The challenge remains to find an acceptable balance between incorporating service-oriented design principles into business analysis environments without having to wait before integrating Web services technologies into technical environments. For many organizations it is therefore useful to view these two approaches as extremes and to find a suitable middle ground.
This is possible by defining a new process that allows for the business-level analysis to occur concurrently with service design and development. Also known as the meet-in-the-middle approach, the agile strategy is more complex than the previous two simply because it needs to fulfill two opposing sets of requirements.
The process steps shown in Figure 10.4 demonstrate an example of how an agile strategy can be used to reach the respective goals of the top-down and bottom-up approaches.
Figure 10.4. A sample agile strategy process.
Step 1: Begin the top-down analysis, focusing first on key parts of the ontology and related business entities
The standard top-down analysis begins but with a narrower focus. The parts of the business models directly related to the business logic being automated receive immediate priority.
Step 2: When the top-down analysis has sufficiently progressed, perform service-oriented analysis
While Step 1 is still in progress, this step initiates a service-oriented analysis phase, such as the one described in Chapters 11 and 12. Depending on the magnitude of analysis required to complete Step 1, it is advisable to give that step a good head start. The further along it progresses, the more service designs will benefit.
After the top-down analysis has sufficiently progressed, model business services to best represent the business model with whatever analysis results are available. This is a key decision point in this process. It may require an educated judgment call to determine whether the on-going top-down analysis is sufficiently mature to proceed with the creation of business service models. This consideration must then be weighed against the importance and urgency of pending project requirements.
Step 3: Perform service-oriented design
The chosen service layers are defined, and individual services are designed as part of a service-oriented design process, such as the one described in Chapters 13 to 16.
Steps 4, 5, and 6: Develop, test, and deploy the services
Develop the services and submit them to the standard testing and deployment procedures.
Step 7: As the top-down analysis continues to progress, revisit business services
Perform periodic reviews of all business services to compare their design against the current state of the business models. Make a note of discrepancies and schedule a redesign for those services most out of alignment. This typically will require an extension to an existing service for it to better provide the full range of required capabilities. When redesigned, a service will need to again undergo standard development, testing, and deployment steps.
To preserve the integrity of services produced by this approach, the concept of immutable service contracts needs to be strictly enforced. After a contract is published, it cannot be altered. Unless revisions to services result in extensions that impose no restrictions on an existing contract (such as the addition of new operations to a WSDL definition), Step 7 of this process likely will result in the need to publish new contract versions and the requirement for a version management system.
10.4.2. Pros and cons
This strategy takes the best of both worlds and combines it into an approach for realizing SOA that meets immediate requirements without jeopardizing the integrity of an organization's business model and the service-oriented qualities of the architecture.
While it fulfills both short and long-term needs, the net result of employing this strategy is increased effort associated with the delivery of every service. The fact that services may need to be revisited, redesigned, redeveloped, and redeployed will add up proportionally to the amount of services subjected to this retasking step.
Additionally, this approach imposes maintenance tasks that are required to ensure that existing services are actually kept in alignment with revised business models. Even with a maintenance process in place, services still run the risk of misalignment with a constantly changing business model.
In response to new business requirements, TLS identifies the need for five, possibly six new services. These must be delivered relatively soon to address a major problem TLS is facing, resulting from glaring discrepancies between outsourced employee timesheets and corresponding customer invoices.
In Chapter 12 we walk through a step-by-step process during which we model these services as part of a new extension to our existing TLS case study. The approach taken to eventually build and deliver these new services will be based on the agile strategy.
SUMMARY OF KEY POINTS
Part I: SOA and Web Services Fundamentals
The Evolution of SOA
Web Services and Primitive SOA
Part II: SOA and WS-* Extensions
Web Services and Contemporary SOA (Part I: Activity Management and Composition)
Web Services and Contemporary SOA (Part II: Advanced Messaging, Metadata, and Security)
Part III: SOA and Service-Orientation
Principles of Service-Orientation
Part IV: Building SOA (Planning and Analysis)
SOA Delivery Strategies
Service-Oriented Analysis (Part I: Introduction)
Service-Oriented Analysis (Part II: Service Modeling)
Part V: Building SOA (Technology and Design)
Service-Oriented Design (Part I: Introduction)
Service-Oriented Design (Part II: SOA Composition Guidelines)
Service-Oriented Design (Part III: Service Design)
Service-Oriented Design (Part IV: Business Process Design)
Fundamental WS-* Extensions
Appendix A. Case Studies: Conclusion