Why should I isolate my domain entities from my presentation layer?

Question

One part of domain-driven design that there doesn\'t seem to be a lot of detail on, is how and why you should isolate your domain model from your interface. I\'m trying to c

Answer 1

Here's a real example as to why I find it good practice to separate domain entities from the view.

A few months ago I created a simple UI to show the values of Nitrogen, Phosphorus and Potassium in a soil sample through a series of 3 gauges. Each gauge had a red, green and red section, i.e. you could either have too little or too much of each component, but there was a safe green level in the middle.

Without much thought, I modelled my business logic to supply data for these 3 chemical components and a separate data sheet, containing data about the accepted levels in each of the 3 cases (including which measurement unit was being used, i.e. moles or percentage). I then modelled my UI to use a very different model, this model was concerned about gauge labels, values, boundary values and colours.

This meant when I later had to show 12 components, I just mapped the extra data into 12 new gauge view models and they appeared on the screen. It also meant I could reuse the gauge control easily and have them display other data sets.

If I'd had coupled these gauges directly into my domain entities, I would not have any of the above flexibility and any future modifications would be a headache. I've come across very similar issues when modelling calendars in the UI. If there is a requirement for a calendar appointment to turn red when there are 10+ attendees, then the business logic to handle this should remain in the business layer and all the calendar in the UI needs to know, is that it has been instructed to turn red, it should not need to know why.

Answer 2

We are using the same model in the server and on the ui. And it's a pain. We have to refactor it some day.

The problems are mainly because the domain model needs to be cut into smaller pieces to be able to serialize it without having the whole database referenced. This makes it harder to use on the server. Important links are missing. Some types are also not serializable and can't be sent to the client. For instance 'Type' or any generic class. They need to be non-generic and Type needs to be transferred as string. This generates extra properties for serialization, they are redundant and confusing.

Another problem is that the entities on the UI don't really fit. We are using databinding and many entities have lots of redundant properties only for ui purposes. Additionally there are many 'BrowsableAttribute' and others in the entity model. This is really bad.

At the end, I think it is just a matter of which way is easier. There might by projects where it just works fine and where is no need to write another DTO model.

Answer 3

Quite simply, the reason is one of implementation and drift. Yes, your presentation layer needs to know about your business objects to be able to represent them properly. Yes, initially it looks like there is a lot of overlap between the implementation of the two types of objects. The problem is, as time goes on, things get added on both sides. Presentation changes, and the needs of the presentation layer evolve to include things that are completely independent of your business layer (color, for example). Meanwhile, your domain objects change over time, and if you don't have appropriate decoupling from your interface, you run the risk of screwing up your interface layer by making seemingly benign changes to your business objects.

Personally, I believe the best way to approach things is through the strictly enforced interface paradigm; that is, your business object layer exposes an interface that is the only way that it can be communicated with; no implementation details (i.e. domain objects) about the interface are exposed. Yes, this means that you have to implement your domain objects in two locations; your interface layer and in your BO layer. But that reimplementation, while it may initially seem like extra work, helps enforce the decoupling that will save TONS of work at some point in the future.

Answer 4

Perhaps you are not conceptualizing the UI layer in broad enough terms. Think in terms of multiple forms of response (web pages, voice response, printed letters etc) and in terms of multiple languages (English, French etc.).

Now suppose that the speech engine for the telephone call-in system runs on a completely different type of computer (Mac for example) from the computer that runs the website (Windows perhaps).

Of course it is easy to fall into the trap "Well in my company we only care about English, run our website on LAMP (Linux, Apache, MySQL and PHP) and everyone uses the same version of Firefox". But what about in 5 or 10 years?

Answer 5

With the help of tool like 'Value Injecter' and the concept of 'Mappers' in the presentation layer while working with views, it's much more easy to understand each piece of code. If you have a little bit of code, you will not see the advantages immediately but when your project will growing more and more, you will be very happy while working with the views to don't have to enter into the logic of the services, repositories to understand the view model. View Model is another guard in the vast world of anti-corruption layer and worth its weight in gold in a long term project.

The only reason why I see no advantage of using view model is if your project is small and simple enough to have views binded directly to each property of your model. But if in the futur, the requirement change and some controls in the views will not be binded to the model and you don't have a view model concept, you will start adding patches in many places and you will begin having a legacy code that you will not appreciate. Sure, you can do some refactoring to transform your view-model in view-viewmodel and follow the YAGNI principle while not adding code if you don't need it but for myself, it's much more a best practice that I must follow to add a presentation layer exposing only view-model objects.

Answer 6

Answer depends on scale of your application.

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Simple CRUD (Create, Read, Update, Delete) application

For basic crud applications you do not have any functionality. Adding DTO on top of entities woudl be a waste of time. It would increase complexity without increasing scalability.

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Moderately complicated Non-CRUD application

In this size of application you will have few entities which have true lifeclycle and some business logic associated with them.

Adding DTOs on this case is a good idea for few reasons:

• Presentation layer can see only subset of fields which entity has. You encapsulate Entities
• No coupling between backend and frontent
• If you have business methods inside entities, but not in DTO then adding DTOs means that outside code can't ruin state of your entity.

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Complicated Enterprise Application

Single entity might need multiple ways of presentation. Each of them will need different set of fields. In this case you encounter the same issues as in previous example plus need to control amount of fields visible for each client. Having separate DTO for each client will help you to chose what should be visible.