In a 1 tier application i.e Mvc, you get a folder called models and you build and store your classes in there, I'm aware when it comes to a three tier application from what I have read it seems correct to store the models inside the business layer (2nd tier), and from the UI (first tier) I would add a project reference to the 2nd tier which will allow me to use the models and make calls to methods.
From the second tiers perspective it would call the data layer (third tier) and perform crud operations against the database, but the data layer would require the models from the business layer so when I try to add a project reference from data layer to business layer i get the error
A Reference to "Business Layer" could not be added, adding this project as a reference would cause a circular dependency
Which I understand as a reference has already been made via business layer to data layer
How would I get around this? do I create additional models in the data layer and populate them with results from the database and pass that back to the business layer which then passes it back up to the UI? I'm slightly confused on this.
** Update **
From what I have read for the data layer to reference models inside the Business layer I would need to do model mapping, My model mapping will be quite large so I'm thinking of including a 4th tier which will be a shared library and that will consist of all the models that way the data layer and the business layer can access the models as and when is required.
A little off topic but ...
Depending on the size of your application, there may be no reason to introduce unnecessary complexities to try and follow pattern you may not necessarily understand. Doing so, will cause you extra headache.
That being said, if your project is of a large size and requires some good organizing, I would strongly suggest more research and maybe a few sample project in where you try out the architecture you came up with. No way you'll get it right the first time.
I would personally suggest looking into "onion architecture" instead of "n-tier", and of course you'll find many different views on it. You'll have to make your own decisions.
Here's a generic set-up I would start off with.
Core. It knows about no other project. This is where your classes with your "business" go. Such as Customer, Product, Order, etc.
Data. It knows about Core. It is responsible for taking in a Core object and storing it somewhere. That is all.
Service. It knows about Core AND Data. It's job is to expose methods such as "Create Customer", it will be responsible for creating a customer and storing it.
Web / Api / Mvc. This will know about Service. It's job is to expose your service to the user. (UI and such). (It may also know about Core / Data ... but that's a larger discussion.)
You're on the right track but I find it easier to think of the model layer as the data (third tier), the controller (business layer, second tier) manages the flow of the data between the ui (first tier) and the data layer.
If you modify your architecture this way, you should get rid of the circular references.
This also allows you to map your data layer objects to the appropriate less/more complex structures of the middle tier in a way which simplifies the interface shown to the UI and encapsulates the business logic there too.
Related
My project has 4 layers: Presentation, application, domain, infrastructure.
I am using ORM for the write side. Read side, sometimes I have cases that have a complex query. I process by using a native query.
I write it at the infrastructure layer. But, I see some projects such as "ESHOPONWEB" write a native query in application layers.
Which layer should contain the native query?
I wasn't familiar with ESHOPONWEB aka eShopOnWeb, a reference architecture & implementation that Microsoft put out, but I see it has an accompanying eBook which includes an "Architectural principles" section, another on "Common web application architectures", and at a glance other sections explain the architecture in more detail.
Using the book and code should give you a good way to learn the relevant concepts at your own pace.
Which layer should contain the native query?
I'm not 100% sure what you mean by "native query". I did a quick search through the eBook and didn't see any obvious references to that.
Typical best practice architecture (like what is in the eBook) says that queries in the Application / Business Logic layer should be agnostic - meaning they should not contain any code or dependencies that tightly-couples the Business Logic to a specific Data Access provider.
So, if you need to query a data base, the database specific query code (or reference to a Stored Procedure in the database or any other database specific concept) should be in the Data Access code, which is in the Data Access (Infrastructure) layer.
If something in your Business Logic layer wants to do a query, there's three approaches that come to mind:
It wants to query itself, which it is free to do, using agnostic code against objects it already has.
It wants to query data from the database, it gets the Data Access to do that. The query inputs and results need to be transported between these layers agnostically. E.g. query results might come back as a collection/array/list of Data Transfer Objects (DTO) or Plain Old CLR/Class Objects (POCO).
You take some section of data from your database / Data Access layer, pre-load that as a database agnostic data model in the Business Logic layer, which it can query just like #1.
I'm not sure, but your Domain layer might be what you can use to do #3?
Does that make sense, and answer your question?
