Question

In some enterprise-like project (.NET, WCF) i saw that all service contracts accept a single Request parameter and always return Response:

[DataContract]
public class CustomerRequest : RequestBase {
        [DataMember]
        public long Id { get; set; }
}

[DataContract]
public class CustomerResponse : ResponseBase {
        [DataMember]
        public CustomerInfo Customer { get; set; }
}

where RequestBase/ResponseBase contain common stuff like ErrorCode, Context, etc. Bodies of both service methods and proxies are wrapped in try/catch, so the only way to check for errors is looking at ResponseBase.ErrorCode (which is enumeration).

I want to know how this technique is called and why it’s better compared to passing what’s needed as method parameters and using standard WCF context passing/faults mechanisms?

Answer 1

The pattern you are talking about is based on Contract First development. It is, however not necessary that you use the Error block pattern in WCF, you can still throw faultexceptions back to the client, instead of using the Error Xml block. The Error block has been used for a very long time and therefore, a lot of people are accustom to its use. Also, other platform developers (java for example) are not as familiar with faultExceptions, even though it is an industry standard.
http://docs.oasis-open.org/wsrf/wsrf-ws_base_faults-1.2-spec-os.pdf

The Request / Response pattern is very valuable in SOA (Service Oriented Architecture), and I would recommend using it rather than creating methods that take in parameters and pass back a value or object. You will see the benefits when you start creating your messages. As stated previously, they evolved from Contract First Development, where one would create the messages first using XSDs and generate your classes based on the XSDs. This process was used in classic web services to ensure all of your datatypes would serialize properly in SOAP. With the advent of WCF, the datacontractserializer is more intelligent and knows how to serialize types that would previously not serialize properly(e.g., ArrayLists, List, and so on).

The benefits of Request-Response Pattern are:

• You can inherit all of your request and responses from base objects where you can maintain consistency for common properties (error block for example).
• Web Services should by nature require as little documentation as possible. This pattern allows just that. Take for instance a method like public BusScheduleResponse GetBusScheduleByDateRange(BusDateRangeRequest request); The client will know by default what to pass in and what they are getting back, as well, when they build the request, they can see what is required and what is optional. Say this request has properties like Carriers [Flag Enum] (Required), StartDate(Required), EndDate(Required), PriceRange (optional), MinSeatsAvailable(Option), etc… you get the point.
• When the user received the response, it can contain a lot more data than just the usual return object. Error block, Tracking information, whatever, use your imagination.
  In the BusScheduleResponse Example, This could return Multiple Arrays of bus schedule information for multiple Carriers.

Hope this helps.

One word of caution. Don’t get confused and think I am talking about generating your own [MessageContract]s. Your Requests and Responses are DataContracts. I just want to make sure I am not confusing you. No one should create their own MessageContracts in WCF, unless they have a really good reason to do so.