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Services are autonomous
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Autonomy requires that the range of logic exposed by a service exist within an explicit boundary. This allows the service to execute self-governance of all its processing. It also eliminates dependencies on other services, which frees a service from ties that could inhibit its deployment and evolution. Service autonomy is a primary consideration when deciding how application logic should be divided up into services, and which operations should be grouped together within a service context.
Deferring the location of business rules is one way to strengthen autonomy and keep services more generic. Processes generally assume this role by owning the business rules that determine how the process is structured and, subsequently, how services are composed to automate the process logic. (This is another aspect of orchestration explored in the Orchestration service layer in Chapter 9.)
 Autonomous services have
control over underlying resources.
Note that autonomy does not necessarily grant a service exclusive ownership of the logic it encapsulates. It only guarantees that at the time of execution the service has control over whatever logic it represents. We can therefore make a distinction between two types of autonomy:
Service-level autonomy � Service boundaries are distinct from each other, but the service may share underlying resources. For example, a wrapper service that encapsulates a legacy environment that is also used independently from the service has a service-level autonomy. It governs the legacy system, but also shares resources with other legacy clients.
Pure autonomy � The underlying logic is under complete control and ownership of the service. This is typically the case when the underlying logic is built from the ground up in support of the service.
This page contains excerpts from:
Service-Oriented Architecture:
Concepts, Technology, and Design
by Thomas Erl
(ISBN: 0131858580, Prentice Hall/PearsonPTR, Hardcover, 792 pages).
For more information, visit www.soabooks.com.
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