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Service-Orientation with .NET : Standardized Service Contract

4/16/2011 3:38:04 PM
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This principle advocates the standardization of service contracts that exist within a given service inventory boundary (Figure 1). Within this context, standardization can refer to the usage of industry standards (such as WSDL and XML Schema), but primarily the focus is on custom design standards that are pre-defined and regulated (such as canonical data models).
Figure 1. The application of the Standardized Service Contract  principle relies heavily on the use of design standards that regulate contract design across services within a given service inventory.


Contract-First

To ensure that contracts are consistently standardized requires that we seize programmatic control over how contracts are created, which is the basis of the contract-first approach that is predominantly used when applying this principle.

There are various ways of going about contract-first development with .NET. The primary consideration is in determining in what sequence to define the various parts of a service contract.

For example, here is a common three-step process:

1. Create or Reuse Data Contract

A WCF data contract most commonly exists as an XML schema that defines the data structure and data types (as part of the overall data model) for a given set of data that is exchanged by a service capability.

Following patterns, such as Canonical Schema and Schema Centralization, this step may involve creating a new data contract to be used by one or more services or it may involve reusing an already existing (and standardized) data contract. The latter circumstance may be due to a data contract that was previously customized or there may be a requirement to use a data contract that exists as an industry standard XML Schema (such as HR-XML or LegalML).


2. Create Message Contract

The body content of a given message transmitted or received by a service capability is primarily pre-defined by the data contract. The message contract encompasses the data contract and further defines metadata in the message header, as per Messaging Metadata . Message contracts within WCF are primarily built using the SOAP Body and Header constructs. Part of this step may also involve pre-defining fault contracts for exception conditions.

Service Modeling & Service Candidates

Prior to applying any contract-first development approach, it is generally assumed that some extent of service modeling has already been completed. The service modeling process is part of the service-oriented analysis stage within a service’s overall delivery cycle. Service modeling produces conceptual services called service candidates that form the basis of service contracts. Often significant up-front analysis is carried out in order to define several service candidates for a specific service inventory before physically building any one service contract. This effectively creates a service inventory blueprint that allows the basic parts of service contracts to be well-defined and further refined through iteration, prior to entering the design and development phases.


3. Create Interface Contract

The interface contract is commonly equated to the abstract description of a WSDL document wherein operation contracts are defined. When working with REST services, the interface contract can be considered the subset of HTTP methods that are supported as part of the overall uniform contract for a given service.

The interface contract and its operations or methods express the externally invokable functionality offered by a service. An interface contract pulls together the data and message contracts and associates them with appropriate operations or methods.


Standardized Service Contract  and Patterns

Beyond carrying out a contract-first approach to service design, there are many more facets to applying the Standardized Service Contract principle. Several of these additional aspects will come up in the subsequent sections exploring SOA design patterns related to service interoperability.

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