Regarding Layers
As the eShopOnWeb book explains, you want to keep various "concerns" (like business logic, UI, and data access all separate. The separation is to make maintenance and change easier to manage, especially over a long time. E.g.:
Keeping the business logic layer/code separate from the UI layer/code means that you should be able to add new types of UI without having to re-write the application code.
Keeping the business logic layer/code separate from data access layer/code means that you should be able to add/swap data providers (e.g. different types of database), and the business logic should be less impacted by database changes.
In the two points, above I say "should" because it depends on how clean and effective your separation is.
To translate that to your layers, UI is obviously your Presenation layer, Business Logic sounds like your Application and Domain layers, Data Access is your Infrastructure layer.
In the eBook they give an example (fig 5-1, pg 16) where they logically have those four layers you use but in the one code-base / package (it will all get deployed together).
In typical DDD architecture we have 3 layers:
Domain - no references
Application - it has reference to Domain layer
Infrastructure - it has reference to Domain layer
(+ Web / UI project)
Domain models live of course in Domain layer. But in which layer should live read models (projections) for read database, for example MongoDb?
Short answer, both Application Services (Application layer) and Repositories (Infrastructure layer) know about the READ models. The domain layer remains transparent to underlying persistence and loading mechanisms.
Long answer, the exact usage mechanism depends on how you use the read models. You could either be using them to construct objects used in your domain layer or more typically, only as responses to API queries.
First case: Use Read Models as objects in the domain layer
The Application service loads the READ model from the repository into the domain entity. It is the repository's responsibility to populate the READ model correctly into the domain entity. The repository is also responsible for transforming the domain entity into the WRITE model to persist in the primary database.
By the time you get to the Domain model, objects are already loaded into memory with the help of repositories. So the domain layer does not even know about the READ model and a WRITE model; it only deals with the domain entity.
Second case: Use Read Models for storing pre-built responses to API queries
This scenario is a more typical use of the READ models. Usually, there are more than one read models for the same Entity/Aggregate because they are custom-built for specific API requests.
In this case, we don't even touch the domain layer. The Application Service accepts the request, uses the READ model repository to load the object, and return a response to the application server.
There's no written law that dictates in which project a read model should live. In my personal opinion, I think having a separate read model project has its benefits. With command query responsibility segregation things tend to get pretty confusing if the command-part of the application can access the query-part of the application. I think the two should be clearly separated.
I've spent some time working on an example project that demonstrates how to set up your DDD/ports-and-adapters/CQRS application. I've dropped the code on GitHub: https://github.com/appie2go/steal-this-code
I've also spent some time to explain the choices I've made in detail in the following articles:
https://medium.com/#abstarreveld/implementing-dddomain-models-ports-adapters-and-cqrs-with-c-2b81403f09f7 and,
https://medium.com/#abstarreveld/dddomain-models-ports-adapters-and-cqrs-reference-architecture-c-504817df65ec
Hope it helps!
Cheers
To be honest, it doesn't really matter. There's no default structure for neither DDD-oriented implementation or event sourcing-oriented implementation.
You can perfectly have a single project if the system is small. If you want to keep your domain clean of external references - you can keep it in a separate project and take care of having zero references except for something you need to support the domain model basis, like entity base class and so on.
Read models and projections are completely orthogonal to the domain model and you usually need them for the query API or query services. You will benefit from keeping read models (documents in case of MongoDB) and projections in one place. You can either reference this project from your API project or keep the query API, query services, query models, read models and projections together.
Again, I would argue that such a thing as a "typical DDD architecture" doesn't exist, because DDD is not architecture to start with. Splitting projects is more a developer convenience and discipline concern and splitting the system is the architectural concern, which is not DDD-specific.
One thing that also comes to my mind is that if you really think DDD, you might first want to find out what is your context map, how many domain models you really need and maybe there you can find some ideas about separation, not based on technical concerns really.
I Am creating a web application and first use Entity Framework. I created Entity Data Model and now I am not sure, how to proceed now.
Premise: My database is really simple (Rating, WebPage, Visitor) and database tables corresponds to the business objects.
My suggestion is 3tier architecture but how to make it?
It is good idea create partial classes with the same name as Entity Framework objects (Rating, Visitor) and declare here new methods (GetAverageRating()...)? Or is better create some VisitorProvider, RatingProvider and place logic here?
