it seems like in my model folder, all of my classes fits into one of 3 buckets:
Pure POCO data objects / business Objects
Repository implementation code to query databases and external services
Helper code.
Is this normal and is there a best practices on how to organize this. should i have subfolders for 1, 2 + 3? any recommendations?
If you look at http://www.sharparchitecture.net which tries to provide a best practices framework, you'll see that POCO/entities, repositories and helper code are not only separated, they also exist in different assemblies. This is so that it's physically impossible to leak from one container to another. This framework grew from http://www.codeproject.com/KB/architecture/NHibernateBestPractices.aspx article which is also good to read.
Related
How can I split domain logic and data access in Grails (and is it a good idea)?
Many software applications we write are rather data(base) centered and in Grails one often persist from service classes or controllers directly to a database configured in DataSource.groovy. It is easy to change database, but we are not really independent of the persistence implementation in the code.
I am trying to write an application that opens for different persistence and data source (not only database) implementations and focus on the business domain instead of database entities. This is also a plus when testing (easy to write fake/mock persistence)
Initially I have only one persistence implementation - Grails domain classes, using GORM. But it is a possibility that I in the future would like to have other data sources than a database, for example rest services or something else.
For now, I only have the database as data source though and do mostly crud stuff (and some domain logic). I think I am still in a way stuck in "old" thinking, focused on database persistence, because most of my business domain classes, have a Grails domain class equivalent that is a copy of it. When domain classes are to be persisted, I just copy the properties to the Grails domain class.
I am not very happy with this solution. I can think of at least two possible improvements/changes:
My Grails domain classes could be organised more differently from the business domain classes, so I don't just copy properties from one class to the other. This will still involve a lot of property mapping from one class to the other when reading or writing from/to the database though.
Maybe there is a way to use business domain classes, from a regular src/main/groovy package and decorate with GORM stuff? Or in some other way split the domain logic and persistence? I have seen it is possible to do this by using hibernate conf over the domain classes. Is this the only way?
I have seen some interesting discussions of Grails architecture, including clean architecture, hexagonal architecture and ddd, but I have not found any examples yet. Are there any?
At this point, as I said, much of the functionality is CRUD stuff, but not everything. And further on, the application may have more business logic, so I would prefer not to use the "default" architecture of Grails with views, controllers, services, domain. I want a "core" application that is in a way independent of grails view/controllers and domain/GORM
It's been some time since you posted your question, but this is a very interesting topic for me...
I currently work in big-ish Java8 projects that implement principles of clean architecture, ddd, cqrs and hexagonal architecture among others. I also have limited experience with Grails 1.x projects and I remember asking the same questions as you are now.
Now that I have a broader perspective, I honestly think that it doesn't make sense to force Grails into a clean architecture. You're going to have a very painful time trying to achieve it and you probably won't be pleased with the result.
Everything in Grails is designed to be used in an opinionated, convention based way. Starting with GORM being an ActiveRecord implementation and following by every little decision that they've made about directory structure, semantics on artifacts that you need to define (controllers, services, models...), etc.. I'm not saying this is bad. In fact, this is great when you're developing something that fits into this schema-of-things.
This coupling and implicit behavior between your artifacts makes really hard to model your business logic apart from your data access (or your http interaction, or any other interaction with third parties for that matter).
From a DDD point of view you should favor data or collection based Repositories over ActiveRecord implementations. Then you can start separating your persistence logic from your Domain model. Trying to do this while maintaining ActiveRecord-like interaction with your persistence layer is going to produce a very "dirty" layer of adaptation with lots of repetition.
You will have a really hard time especially while trying to adapt complex Domain with aggregate objects that should go into different database tables, for example.
Now, addressing the two improvements that you suggest, this is what I can tell you about them:
My Grails domain classes could be organised more differently from the business domain classes, so I don't just copy properties from one class to the other. This will still involve a lot of property mapping from one class to the other when reading or writing from/to the database though.
You can indeed do what you say. Just place some code on src/groovy folder. The main problem that you will face here is dependency injection. Grails automagically injects dependencies on your services and controllers when they're defined in the standard directories. For everything else, you need to explicitly tell Grails how to take dependencies and pass them to your custom artifacts.
Maybe there is a way to use business domain classes, from a regular src/main/groovy package and decorate with GORM stuff? Or in some other way split the domain logic and persistence? I have seen it is possible to do this by using hibernate conf over the domain classes. Is this the only way?
If you decorate your Domain objects defined in src/groovy with GORM (if it is even possible) you will have the same problem. Your mission here is to isolate your Domain from the persistence logic. Doing so by having any GORM in it fails its purpose.
