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Secrets of Designing a Service

Applying basic principles

Software design has always been a focus for developers, but as we cycled through different approaches, standards, and architectures over the years, I think we've had a tendency not to pay enough attention to the fundamentals of software engineering. Clearly I've seen a decline in software quality because of this, not from a lack of programming talent, but a lack of upfront architecture and design. Skip this step and your service will cost much more to build and deploy, as you find yourself in an interactive death spiral that's difficult to recover from.

With the movement towards SOA, and the use of services to assemble and integrate software, we have to pay particular attention to design. Indeed, services have many unique design patterns, a bit different than traditional software systems, these include:

  • Reuse
  • Heterogeneity
  • Polymorphism
  • Aggregation
  • Limited scope
  • Standards based
First and foremost, services should be designed for reuse. Services become apart of any number of other applications, and thus must be designed to provide behavior and information, but not be application specific.

Services have to be designed for heterogeneity. Web services should be built so that there are no calls to native interfaces or platforms. This is due to the fact that a Web service, say one built on Linux, may be leveraged by applications on Windows, Macs, even mainframes. Those that leverage your service should do so without regard for how it was created, and should be completely platform independent.

Polymorphism, although an object-oriented programming term, means that we need to design a service with the ability to process services differently depending on their data or context. This facilitates reuse.

Also, when we build or design services we need to account for aggregation. Many services will become parts of composite services leveraged by an application, and thus you must consider that in their design.

Services are not applications and should have limited scope. In other words, they do simple things such as checking inventory or calculating reorder points. If your needs are more complex you simply write more services and don't overload a single service with too much functionality. Services with too much functionality are considered heavy and are difficult to reuse since you may deploy a service where you're only leveraging 10% of its functions. Lighter, or more granular, services are much easier to reuse.

Finally, services need to be designed as standards based. While in the world of Web services this seems like a no-brainer, many developers and architects ignore compliance with standards and thus limit interoperability.

Checklist for Designing a Service
Now that we understand the common design patterns we must follow, the question is, how do you design a service? There are certain steps architects and developers can follow; here are some suggestions.

First, you need to define the purpose of the service. What the service will do, and who is the intended user.

Possible Artifact: Service definition document

Second, you need to determine the information to be bound to the service including both metadata and schemas. This means you need to understand how information is leveraged by the service, and what functions require what data. Typically services are storing information outside of themselves, thus you also need to design the mechanisms for accessing outside data sources. Moreover, you need to define data policies here, including data validation constraints and dependencies.

Possible Artifact: Metadata and Schema Document

Third, you need to determine the functions (methods) encapsulated inside the service, in other words the behaviors you would like to expose. For instance, if our service checks inventory it may expose:

CheckInvenotoryOnHand(product_ID)
CheckInventoryReorderPoint(product_ID)
CheckInventoryOnOrder(product_ID)

It's also at this step that we define each function, including how the function breaks down using a traditional functional decomposition chart. This means that we define the higher-level function by defining all lower-level functions. Structure charts or decomposition diagrams are very helpful here.

Possible Artifact: Structure chart

Fourth, we need to define any interfaces into the service as both machine and human. This means we need to determine how the service will interact with the calling applications, and through what mechanisms. While Web services define the mechanisms for both interface discovery and communications (e.g., WSDL and SOAP), we need to determine what those interfaces are and what they do. In many instances they map directly back to the functions, but not always. Moreover, with the use of rich client interfaces in many instances you may be interacting with a human through a portal-type interface; you need to define that here as well, including design of the user interface.

Possible artifact: API design, user interface design

Finally, we need to define how the service is to be tested. This is an important but often neglected step where you define how those leveraging the service will test the service within the context of their usage pattern. You need to define test information, service invocation, and validity of results. Even performance profiling should be included in this step.

Possible Artifact: Test plan

Other Design Considerations
Of course there are other things you need to design into a service, including management and security.

You need to design a service with points of management to allow those that leverage the service to include it into their management infrastructure. Designing for management means designing and building points of management into the service, management APIs really, and defining their usage to those that leverage the service.

Security is a bit more of a systemic design issue, meaning you need to define how your service is accessible and how you intend on authenticating users. Typically this means identity management facilities, but what's most important is that you define the security parameters to those who are looking to leverage the service and that security is designed in.

Of course I can't discuss all of the detailed design procedures one would employ in designing a service, but I think I've hit on the more obvious things here. What's important is that you actually apply some fundamental software engineering and design principles here so you create a service that's able to meet your needs the first time. Those who skip that step eventually have to return to it to fix or add to the service later. This interactive approach gets costly quickly and only delays the delivery of the working service. So, always focus on the fundamentals.

More Stories By David Linthicum

Dave Linthicum is Sr. VP at Cloud Technology Partners, and an internationally known cloud computing and SOA expert. He is a sought-after consultant, speaker, and blogger. In his career, Dave has formed or enhanced many of the ideas behind modern distributed computing including EAI, B2B Application Integration, and SOA, approaches and technologies in wide use today. In addition, he is the Editor-in-Chief of SYS-CON's Virtualization Journal.

For the last 10 years, he has focused on the technology and strategies around cloud computing, including working with several cloud computing startups. His industry experience includes tenure as CTO and CEO of several successful software and cloud computing companies, and upper-level management positions in Fortune 500 companies. In addition, he was an associate professor of computer science for eight years, and continues to lecture at major technical colleges and universities, including University of Virginia and Arizona State University. He keynotes at many leading technology conferences, and has several well-read columns and blogs. Linthicum has authored 10 books, including the ground-breaking "Enterprise Application Integration" and "B2B Application Integration." You can reach him at [email protected] Or follow him on Twitter. Or view his profile on LinkedIn.

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