It is better use EF objects in BLL and Presentation Layer or I should create my own BO objects on my BLL layer and transform EF object to BO?
I'm think, it is more practical use static methods on my DAL than instantiate classes on BLL. Do you agree?
Can you recommend me some best practices? I have many ideas how to create it, but I do not know what is the right.
3 layer architecture is quite popular but what it really means?
Presentation layer
Application layer
Database layer
If you ask what each layer means you can be pretty sure you will get several different answers. You can further divide each layer into sublayer and build layered hell like:
Client side presentation layer
Server side view layer
Controller layer
Service facade layer
Service layer
Domain objects layer
Repository + Factory layer
ORM layer
Stored procedure layer
Database view layer
Database table layer
WTF? That is just example that application can be easily over architected. It can go even worse if you insist that only neighbours can exchange data and if you decide to add special type of objects to be exchanged between layers instead of flowing sing set of objects through multiple layers.
Add layers which you need to make you more comfortable with developing the application and which will do reasonable separation of concerns and maintainability needed for the scale of your application. You can simply do the most simplest application which will be used just few weeks and must be developed as fast as possible. In such case you can do that within few days simply by using ASP.NET web forms and data source controls (or ASP.NET dynamic data). It can be badly extensible but in such situation it is exactly what you need to implement application quickly. Writing layers and doing all the stuff around maintainability and extensibility is reasonable if you need it. Another quick prototyping technique is ASP.NET MVC Scaffolding which can create quick multilayered skeleton of the application which can be further modified.
Both approaches are correct and it only depends on the approach you like. The first is called active record pattern but it is not used very often with entity framework. The second approach is more popular. You can either use EF directly in some middle class which you called Provider (common name is also Service). This class will do both data access logic and business logic. In more complex applications developers like to somehow wrap EF to separate class following repository pattern and call the repository either from service or directly from web app. code behind or controller (depending on amount of business logic). Try to do it without repository first. My personal opinion is that people should start to use repository only once they understand EF itself.
Again both approaches are correct. In a simple application it is fully acceptable to create EF model with POCO classes (EFv4.x) and use them in all layers. If you are using ASP.NET MVC you can find that you need special classes as view models to fully represent needs of your individual views. In a more complex application you can have separate objects exposed from a business layer - this is especially used if the business layer is exposed as a remote service (WCF).
It depends how you write these DAL methods - it is absolutely necessary to not share the EF context among requests! It also depends if you want to write some test or not. Layer defined by static methods is something which goes directly against testable architecture where you want unit test just single layer (unit testing with EF can be hard). It also depends if you want to use dependency injection which is based on instances.
I have an ASP.NET app with a three layer architecture:
Presentation layer: ASP.NET
Bussiness Layer: C# library.
Data Access Layer: C# library with
ADO.Net Entity Framework objects.
Some methods on Bussiness layer would return ADO.NET entity objects but, data access layer is not visible at Presentation layer I can't do that.
My question is: On a design view, Is it correct to expose Entity Objects in the Presentation Layer? I think I only have to link Data Layer library with ASP.NET app.
Thank you!
It's absolutely desirable to have your entity objects available for use and consumption in your presentation tier. That's what all the work is for.
Binding collection of objects to a grid/listview/dropdown
Splashing a single object (i.e. customer) onto a form for read/update/delete
This makes your life easier by far. Otherwise you'd have to pass string after int after double after string between your presentation and business layers.
These may be Entity objects or even your own POCO objects that were hydrated from the Entity objects.
I would even go so far as to say that your Entites should be in their own assembly separate from the DAL.
I suggest that you look into the concepts of View objects...or Data Transfer Objects (DTO). You might consider using a tool like AutoMapper or similar which will create a view specific domain object out of your entities. In general you may have screens that need an entity present to perform its work. But more often than not you will need to pass several different entities. In this case you are better off creating one DTO that contains all of these entities. By doing this you are adding a layer of separation between your presentation layer and your business layer. Often times your entities have more power than you might want to expose to your presentation layer. And...vice versa. Frequently you may need to get some UI messages out to the presentation layer based on some validation flagged in your business layer. Rather than make your ui more complex than it needs to be (by passing in your full entities) you can only pass in what the UI needs in the form of the DTO. Also, there is never a need for your business objects to care about anything specific to the presentation layer. I suggest that you not databind directly to anything as far back as the data access layer. Technically your presentation layer should know as little as possible about your business layer. In the case of MVP or MVC this is very easy to achieve by disconnecting the front end and the back end by way of this additional separation!