Everything considered my advice here would be to:
switch to other less coupled libraries that let you desing your own architecture (i.e. Ratpack, Jooq) or
if that is not an option, just embrace the Grails-way-to-do-things completely.
There is a very comprehensive list of libraries that you can browse for inspiration: Awesome Java
I'm new to asp.net MVC and EF, so excuse me if my question is not clear or easy.
I have read some tutorials about EF Code-First approach and now I'm trying this tutorial Getting Started with EF using MVC.
Using Code-First approach I'm using the POCO classes to define my DB model, DB logic, DB validation, UI validation and some UI logic. So I can use these classes(or objects) in my presentation layer or use them as JSON objects when dealing with Web-Services(or in JavaScript code).
My question: Isn't that mixing some logic together? I mean shouldn't I use like special view-model classes for presentation and these classes should have the UI logic and validation ?!
Is it good practice to send the POCO object to the view(or to the client in general) ?
Finally i need some guidance on how to organize my project layers or folders? I see many ways and I'm confused about what to choose or what format I should base my own on?!!
You should not use your data entities in higher layers of your application. Transform/compose/copy data from DAL into view model or business classes using a library like Automapper. This keeps the layers of your application independent and hides your database schema from end users.
Keep in mind that MVC is only concerned with presentation, all real work should happen in other layers of your app. I try to keep layers independent and use a little bit of glue code to bundle them into an app. I may write data access and business logic layers when working on MVC project, but then re-use in a command line utility. There are a lot of books that talk about writing quality code. I personally found Clean Code, Code Complete 2, and Design Patterns extremely helpful.
As LeffeBrune said, is bad practice to show directly your data entities to the presentation layer, you can try to expose some interfaces in form of project services that return the view model to the controller. This can help to keep the layers separate, and implement Unit of Work pattern and some other cool stuff.
You can start reading the Scott Millet Book
ASP.NET Design Patterns
for a starting point in designing a good layered application, here his blog.
You can define interfaces in your business layer which your EF entities can implement; the business logic doesn't know about the actual implementation. To do this you need the data layer to reference the business layer which means you're inverting the dependencies - you then use an IoC container to bind the interfaces to their implementations. ...but yeah that's one of many, many ways to go about it.
The thing is, with single responsibility in mind, your entities shouldn't worry about UI stuff - this can mean your interfaces also need to be implemented by "view model" classes, which implement things like validation and other business & presentation concerns.
Your questions require a lot of discussions.
Choosing infrastructure for your project is a complicated issue that depends on lots of factors. There are some design patterns that address different requirements in a project which involve you or your team in multiple concepts and technologies. I recommend you two valuable resources that help you understanding software architecture focused on .Net technologies.
CSLA.NET : The CSLA .NET framework is an application development framework that reduces the cost of building and maintaining applications. Rockford Lhotka, createor of CSLA.NET, has some books that deeply describe an optimal infrastructure of a project; all of your questions have answered in his books.
Microsoft Spain - Domain Oriented N-Layered .NET 4.0 Sample App: the project/sample is willfully restricted to .NET implementation of most used patterns in N-Layered Domain Oriented Architectures based on simple scenarios easy to understand (Customers, Orders, Bank Transfers, etc.). The project/sample is very well documented and all of your questions , also, have answered in the project.
All in all, view-models, POCOs, layers, etc. are concepts that have root in software architecture and I believe that they can not be described in short.
What I normally use is a POCO layer to use in my data access and a view model layer to use in my views just as LeffeBrune said.
All my layers use POCOs and the controllers are responsible to construct the view model that each view needs.
To get this work more automated I use automapper.
So my solution structure gets usually to be like this:
Model (POCO)
Data access (NHibertante)
Service (Business Layer)
Web (UI)
ViewModel
I'm building an MVC3 app, trying to use IoC and constructor injection. My database has (so far) about 50 tables. I am using EF4 (w/ POCO T4 template) for my DAC code. I am using the repository pattern, and each table has its own repository. My service classes in my service layer are injected w/ these repositories.
Problem: My service classes are growing in the number of repositories they need. In some cases, I am approaching 10 repositories, and it's starting to smell.
Is there a common approach for designing repositories and service classes such that the services don't require so many repositories?
Here are my thoughts, I'm just not sure which one is right:
1) This is a sign I should consider combining/grouping my repositories into related sections of tables, reducing the number or dependent repositories per service class. The problem with this approach, though, is that it will bloat and complicate my repositories, and will keep me from being able to use a common interface for all repositories (standard methods for data retrieval/update).
