Onion Architecture - Repository Vs Service?

While I'm still struggling with this, I want to post as an answer but also I accept (and want) feedback about this.

In the example GetProductsByCategory(int id)

First, let's think from the initial need. We hit a controller, probably the CategoryController so you have something like:

public CategoryController(ICategoryService service) {
    // here we inject our service and keep a private variable.
}

public IHttpActionResult Category(int id) {
    CategoryViewModel model = something.GetCategoryViewModel(id); 
    return View()
} 

so far, so good. We need to declare 'something' that creates the view model. Let's simplify and say:

public IHttpActionResult Category(int id) {
    var dependencies = service.GetDependenciesForCategory(id);
    CategoryViewModel model = new CategoryViewModel(dependencies); 
    return View()
} 

ok, what are dependencies ? We maybe need the category tree, the products, the page, how many total products, etc.

so if we implemented this in a repository way, this could look like more or less like this :

public IHttpActionResult Category(int id) {
    var products = repository.GetCategoryProducts(id);
    var category = repository.GetCategory(id); // full details of the category
    var childs = repository.GetCategoriesSummary(category.childs);
    CategoryViewModel model = new CategoryViewModel(products, category, childs); // awouch! 
    return View()
} 

instead, back to services :

public IHttpActionResult Category(int id) {
    var category = service.GetCategory(id);
    if (category == null) return NotFound(); //
    var model = new CategoryViewModel(category);
    return View(model);
}

much better, but what is exactly inside service.GetCategory(id) ?

public CategoryService(ICategoryRespository categoryRepository, IProductRepository productRepository) {
    // same dependency injection here

    public Category GetCategory(int id) {
        var category = categoryRepository.Get(id);
        var childs = categoryRepository.Get(category.childs) // int[] of ids
        var products = productRepository.GetByCategory(id) // this doesn't look that good...
        return category;
    }

}

Let's try another approach, the unit of work, I will use Entity framework as the UoW and Repositories, so no need to create those.

public CategoryService(DbContext db) {
    // same dependency injection here

    public Category GetCategory(int id) {
        var category = db.Category.Include(c=> c.Childs).Include(c=> c.Products).Find(id);
        return category;
    }
}

So here we are using the 'query' syntax instead of the method syntax, but instead of implementing our own complex, we can use our ORM. Also, we have access to ALL repositories, so we can still do our Unit of work inside our service.

Now we need to select which data we want, I probably don't want all the fields of my entities.

The best place I can see this is happening is actually on the ViewModel, each ViewModel may need to map it's own data, so let's change the implementation of the service again.

public CategoryService(DbContext db) {
    // same dependency injection here

    public Category GetCategory(int id) {
        var category = db.Category.Find(id);
        return category;
    }
}

so where are all the products and inner categories?

let's take a look at the ViewModel, remember this will ONLY map data to values, if you are doing something else here, you are probably giving too much responsibility to your ViewModel.

public CategoryViewModel(Category category) {
    Name = category.Name;
    Id = category.Id;
    Products = category.Products.Select(p=> new CategoryProductViewModel(p));
    Childs = category.Childs.Select(c => c.Name); // only childs names.
}

you can imagine the CategoryProductViewModel by yourself right now.

BUT (why is there always a but??)

We are doing 3 db hits, and we are fetching all the category fields because of the Find. Also Lazy Loading must be enable. Not a real solution isn't it ?

To improve this, we can change find with where... but this will delegate the Single or Find to the ViewModel, also it will return an IQueryable<Category>, where we know it should be exactly one.

Remember I said "I'm still struggling?" this is mostly why. To fix this, we should return the exact needed data from the service (also know as the ..... you know it .... yes! the ViewModel).

so let's back to our controller :

public IHttpActionResult Category(int id) {
    var model = service.GetProductCategoryViewModel(id);
    if (category == null) return NotFound(); //
    return View(model);
}

inside the GetProductCategoryViewModel method, we can call private methods that return the different pieces and assemble them as the ViewModel.

this is bad, now my services know about viewmodels... let's fix that.

We create an interface, this interface is the actual contract of what this method will return.

ICategoryWithProductsAndChildsIds // quite verbose, i know.

nice, now we only need to declare our ViewModel as

public class CategoryViewModel : ICategoryWithProductsAndChildsIds 

and implement it the way we want.

The interface looks like it has too many things, of course it can be splitted with ICategoryBasic, IProducts, IChilds, or whatever you may want to name those.

So when we implement another viewModel, we can choose to do only IProducts. We can have our services having methods (private or not) to retrieve those contracts, and glue the pieces in the service layer. (Easy to say than done)

When I get into a fully working code, I might create a blog post or a github repo, but for now, I don't have it yet, so this is all for now.

