How to break apart layers in a strict-layered arch

问题:

I've received the go-ahead to start building the foundation for a new architecture for our code base at my company. The impetus for this initiative is the fact that:

• Our code base is over ten years old and is finally breaking at the seams as we try to scale.
• The top "layers", if you want to call them such, are a mess of classic ASP and .NET.
• Our database is filled with a bunch of unholy stored procs which contain thousands of lines of business logic and validation.
• Prior developers created "clever" solutions that are non-extensible, non-reusable, and exhibit very obvious anti-patterns; these need to be deprecated in short order.

I've been referencing the MS Patterns and Practices Architecture Guide quite heavily as I work toward an initial design, but I still have some lingering questions before I commit to anything. Before I get into the questions, here is what I have so far for the architecture:

(High-level)

(Business and Data layers in depth)

The diagrams basically show how I intend to break apart each layer into multiple assemblies. So in this candidate architecture, we'd have eleven assemblies, not including the top-most layers.

Here's the breakdown, with a description of each assembly:

• Company.Project.Common.OperationalManagement : Contains components which implement exception handling policies, logging, performance counters, configuration, and tracing.
• Company.Project.Common.Security : Contains components which perform authentication, authorization, and validation.
• Company.Project.Common.Communication : Contains components which may be used to communicate with other services and applications (basically a bunch of reusable WCF clients).
• Company.Project.Business.Interfaces : Contains the interfaces and abstract classes which are used to interact with the business layer from high-level layers.
• Company.Project.Business.Workflows : Contains components and logic related to the creation and maintenance of business workflows.
• Company.Project.Business.Components : Contains components which encapsulate business rules and validation.
• Company.Project.Business.Entities : Contains data objects that are representative of business entities at a high-level. Some of these may be unique, some may be composites formed from more granular data entities from the data layer.
• Company.Project.Data.Interfaces : Contains the interfaces and abstract classes which are used to interact with the data access layer in a repository style.
• Company.Project.Data.ServiceGateways : Contains service clients and components which are used to call out to and fetch data from external systems.
• Company.Project.Data.Components : Contains components which are used to communicate with a database.
• Company.Project.Data.Entities : Contains much more granular entities which represent business data at a low level, suitable for persisting to a database or other data source in a transactional manner.

My intent is that this should be a strict-layered design (a layer may only communicate with the layer directly below it) and the modular break-down of the layers should promote high cohesion and loose coupling. But I still have some concerns. Here are my questions, which I feel are objective enough that they are suitable here on SO...

1. Are my naming conventions for each module and its respective assembly following standard conventions, or is there a different way I should be going about this?
2. Is it beneficial to break apart the business and data layers into multiple assemblies?
3. Is it beneficial to have the interfaces and abstract classes for each layer in their own assemblies?
4. MOST IMPORTANTLY - Is it beneficial to have an "Entities" assembly for both the business and data layers? My concern here is that if you include the classes that will be generated by LINQ to SQL inside the data access components, then a given entity will be represented in three different places in the code base. Obviously tools like AutoMapper may be able to help, but I'm still not 100%. The reason that I have them broken apart like this is to A - Enforce a strict-layered architecture and B - Promote a looser coupling between layers and minimize breakage when changes to the business domain behind each entity occur. However, I'd like to get some guidance from people who are much more seasoned in architecture than I am.

If you could answer my questions or point me in the right direction I'd be most grateful. Thanks.

---

EDIT: Wanted to include some additional details that seem to be more pertinent after reading Baboon's answer. The database tables are also an unholy mess and are quasi-relational, at best. However, I'm not allowed to fully rearchitect the database and do a data clean-up: the furthest down to the core I can go is to create new stored procs and start deprecating the old ones. That's why I'm leaning toward having entities defined explicitly in the data layer--to try to use the classes generated by LINQ to SQL (or any other ORM) as data entities just doesn't seem feasible.

回答1:

I actually just started the same thing, so hopefully this will help or at least generate more comments and even help for myself :)

1. Are my naming conventions for each module and its respective assembly following standard conventions, or is there a different way I should be going about this?

According to MSDN Names of Namespaces, this seems to be ok. They lay it out as:

<Company>.(<Product>|<Technology>)[.<Feature>][.<Subnamespace>]
For example, Microsoft.WindowsMobile.DirectX.

