4.4 Guidelines for Using Web Service Contract Technologies
It is not difficult to learn the mechanics of building Web service contracts. Once you get to know the technologies weâ€™ve been discussing in this chapter, you will be able to author custom contracts with relative ease.
As we already established, every primary technology we cover in this book exists as an XML-based, industry-standard, markup language. Even though each serves a different purpose, all share a common syntax and many common conventions.
While learning these languages is a fundamental requirement to customizing your Web service contracts, it is not the most important skill, nor is it the most challenging part of creating effective contractsWriting a Web service contract and designing an effective Web service contract for a given service as part of a federated service inventory are two different things. This is where technology features, design philosophies, and business requirements converge. This book primarily attempts to help you with the first two items.
Keep in mind that these technology languages are merely tools. You can just as easily use them to build a bad Web service contract as you could a good one. It is therefore helpful to remember that the decisions you make when designing the different parts of a contract will eventually impact the usability, flexibility, and governance burden of your services.
Before we proceed to the series of chapters that explore these technologies in detail, here are a few guidelines to keep in mind.
Many development tools allow you to generate XML code for Web service contract languages. While this is useful for learning purposes, it may not be suitable for designing standardized Web service contracts in support of service-orientation.
The Standardized Service Contract design principle requires that service contracts be designed in accordance with existing design standards. The Service Loose Coupling principle further explains that contracts generated from underlying parts of a serviceâ€™s implementation can inherit negative forms of coupling that will make the service burdensome to own and govern.
These considerations also tie directly into the Canonical Schema and Canonical Expression design patterns that promote the use of design standards within the boundary of a service inventory.
Flexibility vs. Restrictiveness
One common dilemma in XML-based design, and indeed in software engineering in general, is the tension between flexibility and restrictiveness. At one end of the spectrum, a contract could be highly flexible in that it might allow just about any consumer to send it any type of content. The advantage of this approach is that you will never have to change the service contract when the message structure changes.
However, this technique also has its weaknesses. If you donâ€™t define a message structure, you canâ€™t pre-define validation logic in the contract layer, and you have no formal documentation of what your service can do. Applications that process the messages will have no idea of what to expect.
On the other hand, you could write a very rigid schema for your contract that must be modified every time there is the slightest change or addition to the message definition. Such contracts are brittle and have a ripple effect on the solutions that consume and form dependencies on the service. You may want this kind of rigidity if you are implementing a service that processes messages requiring strict precision (such as bank transactions), but such rigidity often does more harm than good.
With service-orientation there is the tendency to be less restrictive with technical contracts than with traditional object and component interfaces. The priorities advocated by the Service Abstraction principle and the Validation Abstraction design pattern support this tendency. However, in the end, the ideal usually lies somewhere in the middle. The key is to identify the areas of the contract that require flexibility and to then design them so that they allow the necessary variation.
Modularity and Reuse
Reuse of software saves time and money, and programs that are reused are often more robust and better designed all around. The individual parts that comprise Web service contracts are no exception.
As you will learn in the upcoming chapters, there is plenty of opportunity for reuse within Web service contract languages. Leveraging language features that allow for the creation of modules and provide include mechanisms can significantly streamline all parts of the service contract (and can further end up saving a lot of time that would be spent writing redundant code).
Then, of course, there is the notion of reusing parts of a contract across different contracts. This is where standardization-related patterns, such as Canonical Schema, Schema Centralization, and Policy Centralization, prove extremely valuable in not only getting more ROI out of the service contract code, butâ€”more importantlyâ€”ensuring that these key parts are consistent across all the service contracts that establish a federated endpoint layer within a service inventory.
As with anything, though, overuse of something can lead to the opposite of the intended effect. XML language modules and centralization techniques need to be carefully chosen. Over-modularizing code within or across service contracts will inevitably lead to an inhibitive and complex architecture.
Ultimately, one of the biggest challenges to achieving reuse is not related to technology, but to organizational governance. This is an on-going area that is addressed by all levels of service-orientation and SOA and will be covered in the upcoming book SOA Governance.
Clarity and Ease of Processing
With their hierarchical structure, larger-sized Web service contracts can quickly become unwieldy. Although they are intended to be read by machines, it does not mean that it doesnâ€™t matter if they are difficult to interpret by humans (especially considering that you may not want to rely on auto-generation tools to create and maintain them).
People from both the service provider and consumer sides will need to discover, understand, and work with these contracts, and any misunderstandings will introduce risk. Furthermore, overly complex contracts lead to increased governance effort and a higher cost of ownership.
Of course contracts have a job to do and sometimes this job requires that they be designed to a degree of complexity and sophistication. However, never lose sight of the fact that others will end up having to use (and perhaps evolve) what you are building today. Good contract design requires that you strive for clarity and avoid complexity or ambiguity wherever possible.
Here are some guidelines:
- Consistent Design â€“ Use the same name for parts of the contract that mean the same thing, and different names for parts that are conceptually different. Developing a naming standard (as per the Canonical Expression pattern) may be the only way to guarantee that contracts are consistent in how they express themselves. This includes choosing names that are human-readable and, ideally, provide a business context. Also, when possible, use a consistent order and structure for elements that tend to be reused.
- Limited Depth â€“ Some structural elements can be very useful, but you should not introduce unnecessary constraints or nested levels into the data structures that underlie your contract types. As per the Service Abstraction principle, try to define a balanced level of constraint granularity.
- Good Documentation â€“ Simply providing a name and a structure for elements is not enough for the contract to be communicative to a wide range of project team members. Use annotations to further document elements and types, as per the Service Discoverability principle. (There are tools that will generate human-readable documentation from schema annotations.)
SUMMARY OF KEY POINTS
- The use of auto-generation tools can make it challenging to author standardized Web service contracts.
- Granularity, reusability, and clarity are among the common Web service contract design considerations.