I believe the Repository should be only for CRUD operations.

public interface IRepository<T>
{
    Add(T)
    Remove(T)
    Get(id)
    ...
}

So IRepository would have: Add, Remove, Update, Get, GetAll and possibly a version of each of those that takes a list, i.e, AddMany, RemoveMany, etc.

For performing search retrieval operations you should have a second interface such as an IFinder. You can either go with a specification, so IFinder could have a Find(criteria) method that takes criterias. Or you can go with things like IPersonFinder which would define custom functions such as: a FindPersonByName, FindPersonByAge etc.

public interface IMyObjectFinder
{
    FindByName(name)
    FindByEmail(email)
    FindAllSmallerThen(amount)
    FindAllThatArePartOf(group)
    ...
}

The alternative would be:

public interface IFinder<T>
{
    Find(criterias)
}

This second approach is more complex. You need to define a strategy for the criterias. Are you going to use a query language of some sort, or a more simple key-value association, etc. The full power of the interface is also harder to understand from simply looking at it. It's also easier to leak implementations with this method, because the criterias could be based around a particular type of persistence system, like if you take a SQL query as criteria for example. On the other hand, it might prevent you from having to continuously come back to the IFinder because you've hit a special use case that requires a more specific query. I say it might, because your criteria strategy will not necessarily cover 100% of the querying use cases you might need.

You could also decide to mix both together, and have an IFinder defining a Find method, and IMyObjectFinders that implement IFinder, but also add custom methods such as FindByName.

The service acts as a supervisor. Say you need to retrieve an item but must also process the item before it is returned to the client, and that processing might require information found in other items. So the service would retrieve all appropriate items using the Repositories and the Finders, it would then send the item to be processed to objects that encapsulates the necessary processing logic, and finally it would return the item requested by the client. Sometime, no processing and no extra retrievals will be required, in such cases, you don't need to have a service. You can have clients directly call into the Repositories and the Finders. This is one difference with the Onion and a Layered architecture, in the Onion, everything that is more outside can access everything more inside, not only the layer before it.

It would be the role of the repository to load the full hierarchy of what is needed to properly construct the item that it returns. So if your repository returns an item that has a List of another type of item, it should already resolve that. Personally though, I like to design my objects so that they don't contain references to other items, because it makes the repository more complex. I prefer to have my objects keep the Id of other items, so that if the client really needs that other item, he can query it again with the proper Repository given the Id. This flattens out all items returned by the Repositories, yet still let's you create hierarchies if you need to.

You could, if you really felt the need to, add a restraining mechanism to your Repository, so that you can specify exactly which field of the item you need. Say you have a Person, and only care for his name, you could do Get(id, name) and the Repository would not bother with getting every field of the Person, only it's name field. Doing this though, adds considerable complexity to the repository. And doing this with hierarchical objects is even more complex, especially if you want to restrict fields inside fields of fields. So I don't really recommend it. The only good reason for this, to me, would be cases where performance is critical, and nothing else can be done to improve the performance.

The repository is not a gateway to access Database. It is an abstraction that allow you to store and load domain objects from some form of persistence store. (Database, Cache or even plain Collection). It take or return the domain objects instead of its internal field, hence it is an object oriented interface.

It is not recommended to add some methods like GetAllProductsByCategoryId or GetProductByName to the repository, because you will add more and more methods the repository as your use case/ object field count increase. Instead it is better to have a query method on the repository which takes a Specification. You can pass different implementations of the Specification to retrieve the products.

Overall, the goal of repository pattern is to create a storage abstraction that does not require changes when the use cases changes. This article talks about the Repository pattern in domain modelling in great detail. You may be interested.

For the second question: If I see a ProductRepository in the code, I'd expect that it returns me a list of Product. I also expect that each of the Product instance is complete. For example, if Product has a reference to ProductDetail object, I'd expect that Product.getDetail() returns me a ProductDetail instance rather than null. Maybe the implementation of the repository load ProductDetail together with Product, maybe the getDetail() method invoke ProductDetailRepository on the fly. I don't really care as a user of the repository. It is also possible that the Product only returns a ProductDetail id when I call getDetail(). It is perfect fine from the repository's contract point of view. However it complicates my client code and forces me to call ProductDetailRepository myself.

By the way, I've seen many service classes that solely wrap the repository classes in my past. I think it is an anti-pattern. It is better to have the callers of the services to use the repositories directly.

Repository pattern mediates between the domain and data mapping layers using a collection-like interface for accessing domain objects.

So, repositories is to provide interface for CRUD operation on domain entities. Remember that Repositories deals with whole Aggregate.

Aggregates are groups of things that belong together. An Aggregate Root is the thing that holds them all together.

Example Order and OrderLines:

OrderLines have no reason to exist without their parent Order, nor can they belong to any other Order. In this case, Order and OrderLines would probably be an Aggregate, and the Order would be the Aggregate Root

Business logic should be in Domain Entities, not in Repository layer , application logic should be in service layer like your mention, services in here play a role as coordinator between repositoies.