I think no, it is not, the best way to do that is to separate data classes from behavior, and reference only data classes in presentation level.The good approach I think to use WCF see this link
See Supervising Controller and Passive View
If you pass the Entity, you are essentially Supervising controller. Otherwise you are Passive View.
Supervising controller is less work, but less testable. Supervising Controller also says databinding is OK.
Passive view is testable but a LOT more work. No databinding. Lots of properties.
Typically I stick with Supervising Controller. You typically don't need that level of testability and it isn't worth the extra trouble.
So i am reorganizing a winforms C# solution to help decouple and make it cleaner and more organized. The solution tracks a small business orders,etc . .
I have broken out the projects so far into
App.View - all GUI Related Code
App.Data - just data structures and interfaces. No other implementation code
App.BusinessLogic - all business logic code that has no GUI references
I have some classes that i can't figure out where they belong. Please let me know your thoughts which project each class should go or if there is another project that should get created for this.
A Class that retrieves user preferences from a database
A Class that retrieves static data from our static data server and returns sets of data results.
A Class that brings down user entitlements
A model class that stores a hashtable of orders
A class that emails out messages on a user action
Actually, I think you have things a little off from a traditional layered architecture. Normally, the models of your data that your application works on would be kept in a business layer, along with the code to operate on them. Your data layer would have both the data models of your persistence framework and the code to interact with that framework. I think this might be the source of the confusion between the suggested locations of your classes and your reaction to it based on your comments.
From that perspective anything that retrieves or brings would necessarily be located in your data layer -- it's accessing data in persistent storage. What it retrieves are eventually converted into business layer objects that your business logic operates on. Things are are conceptual models -- like a table of orders -- or business actions belong in the business layer. I would agree with #Adron with, perhaps, the same confusion about where (3) goes depending on what it actually is.
More specifically:
User Preferences are business
objects, the thing that retrieves
them is a data layer object.
The static data maps on to a business
object (table or view or something),
the thing that accesses the external
server is a data layer object.
The user entitlement is a business object, the thing that retrieves it is data layer object.
A table of Orders is a business object
Emailing is a business activity, so the thing that mails people is a business object
[EDIT] My generalized 3-Tier Architecture for (simple) web apps
DataAccessLayer
This would include my TableAdapters and strongly typed DataTables and Factories that turn rows of my DataTables into business objects in pre-LINQ projects. Using LINQ this would include my DataContext and designer generated LINQ entities.
BusinessLayer
This would include any business logic, including validation and security. In pre-LINQ these would be my business objects and any other classes that implement the logic of the application. Using LINQ these are the partial class implementations of my LINQ entities to implement security and validation along with any other classes to implement business logic.
Presentation
These are my web forms -- basically the UI of the app. I do include some of the validation logic in the forms as an optimization, although these are also validated in the BL. This would also include any user controls.
Note: This is the logical structure. The project structure generally mirrors this, but there are some cases, like connections to web services, that may be directly included in the web project even though logically the components are really in the BL/DAL.
Note: I'll probably be moving to MVC over 3-Tier once ASP.NET MVC is in production. I've done some personal projects in Ruby/Rails and I really like the MVC paradigm for web apps.
You have specified that App.Data should contain only data structures and interfaces, no implementation code, which is fine if you want to do that, but that leaves you with nowhere to put your database access code except in your App.BusinessLogic assembly.
Perhaps you really need to rename App.Data to App.Model (or something similar), and have a new App.DataAccess assembly that talks to the database (perhaps implementing a Repository pattern). Having done that, I would split things up like this:
App.DataAccess
App.DataAccess
App.DataAccess
App.Model
App.BusinessLogic
I would probably go with
Data
Data
Data, although I'm not entirely sure what the class is doing
Data
BusinessLogic
-> App.Data
-> App.Data
-> App.BusinessLogic or App.Data - not sure exactly what this means.
-> App.BusinessLogic
-> App.BusinessLogic