Domain Driven Data Access Layer Example
In the past few days I have been refactoring a system that had no concept of an independent Data Access Layer (DAL). All the code to access and manipulate the database was directly in the Domain objects. I set myself the task to extract all this code and introduce a decoupled DAL. There are a fair few articles on this topic available on the net, but I could find very few with a good example to show the implementation of the solution - so this is my reason for me writing this article.
One of the key goals I wanted to achieve with the DAL, was the ability to pass my Domain objects to and from the DAL, rather than passing primitive data (string, int etc). The reason being that if I ever added or removed data to my Domain object, the method signatures to the DAL would not have to change (and for loading up data, I want the DAL to return me a Domain Object rather than a DataSet or something similar).
eg. (the following example uses Data Access Objects or DAO as the implementation of the DAL).
If I have a Customer object:
namespace Example.DomainLayer
{
public class Customer
{
private string _Name;
public string Name { get { return _Name; } }
public void Save()
{
//Validate
CustomerDAO dal = new CustomerDAO();
dal.SaveCustomer(....);
}
}
DAL method using primitive data would look something like :
namespace Example.DataAccessLayer
{
public class CustomerDAO //DAO = Data Access Object
{
public void SaveCustomer(string customerName)
{
//sql to insert customer in db
}
}
}
DAL method using Domain object:
namespace Example.DataAccessLayer
{
public class CustomerDAO
{
public void SaveCustomer(Customer theCustomer)
{
//sql to insert customer in db
}
}
}
Using the above example, the problem with the 'primitive' DAL would arise as soon as I add properties to the Customer object.
eg.
public class Customer
{
private string _Name;
public string Name { get { return _Name; } }
private DateTime _Dob;
public DateTime Dob { get { return _Dob; } }
}
To now be able to save the customers date of birth, I would need to alter the DAL method signature to something like below (causing all code calling this method to change):
public void SaveCustomer(string customerName, DateTime dob);
The DAL method using the Domain object would only have to change the internals of its method, and the method signature could remain intact.
However, this goal introduced the annoying 'Circular Reference' problem. If you are unfamiliar of this issue, then I will explain:
The Domain Layer must be able to use the DAL to load, save and manipulate data in the data storage (database) - therefore the Domain Layer assembly must contain a reference to the DAL assembly. If I want the DAL to deal with Domain objects (and not just primitive data) the DAL must contain a reference to the Domain Layer assembly. Problem: circular reference - domain layer requires the DAL and vica-versa.
The solution: Separated Interface.
Separated interface will remove the circular reference issue as well introduce some very welcome side effects - the main one being you DAL is now completely decoupled from the Domain Layer, meaning its completely interchangeable for different implementations of DAL (which will usually be for different data sources), and can be mocked out for unit testing.
I will explain by continuing on with the example:
First you would need to specifiy your interface, which will define how you communicate with your DAL.
namespace Example.DomainLayer
{
public interface ICustomerDAO
{
void Save(Customer theCustomer);
//load
//delete
//etc...
}
}
Note: the Interface is in the Domain Layer - this is because the domain objects need to know how to talk to the DAL.
Secondly, your DAL implementation will need to implement the ICustomerDAO. So we update it to be:
namespace Example.DataAccessLayer
{
using Example.DomainLayer; //need this reference so it knows
// the ICustomerDAO interface
public class CustomerDAO : ICustomerDAO
{
public void SaveCustomer(Customer theCustomer)
{
//sql to insert customer in db
}
}
}
Thirdly (not sure if thats a word), you will need to change your Domain object to use the Interface now, rather than the direct implementation:
namespace Example.DomainLayer
{
public class Customer
{
private string _Name;
public string Name { get { return _Name; } }
public void Save()
{
//Validate
ICustomerDAO dal = DataAccessFactory.GetCustomerDAO();
dal.SaveCustomer(....);
}
}
Last of all, you may notice the above, I've use a 'DataAccessFactory' to get me the implementation of the DAO to use. The code for the DataAccessFactory is below. It uses .NET reflection to load up your DAL assembly at runtime and instantiate the required DAO.
namespace Example.DomainLayer
{
internal class DataAccessFactory
{
public ICustomerDAO GetCustomerDAO()
{
string assemblyName = "Example.DataAccessLayer";
string typeName = "ExampleDataAccessLayer.CustomerDAO";
Assembly dataAccessAssembly = Assembly.Load(assemblyName);
Type dataAccessType = dataAccessAssembly.GetType(typeName);
return (ICustomerDAO) Activator.CreateInstance(dataAccessType);
}
}
}
At the moment the location of the DAL assembly and type are hardcoded, but you could easily change this to get that information from a configuration file, so you could swap DAL implementations for testing etc.