2) This is a sign I should consider breaking my services into groups based on my repositories (tables). Problem with this is that some of my service methods share common implementation, and breaking these across classes may complicate my dependencies.
3) This is a sign that I don't know what I'm doing, and have something fundamentally wrong that I'm not even able to see.
UPDATE: For an idea of how I'm implementing EF4 and repositories, check out this sample app on codeplex (I used version 1). However, looking at some of the comments there (and here), looks like I need to do a bit more reading to make sure this is the route I want to take -- sounds like it may not be.
Chandermani is right that some of your tables might not be core domain classes. This means you would never search for that data except in terms of a single type of parent entity. In those cases you can reference them as "complex types" rather than full-blown entities, and EF will still take care of you.
I am using the repository pattern, and each table has its own repository
I hope you're not writing these yourself from scratch.
The EF 4.1 already implements the Repository Pattern (DbSet), and the Unit of Work pattern (DbContext). The older versions do too, though the DbContext template can easily be tweaked to provide a clean mockable implementation by changing those properties to an IDbSet.
I've seen several tutorial articles where people still write their own, though. It is strange to me, because they usually don't provide a justification, other than the fact that they are "implementing the Repository Pattern".
Writing wrappers for these repositories for access methods like FindById make it slightly easier to access, but as you've seen is a big amount of effort potentially little payback. Personally, unless I find that there is interesting domain logic or complex queries to be encapsulated, I don't even bother and just use Linq directly against the IDbSet.
My service classes in my service layer are injected w/ these repositories.
Even if you choose to use custom query wrappers, you might choose to simply inject the DbContext, and let the service code instantiate the wrappers it needs. You'd still be able to mock your data access layer, you just wouldn't be able to mock up the wrapper code. I'd still recommend you inject less generic ones though, because complex implementation is exactly the type of thing you'd like to be able to factor out in maintenance, or replace with mocks.
If you look at DDD Aggregate Root pattern and try to see you data in this perspective you would realize that many of the table do not have a independent existence at all. Their data is only valid in context of their parent. Most of the operations on them require you to get the parent as well. If you can group such tables and find the parent entity\repository all other child repository can be removed. The complexity of associating the parent child which till now you would be doing in your business layer (assuming you are retrieving parent and child using independent repo) not would be shifted to the DAL
Refactoring the Service interface is also a viable option, and any common functionality can be moved into a base class and\or can be itself defined as a service which is consumed by all your existing services (Is A vs Has A)
#Chandermani has a good point about aggregate roots. Repositories should not, necessary have a 1:1 mapping to tables.
Getting large numbers of dependencies injected in is a good sign your services are doing too much. Follow the Single Responsibility Principle, and refactor them into more manageable pieces.
are your services writing to all of the repositories? i find that my services line up pretty closely with repositories, that they provide the business logic around the CRUD operations that the repository expose.
Well, not sure if that's exactly the right title, but basically I have been having a lot of problems using repositories in MVC applications in such a way that you can substitute one set of repositories, implementing a different data storage technology, for another.
For example, suppose I want to use Entity Framework for my application. However, I also want to have a set of test data implemented in hard-coded Lists. I would like to have a set of interfaces (IUserRepository, IProductRepository, etc. -- let's not talk about a more generic IRepository<T> for now) that both approaches can instantiate. Then, using (say) a Dependency Injection tool such as Ninject or Castle Windsor, I can switch back and forth between the entity framework provider (accessing the actual database) and the test provider (accessing the lists).
In a nutshell, here's the problem:
-- If you are going to use Entity Framework, you want your repositories returning IQueryable<SomeType>.
-- If you are going to use hard-coded lists, you do NOT want your repositories returning IQueryable, because it adds hugely to the overhead, and plus, Linq to Entities is significantly different from Linq to Objects, causing many headaches in the code that is common to both providers.
In other words, I have found that the best approach isolates all the EF-dependent code within the repositories, so that the repositories themselves return IEnumerable or IList or some such -- then both EF and some other technology can use the same repositories. Thus, all the IQueryable's would be contained WITHIN the EF repositories. That way, you can use Linq to Entities with the EF repositories, and Linq to Objects with the Test repositories.
Yet this approach puts an enormous amount of the business logic into the repositories, and results in much duplicated code -- the logic has to be duplicated in each of the repositories, even if the implementations are somewhat different.
The whole idea of the repositories as this layer that is very thin and just connects to the database is then lost -- the repositories are "repositories" of business logic as well as of data store connectivity. You can't just have Find, Save, Update, etc.
I've been unable to resolve this discrepancy between needing to isolate provider-dependent code, and having business logic in a centralized location.
