainModel doing it's magic on the command side, and create a readonly model of your data for your client on the query side.
This setup can become really elaborate depending on your requirements, but it can be as simple as two ORM models mapped to the same database (the one on the command side is mapping your domain entities, the one on the query sides maps to simple DTOs ).
Personally I do not feel that exposing EF entities by way of IQueryable across all layers is necessarily a bad thing. But this is just my own opinion. Others may not agree especially if you look at it on the encapsulation perspective. But generally the concept of loose-coupling is a trade-off between complexity and practical gain. By encapsulating IQueryable know that you will loose a lot of practical gain, like the ability to lazy-load for instance.
If your application layer is directly on top of your repository layer, I vote to use IEnnumerable instead of IQueryable. But if you have a service layer in the middle (which I personally prefer to contain all business logic so the repository layer can specialize with data access operations) then I will have the repository return IQueryable and have the service layer return IEnnumerable after it has performed its business logic from the IQueryable object returned by the repository.
These are just my own personal rules:
If you need to encapsulate EF, encapsulate it away only from the application layer. Otherwise, use it as excessively as you need on all layers. EF is very powerful, encapsulating it in the repository layer will make you lose a lot of its power
The application layer should be as thin as possible, it should not perform any further processing on the data it receives from the layer below it. It receives the list and it renders it, that's it.
Despite having studied Domain Driven Design for a long time now there are still some basics that I simply figure out.
It seems that every time I try to design a rich domain layer, I still need a lot of Domain Services or a thick Application Layer, and I end up with a bunch of near-anemic domain entities with no real logic in them, apart from "GetTotalAmount" and the like. The key issue is that entities aren't aware of external stuff, and it's bad practice to inject anything into entities.
Let me give some examples:
1. A user signs up for a service. The user is persisted in the database, a file is generated and saved (needed for the user account), and a confirmation email is sent.
The example with the confirmation email has been discussed heavily in other threads, but with no real conclusion. Some suggest putting the logic in an application service that gets an EmailService and FileService injected from the infrastructure layer. But then I would have business logic outside of the domain, right? Others suggest creating a domain service that gets the infrastructure services injected - but in that case I would need to have the interfaces of the infrastructure services inside the domain layer (IEmailService and IFileService) which doesn't look too good either (because the domain layer cannot reference the infrastructure layer). And others suggest implementing Udi Dahan's Domain Events and then having the EmailService and FileService subscribe to those events. But that seems like a very loose implementation - and what happens if the services fail? Please let me know what you think is the right solution here.
2. A song is purchased from a digital music store. The shopping cart is emptied. The purchase is persisted. The payment service is called. An email confirmation is sent.
Ok, this might be related to the first example. The question here is, who is responsible for orchestrating this transaction? Of course I could put everything in the MVC controller with injected services. But if I want real DDD all business logic should be in the domain. But which entity should have the "Purchase" method? Song.Purchase()? Order.Purchase()? OrderProcessor.Purchase() (domain service)? ShoppingCartService.Purchase() (application service?)
This is a case where I think it's very hard to use real business logic inside the domain entities. If it's not good practice to inject anything into the entities, how can they ever do other stuff than checking its own (and its aggregate's) state?
I hope these examples are clear enough to show the issues I'm dealing with.
Dimitry's answer points out some good things to look for. Often/easily you find yourself in your scenario, with a data shoveling from db up to GUI through different layers.
I have been inspired by Jimmy Nilsson's simple advice "Value objects, Value objects and more Value objects". Often people tend to focus to much on Nouns and model them as entity. Naturally you often having trouble in finding DDD behavior. Verbs are easier to associate with behavior. A good thing is to make these Verbs appear in your domain as Value objects.
Some guidance I use for my self when trying to develop the domain (must say that it takes time to construct a rich domain, often several refactoring iterations...) :
Minimize properties (get/set)
Use value objects as much as you can
Expose as little you can. Make you domain aggregates methods intuitive.
