A while back I wrote a blog post about DataSets and why you shouldn’t use them on service boundaries. The fundamental issues are:
So when I saw that Entity Framework 4.0 supports Self Tracking Entities (STEs) I was interested to see how they would work – after all, automated change tracking is one of the reasons people wanted to use DataSets in service contracts. The idea is that as you manipulate the state the object itself tracks the changes to properties and whether it has been created new or marked for deletion. It does this by implementing an interface called IObjectWithchangeTracker which is then used by the ObjectContext to work out what needs to be done in terms of persistence. There is an extension method on the ObjectContext called ApplyChanges which does the heavy lifting.
The Entity Framework team has released a T4 Template to generate these STEs from an EDMX file and the nice thing is that the generated entities themselves have no dependency on the Entity Framework. Only the generated context class has this dependency and, so the story goes, the client needs to know nothing about Entity Framework, only the service does. The client, and the entities, remain ignorant of the persistence model. For this reason STEs have been touted as a powerful tool in n-tier based architectures
All of this seems almost too good to be true … and unfortunately it is.
To understand what the issue is with STEs we have to remember what two of the main goals of Service based systems are:
So to understand the problem with STEs we need to look at the generated code.Here’s the model I’m using:
Now lets look at the T4 Template generated code for, say, the OrderLine
[DataContract(IsReference = true)]
[KnownType(typeof(Order))]
public partial class OrderLine: IObjectWithChangeTracker, INotifyPropertyChanged
{
#region Primitive Properties
[DataMember]
public int id
get { return _id; }
set
if (_id != value)
ChangeTracker.RecordOriginalValue("id", _id);
_id = value;
OnPropertyChanged("id");
}
private int _id;
// more details elided for clarity
A few things to notice: firstly this is a DataContract and therefore is designed to be used on WCF contracts – that is its intent; secondly a bit of work takes place inside the generated property setters. The property setters check to see if the data is actually changed, then it records the old value and raises calls OnPropertyChanged to raise the PropertyChanged event (defined on INotifyPropertyChanged). Lets have a look inside OnPropertyChanged:
protected virtual void OnPropertyChanged(String propertyName)
if (ChangeTracker.State != ObjectState.Added && ChangeTracker.State != ObjectState.Deleted)
ChangeTracker.State = ObjectState.Modified;
if (_propertyChanged != null)
_propertyChanged(this, new PropertyChangedEventArgs(propertyName));
Ok so maybe a bit more than raising the event. It also marks the object as modified in the ChangeTracker. The ChangeTracker state is partly how the STE serializes its self tracked changes. It is this data that is used by the ApplyChanges extension method to work out what has changed. So the thing to remember here is that the change tracking is performed by code generated into the property setters.
Well the T4 Template has done its work so we create our service contract using these conveniently generated types and the client uses metadata and Add Service Reference to build its proxy code. It gets the OrderLine from the service, updates the quantity and sends it back. The service calls ApplyChanges on the context and then saves the changes and … nothing changes in the Database. What on earth went wrong?
At this point we have to step back and think about what those types we use in service contracts are actually doing. Those types are nothing more than serialization helpers – to help us bridge the object world to the XML one. The metadata generation uses the type definition and the attribute annotations to generate a schema (XSD) definition of the data in the class. Notice we’re only talking about data – there is no concept of behavior. And this is the problem When the Add Service Reference code generation takes place its based on the schema in the service metadata – so the objects *look* right, just the all important code in the property setters is missing. So you can change the state of entities in the client and the service will never be able to work out if the state has changed – so changes don’t get propagated to the database.
There is a workaround for this problem. You take the generated STEs and put them in a separate assembly which you give to the client. Now the client has all of the change tracking code available to it, changes get tracked and the service can work out what has changed and persist it.
But what have we just done? We have forced the client to be .NET. Not only that, we’ve probably compiled against .NET 4.0 and so we are requiring the client to be .NET 4.0 aware – we might as well have taken out a dependency on Entity Framework 4.0 in the client at this point. In addition, changes I make to the EDMX file are going to get reflected in the T4 generated code – I have coupled my client to my data access layer. Lets go back to the problems with using DataSets on service boundaries again. We’re back where we pretty much started – although the data being transmitted is more controlled.
So STEs at first glance look very attractive, but in service terms they are in fact similar to using DataSets in terms of the effect on the service consumer. So what is the solution? We’ll we’re back with our old friends DTOs and AutoMapper. To produce real decoupling and allow heterogeneous environments we have to explicitly model the *data* being passed at service boundaries. How this is patched this into our data access layer is up to the service. Entity Framework 4.0 certainly improves matters here over 1.0 as we can use POCOs which aid testability and flexibility of our service
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