Any ideas? If anyone could point me to an example of an implementation that addresses this concern, I would be most appreciative. (I've read a lot, but can't find anything that specifically talks about these issues.)
UPDATE:
I guess I'm starting to feel that it's probably not possible to have repositories that can be swapped out for different providers -- that if you are going to use Entity Framework, for example, you just have to devote your whole application to Entity Framework. Unit tests? I'm struggling with that. My practice to this point has been to set up a separate repository with hard-coded data and use that for unit testing, as well as to test the application itself before the database is set up. I think I will have to look to a different solution, perhaps some mocking tool.
But then that raises the question of why use repositories, and especially why use repository interfaces. I'm working on this. I think determining the best practice is going to take a bit of research.
What I can say? Welcome to the club ...
What you found is problem reached by many developers who followed "repository boom" with EFv4. Yes it is the problem and the problem is really complex. I discussed this several times:
ASP.NET MVC 3 and Entity Framework code first architecture
Organizationally, where should I put common queries when using Entity framework
Separate topic is why to use repositories:
Generic repository, what is the point
Basically your proposed way is a solution but do you really want it? In my opinion the result is not repository but the Data Access Object (DAO) exposing plenty of access methods. Repository definition by Martin Fowler is:
A Repository mediates between the
domain and data mapping layers, acting
like an in-memory domain object
collection. Client objects construct
query specifications declaratively and
submit them to Repository for
satisfaction. Objects can be added to
and removed from the Repository, as
they can from a simple collection of
objects, and the mapping code
encapsulated by the Repository will
carry out the appropriate operations
behind the scenes. Conceptually, a
Repository encapsulates the set of
objects persisted in a data store and
the operations performed over them,
providing a more object-oriented view
of the persistence layer. Repository
also supports the objective of
achieving a clean separation and
one-way dependency between the domain
and data mapping layers.
I believe exposing IQueryable fulfils this 100 times better then creating a public interface similar to repositories from Stored procedures era - one access method per stored procedure (fixed query).
The problem can be summarized by the rule of leaky abstraction. IQueryable is an abstraction of the database query but the features provided by IQueryable are dependent on the provider. Different provider = different feature set.
What is a conclusion? Do you want such architecture because of testing? In such case start using integration tests as proposed in first two linked answers because in my opinion it is the lest painful way. If you go with your proposed approach you should still use integration tests to verify your repositories hiding all EF related logic and queries.
So I have a layered ASP.NET MVC proof-of-concept application with good separation between presentation concerns, business logic, and infrastructure concerns. Right now it is operating off of a fake repository (i.e. LINQ queries against static IQueryable objects). I would like to create a functional SQL repository now.
That said, I don't want to simply tie it into a database that has a 1-1 mapping between tables and entities. That wouldn't meet the business need I am hoping to solve (partial integration with existing database - no hope for convention over configuration).
Do you have suggestions for which ORM / mapping tools I should consider and/or avoid?
Do you have suggestions for articles/books I could look at to help me approach this topic?
Would it be better to simply use parameterized queries in this scenario?
Entity Framework in version 4 would definitely allow you to:
have a mapping between the physical database schema and your conceptual schema, e.g. having an entity mapped to several tables, or several tables joined together forming a single business entity
grab data from views (instead of tables directly)
use stored procedures (where needed and appropriate) for INSERT, UPDATE, DELETE on every entity
NHibernate sounds like a good fit for what you are looking for. You will be able to make your repositories call queries in either HQL or using the API, either way you can get to your database and shape the data to fit the way your repository is being used. It will always be hard to make a square peg fit into a round hole though. SO has lots of nice support when you get into using NHibernate, good luck.
As you mentioned in the question, it is very debatable to choose an ORM. Different people will have different project needs. I am not exactly sure what will take priority for you. Here is what I have tried myself.
NHibernate seems to be the most commonly used ORM in DotNet projects. I feel it suffers from a typical open source problem. It offers so many features but the documentation really sucks. If you have lots of time at your disposal you can give it a shot.
Another options is to go for something like Entity Framework. Its very easy to set up and get up and running. With version 4.0 and the CTP there is provison for code first as well as fluent mapping and configuration. Since you have said you would want to keep the domain model separated EF 4 will help you because it has a notion of conceptual model which is an abstraction over the mapping layer.
You can refer to few links below for the blogs I had written based on my experience
http://nileshgule.blogspot.com/2010/08/entity-framework-hello-world.html
http://nileshgule.blogspot.com/2010/09/nhibernate-code-first-approach-with.html
http://nileshgule.blogspot.com/2010/09/entity-framework-first-query-using.html
http://nileshgule.blogspot.com/2010/09/entity-framework-learning-series.html