Windows Communication Foundation: Steroids for your Enterprise Service-Oriented Architecture (Part II)
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by Tom Fuller
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Sign Here on the Dotted Line

Contracts are another critical component of the policy that must be agreed upon between clients and services. Contracts up until today were written with either IDL (Interface Definition Language) or WSDL (Web Service Description Language). WSDL is the most recent attempt at answering the what a service is supposed to do. WSDL has the same information you see in IDL plus much more. WSDL is not really changing with the introduction of WCF services; what does change is how we go about defining what our WSDL should look like.

WCF services have defined a variety of contract types that are used to put together the WSDL document that a service exposes. Service contracts, data contracts, and message contracts all help to define the WSDL a service will expose. I will walk us through the different contract types and map them to those elements of WSDL we are already familiar with. This will help to show some of the obvious parallels between WSDL and WCF service definitions.

Service Contracts

Service contracts are the API for your service. These contracts can be defined using interfaces, or they can be defined on the actual class that implements the service behavior. The following is an example of a service contract using a C# interface.

IFamilyGuyGoodies.cs

[ServiceContract(Namespace = "http://familyguyquotes")]

public interface IFamilyGuyGoodies

{

[OperationContract]

string GetQuote(string familyGuyCharacter);

}

As you have probably already realized, this service contract does not specify much. This really just demonstrates the use of an interface instead of a class to define a service. You should take note that the access modifiers have been left off intentionally. The concept of public and private are meaningless when identifying methods on a service contract. If you expose that functionality using the [OperationContract] then it will be capable of being invoked externally. This again goes back to making explicit decisions about the behavior that is exposed.

It is also important to note that within the service contract and operation contract there are a number of declarative settings that can be enforced. For example, if this method was intended to use simplex messaging then you would specify IsOneWay=True. It is also within this service contract definition that I can make explicit assertions about my binding requirements. I would add an attribute [BindingRequirements()] to specify transaction flow requirements or reliable messaging requirements.

The service contract maps to the portType section of the WSDL document. With the current ASMX stack the [webMethods()] that you define are what make up this section. With WCF, you are capable of making specific decisions about how you want consumers to interact with your service by building interfaces first before diving right into implementation. This decision to focus on API first can lead to much better decisions about what is truly required to invoke a service operation.

Data Contracts

WCF gives us a whole new level of control for how we specify the data that is passed back and forth between the client and the service. A data contract allows us to specify what members of a custom type are serialized and how those members are serialized. The data contracts are analogous to the type element in WSDL today. The following is an example of a data contract in WCF.

Quote.cs

[DataContract]

class Quote

{

[DataMember] string quoteText;

[DataMember(VersionAdded=2)]string episodeFound;

}

I have again used a simple type to demonstrate the most obvious advantages of the data contracts with WCF. Notice the [DataMember] attribute; this is the only way to identify to the serializer that this member should be expected as part of the SOAP payload. As with service contracts, the access modifiers are meaningless.

I also used this simple example to showcase the implicit versioning available in the data contract. Notice that the episodeFound member above was added in a later version of the data contract. This in turn allows a client to specify that it is using version 1 of this service and that that portion of the data is not required to communicate successfully with the service.

WCF data contracts give developers their first set of built-in versioning capabilities. Abiding by some very simple rules will allow for the flexibility to add and remove portions of your data being sent back and forth to a service. This reinforces a couple of service tenets. First, you are obviously exchanging contracts and schema, not types. If you understand how this code translates to WSDL, this becomes even more apparent. I also believe that through data contract versioning you are able to remain autonomous. If the services we develop today can supply a variety of versions to a variety of consumers then the service can remain in control of how to handle those various versions and maintain self-governance.

Message Contracts

One of the types of services that we discussed earlier was a typed message service. This type of service uses typed messages to invoke a service and receives typed messages as responses. One of the common needs of these types of services is to manipulate the format of the actual SOAP message that is being passed between the client and the service. In the past this would have to be done with complicated SOAP extensions. With WCF message contracts, we are able to easily specify the portions of a message that should live in the SOAP header and those that should live in the body. Often, these two sections of the message will have different encryption requirements, and the message contract is a good way to provide flexibility in how we structure that message. The message contract maps to the message section of WSDL today. The following is an example of a WCF message contract.

FamilyGuyQuoteRequest.cs

[MessageContract]

public class FamilyGuyQuoteRequest

{

[MessageBody]

public string characterName;

[MessageHeader(MustUnderstand=True)]

public string requestedBy;

}

You will notice a couple of interesting things in the simple message contract defined above. First of all, notice how simple it is to specify where a member would live in a SOAP message. When using the [MessageHeader] attribute, I am telling the serializer to expect this field in the SOAP header. I also have used the MustUnderstand enumeration to tell the serializer that this is a required field in my SOAP header.

Message contracts round out the primary contract types that go into specifying exactly what a service will be passing on the wire. This takes the guesswork out of service definition. As I mentioned above, this is a vast improvement on the implicit approaches today. We really do not have this type of flexibility with ASMX. Our current approaches would require us to plug into the HTTP pipeline and intercept messages using SOAP extensions.

So Am I Truly Contract-First Now?

An interesting ongoing debate amongst those in the industry that are designing, delivering, and teaching service-orientation is whether or not a code-first approach will ever allow us to design and deliver services that are truly loosely coupled, interoperable, and ultimately long-lived. What we just looked at was a very distinct set of technologies that map directly to the WSDL contract that is generated when this service is exposed. Were we actually doing contract-first development with a code-first approach?

The purists would say that contract-first means focusing on schema (XSD) and behavior (interface) before ever writing any code. What I would say is that we do this now by specifying data contracts (schema/XSD), service contract (behavior/interface), and message contracts when necessary. These approaches are still very code-centric and will require some level of governance to avoid developers diving right into implementation before a contract has been agreed upon. However, with the right level of discipline, I think WCF has found an interesting middle ground for contract-first service development.


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User Comments

Title: out of date article   
Name: Rocky
Date: 2008-02-07 3:40:37 AM
Comment:
Please update the article it is now out of date
Title: Great Work!   
Name: Jani
Date: 2005-11-29 3:23:59 PM
Comment:
Very good article. Clarified a lot of confusions on SOA and WCF.

Keep up the good work.

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