So as you can see, by introducing a DAL you have a clearer separation of concerns, your Domain objects are completely independent of their data source (and data access implementation), and your system has just become a whole lot more testable.
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12 December 2007, 6:52 PM
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15 January 2008, 6:33 PM
mattcalla.com wrote:
I have been reading about NHibernate for about a year now and have been thinking about converting to it for many of those months.
Good Article.
Just concerned about the implementation of DataAccessFactory. If i mark the class as Singleton, It will not work as desired. Another thing is i have been using the my DAL code the way it is shown in the example but never needed to use primitive methods for data access. I just created the interface for Save method and implemented it in Business Object which i used to change if i need to add or remove the field from the sql statement.
I think creating the object by Reflection will not give much benifit.
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Thanks for the comment Salim. Can you please explain why the DataAccessFactory as a singleton will not work? I have used it before as a singleton, and as long as I made it threadsafe (meaning the underlying data access was threadsafe), it worked fine.
Also, I had to use Reflection to instantiate the DAL object because I could not have a direct reference to the DAL assembly (circular reference problem). I'm welcome to suggestions and alternative ways of achieving this if you think there is a better way.
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You would achieve better "loose coupling" if your domain objects didn't even have a save method. Because, now your domain objects have to know that there is a DAL. Just have the calling code pass the domain object to the DAL directly, and the DAL only has to know what to do to save the data. Then you don't have to worry about the circular reference problem either. Enjoyed the article though.
Troy
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Hi Troy,
Thanks for the comment and advice. I have thought about this issue of the domain objects knowing about a DAL too. Are you saying they should be saved from some sort of Service Layer which interacts both with the model and the data access layer? Or are you saying the front end (Website or Windows app) should be interacting with the DAL?
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Sure, I hope it provides value.
In most cases, I would just have the website, win app, console app, etc. call the DAL for data. And the DAL provides data in the form of domain objects defined in the domain logic layer. I know some don't like the UI talking to the data layer, but it's a common pattern with people that use NHibernate. That way your web app references your domain layer, and DAL. Your DAL references your domain layer. And your domain layer doesn't have to reference anything--it can remain ignorant of the integration layers.
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Matt -- this approach seems pretty solid... and I'm trying to be sold on it, but then I found an issue that pushes me back towards Troy's approach (which is the way I'm doing things now).
What if I want to have a DAO method on my CustomerDAO called findByState(string state)?
This method returns an ArrayList of Customers, and doesn't make sense for a customer instance object to have this method on it. Thoughts?
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***Adam,
In cases where I need to load collections of data or just any operation involving the 'class' but not specific to one instance, I've usually created a static method on the class which would call the DAO.
So in this case you could have:
public class Customer
{
public static List<Customer> findByState(string state)
{
ICustomerDAO dal = DataAccessFactory.GetCustomerDAO();
return dal.findByState(state);
}
}
Having said that, since I originally wrote this article, I've done a project leaning more towards Troys solution, by having a DomainLayer, DataAccessLayer, and a ServiceLayer/ApplicationLayer that sits on top of these and co-ordinates both.
Either way, I think it's a clean solution and really depends on how complex your application is. I think for less complex applications, a static method on the domain object itself (like the example above) works fine. If it starts to get more complex and you need a complete separation of the domain and the data access, then introducing a separate 'Service' or 'Application' layer is worthwhile. (This is almost a topic of conversation for another blog post).
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Just concerned about the implementation of DataAccessFactory. If i mark the class as Singleton, It will not work as desired. Another thing is i have been using the my DAL code the way it is shown in the example but never needed to use primitive methods for data access. I just created the interface for Save method and implemented it in Business Object which i used to change if i need to add or remove the field from the sql statement.
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Thanks for the code. It was usefull for my job.
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