2.Is it beneficial to break apart the business and data layers into multiple assemblies?

I definitely think its beneficial to break apart the business and data layers into multiple assemblies. However, in my solution, I've create just two assemblies (DataLayer and BusinessLayer). The other details like Interfaces, Workflows, etc I would create directories for under each assembly. I dont think you need to split them up at that level.

3.Is it beneficial to have the interfaces and abstract classes for each layer in their own assemblies?

Kind of goes along with the above comments.

4.Is it beneficial to have an "Entities" assembly for both the business and data layers?

Yes. I would say that your data entities might not map directly to what your business model will be. When storing the data to a database or other medium, you might need to change things around to have it play nice. The entities that you expose to your service layer should be useable for the UI. The entities you use for you Data Access Layer should be useable for you storage medium. AutoMapper is definitely your friend and can help with mapping as you mentioned. So this is how it shapes up:

Service Layer Details http://i.msdn.microsoft.com/dynimg/IC350997.png

回答2:

I would disagree with this standard layered architecture in favor of a onion architecture.

According to that, I can give a try at your questions:

1. Are my naming conventions for each module and its respective assembly following standard conventions, or is there a different way I should be going about this?

Yes, I would agree that it is not a bad convention, and pretty much standard.

2. Is it beneficial to break apart the business and data layers into multiple assemblies?

Yes, but I rather have one assembly called Domain (usually Core.Domain) and other one called Data (Core.Data). Domain assembly contains all the entities (as per domain-driven-design) along with repository interfaces, services, factories etc... Data assembly references the Domain and implements concrete repositories, with an ORM.

3. Is it beneficial to have the interfaces and abstract classes for each layer in their own assemblies?

Depending on various reasons. In the answer to the previous question, I've mentioned separating interfaces for repositories into the Domain, and concrete repositories in Data assembly. This gives you clean Domain without any "pollution" from any specific data or any other technology. Generally, I base my code by thinking on a TDD-oriented level, extracting all dependencies from classes making them more usable, following the SRP principle, and thinking what can go wrong when other people on the team use the architecture :) For example, one big advantage of separating into assemblies is that you control your references and clearly state "no data-access code in domain!".

4. Is it beneficial to have an "Entities" assembly for both the business and data layers?

I would disagree, and say no. You should have your core entities, and map them to the database through an ORM. If you have complex presentation logic, you can have something like ViewModel objects, which are basically entities dumbed down just with data suited for representation in the UI. If you have something like a network in-between, you can have special DTO objects as well, to minimize network calls. But, I think having data and separate business entities just complicates the matter.

One thing as well to add here, if you are starting a new architecture, and you are talking about an application that already exists for 10 years, you should consider better ORM tools from LINQ-to-SQL, either Entity Framework or NHibernate (I opt for NHibernate in my opinion).

I would also add that answering to as many question as there are in one application architecture is hard, so try posting your questions separately and more specifically. For each of the parts of architecture (UI, service layers, domain, security and other cross-concerns) you could have multiple-page discussions. Also, remember not to over-architecture your solutions, and with that complicating things even more then needed!

回答3:

1) The naming is absolutely fine, just as SwDevMan81 stated.

2) Absolutely, If WCF gets outdated in a few years, you'll only have to change your DAL.

3) The rule of thumb is to ask yourself this simple question: "Can I think of a case where I will make smart use of this?".
When talking about your WCF contracts, yes, definitely put those in a separate assembly: it is key to a good WCF design (I'll go into more details).
When talking about an interface defined in AssemblyA, and is implemented in AssemblyB, then the properties/methods described in those interfaces are used in AssemblyC, you are fine as long as every class defined in AssemblyB is used in C through an interface. Otherwise, you'll have to reference both A, and B: you lose.

4) The only reason I can think of to actually move around 3 times the same looking object, is bad design: the database relations were poorly crafted, and thus you have to tweak the objects that come out to have something you can work with.
If you redo the architecture, you can have another assembly, used in pretty much every project, called "Entities" that holds the data objects. By every project i meant WCF as well.

On a side note, I would add that the WCF service should be split into 3 assemblies: the ServiceContracts, the Service itself, and the Entities we talked about. I had a good video on that last point, but it's at work, i'll link it tomorow!

HTH,

bab.

EDIT: here is the video.