What is a service after all ?
I could not find a concrete definition of service. Many authors give different definitions for the same thing but all of them agree on describing a service through the famous tenets of service orientation:
1. Services are autonomous
2. Service have explicit boundaries
3. Services share schema and contract, but do not share implementation
4. Service compatibility is determined by a policy
After reading these tenets I can conclude the following:
- Each service is independent of other services, for instance, if one service crashes, the other services are not affected.
- At first glance, I know where a service ends and another starts.
- A service can be considered as an unit of design and deployment.
- Schema describes the format and the content of the messages. Contract describes message sequences allowed in and out of the service. This is more related to the concept of loose-coupling through messaging (The client and the service are decoupled), the communication between services is performed by means of a message interchange pattern. For instance, this is not true for a .NET remoting object where the client must have a reference to the server type.
- The use of a service is governed by metadata or policies. In other words, each service has a set of requirements to be used, and these requirements are expresses through metadata. For instance security.
This is nice in theory, but not-so-clever in practice.
As you can see, these tenets do not mention anything about XML, SOAP or any of the available WS-* standards. Therefore, I assume that a Soap Service is a concrete implementation that adhere to these principles. Let's discuss more about soap service in the following paragraphs.
Soap Services interchange xml messages (Soap Messages) which basically contains data (The payload expected by the service) and metadata (Headers required to execute the service, most of them are part the WS-* Family). The service policy in this kind of service is expressed through the WS-Policy standard, and the contract through WSDL.
The concept of service becomes evident in WCF, where we can build real soap services. According to the Don Box's article "A Guide to Developing and Running Connected Systems with Indigo",
Service-oriented development focuses on systems that are built from a set of autonomous services. This difference has a profound impact on the assumptions one makes about the development experience.
In Indigo, a service is simply a program that one interacts with via message exchanges. A set of deployed services is a system.
In addition, a web service is an specific type of Soap Service that uses Http as transport for interchanging messages. The Web services (Asmx) provided by default in the .NET framework (Basic Profile 1.1) does not implement the concept of policy at all, unless you decided to use WSE or WCF.
Existing techniques to design and implement a soap service
Once we know the business requirements for a service, it is time to start thinking in the design and implementation. Nowadays, the most common paths in that direction are "Development-First" and "Contract-First".
In the first approach or technique, which is the approach used by default when you develop an asmx service with Visual Studio, the steps to design the XSD and WSDL are totally hidden for the developer.
The developer begin authoring the service class and its operations or methods (A asmx for simple web services or a service contract in WCF). In addition, he decorates the service with attributes that the framework will use later to automatically generate the WSDL for the service. In this way, the developer does not have to deal with complexities of WSDL or XSD and he can get the service running with some minimal efforts.
On the other hand, the "Contract-First" approach has a completely different starting point. As first step in this approach, the developer usually defines the XSD types and designs the service messages.
Once the service messages are ready, as second step, he begin authoring the service WSDL or contract. Finally, the latest service's WSDL version is used to generate the code and implementation. This can be easily done in .NET with the WSDL.exe tool, but other platforms such as JAVA also provides tools to perform the same task. Most developers use this approach from a client side perspective to generate the proxy classes required to consume the service.
As you can imagine, creating a WSDL by hand can be a complicated and error prone task, many developers (including me) are not familiar at all with the complete specification. However, it provides a explicit control of the generated contracts, which is in my opinion, it is a crucial point for interop scenarios.
Unfortunately, these is not a good tooling support in Visual Studio to use this approach, I mean VS does not provide the necessary tools to increase the developer productivity.
As result, the .NET community has started developing tools to support his approach as well. One of the most well-know projects in this are is WSCF created by Thinktecture, this tool basically provides a set of wizards and designers to generate the XSD types and the final WSDL.
The Service Factory Project from Patterns & Practices supports both approaches as well, if you are not familiar with project, the P&P team has lately beginning to provide software factories for different purposes, most of them are based on the Guidance Automation Toolkit (GAT). The aim of this project is to create web services using the best practices of the market and facilitate the developer work through a set of tools, wizards and code generators totally integrated in Visual Studio. Don Smith has published a nice blogcast showing the complete process to create contracts with the wizards provided in Service Factory.
I personally prefer a mix between the two approaches, which can be illustrated with the following steps:
- Design the XSD types and messages
- Create the service interface with the necessary operations
- Decorate the interface and operations with the right attributes
- Use the WSDL generation support provided by ASP.NET or WCF to automatically create the service WSDL
We (Me and rest of team) successfully used this approach in the "WS-I Sample Application" project. The main objective here was interop between different platforms, and I can say it worked perfect, we only found some minor issues without importance.
For more information about this topic, I recommend these two articles written by Aaron Skonnard:
For this section, I decided to put a link to this excellent article "Versioning Web Services" written by Don Smith. This article mainly discusses well-know practices to the versioning problem in Soap Services.
This is all for now, I will continue discussing more topics about service design in my next posts.