Don't forget that your Domain can be rich by doing Validation. It's only your domain that knows how to conduct a purchase, and what's required.
Your domain should also be responsible for validation when your entities make a transition from one state two another state (work flow validations).
I'll give you some examples:
Here is a article I wrote on my blog regarding your issue about anemic Domain http://magnusbackeus.wordpress.com/2011/05/31/preventing-anemic-domain-model-where-is-my-model-behaviour/
I can also really recommend Jimmy Bogard's blog article about entity validations and using Validator pattern together with extension methods. It gives you the freedom to validate infrastructural things without making your domain dirty:
http://lostechies.com/jimmybogard/2007/10/24/entity-validation-with-visitors-and-extension-methods/
I use Udi's Domain Events with great success. You can also make them asynchronous if you believe your service can fail. You also wrap it in a transaction (using NServiceBus framework).
In your first example (just brainstorming now to get our minds thinking more of value objects).
Your MusicService.AddSubscriber(User newUser) application service get a call from a presenter/controller/WCF with a new User.
The service already got IUserRepository and IMusicServiceRepository injected into ctor.
The music service "Spotify" is loaded through IMusicServiceRepository
entity musicService.SignUp(MusicServiceSubscriber newSubsriber) method takes a Value object MusicServiceSubscriber.
This Value object must take User and other mandatory objects in ctor
(value objects are immutable). Here you can also place logic/behavior like handle subscriptionId's etc.
What SignUp method also does, it fires a Domain Event NewSubscriberAddedToMusicService.
It get caught by EventHandler HandleNewSubscriberAddedToMusicServiceEvent which got IFileService and IEmailService injected into it's ctor. This handler's implementation is located in Application Service layer BUT the event is controlled by Domain and MusicService.SignUp. This means the Domain is in control. Eventhandler creates file and sends email.
You can persist the user through eventhandler OR make the MusicService.AddSubscriber(...) method to this. Both will do this through IUserRepository but It's a matter of taste and perhaps how it will reflect the actual domain.
Finally... I hope you grasp something of the above... anyhow. Most important is to start adding "Verbs" methods to entitys and making the collaborate. You can also have object in your domain that are not persisted, they are only there for mediate between several domain entities and can host algorithms etc.
A user signs up for a service. The user is persisted in the
database, a file is generated and saved (needed for the user account),
and a confirmation email is sent.
You can apply Dependency Inversion Principle here. Define a domain interface like this:
void ICanSendConfirmationEmail(EmailAddress address, ...)
or
void ICanNotifyUserOfSuccessfulRegistration(EmailAddress address, ...)
Interface can be used by other domain classes. Implement this interface in infrastructure layer, using real SMTP classes. Inject this implementation on application startup. This way you stated business intent in domain code and your domain logic does not have direct reference to SMTP infrastructure. The key here is the name of the interface, it should be based on Ubiquitous Language.
A song is purchased from a digital music store. The shopping cart
is emptied. The purchase is persisted. The payment service is called.
An email confirmation is sent. Ok, this might be related to the first example. The question here is, who is responsible for orchestrating this transaction?
Use OOP best practices to assign responsibilities (GRASP and SOLID). Unit testing and refactoring will give you a design feedback. Orchestration itself can be part of thin Application Layer. From DDD Layered Architecture:
Application Layer: Defines the jobs the software is supposed to do and directs the
expressive domain objects to work out problems. The tasks this layer
is responsible for are meaningful to the business or necessary for
interaction with the application layers of other systems.
This layer is kept thin. It does not contain business rules or
knowledge, but only coordinates tasks and delegates work to
collaborations of domain objects in the next layer down. It does not
have state reflecting the business situation, but it can have state
that reflects the progress of a task for the user or the program.
Big part of you requests are related to object oriented design and responsibility assignment, you can think of GRASP Patterns and This, you can benefit from object oriented design books, recommend the following
Applying UML and Patterns