Composition Autonomy

How can compositions be implemented to minimize loss of autonomy?

Problem Composition controller services naturally lose autonomy when delegating processing tasks to composed services, some of which may be shared across multiple compositions.
Solution All composition members can be isolated to maximize the autonomy of the composition as a whole.
Application The agnostic member services of a composition are redundantly implemented in an isolated environment.	Impacts Increasing autonomy on a composition level results in increased infrastructure costs and operational expenses.
Principles Service Autonomy, Service Reusability, Service Composability	Architecture Composition

Table 6.4 – Profile summary for the Composition Autonomy pattern.

Problem

Services are ideally individually autonomous so that they can provide a high degree of behavioral predictability when repeatedly reused. This allows a single, agnostic service to be shared across multiple compositions in support of automating multiple business processes.

However, a natural result of any distributed service composition is a loss of autonomy by any service composing another simply due to the fact that the service is required to invoke solution logic that resides outside of its controlled execution environment.

When individual services provide a high level of autonomy, the collective autonomy of the composition is correspondingly elevated and this is generally adequate for most service compositions. But when one or more composition members have a poor level of autonomy or when the quality of service requirements of the composition as a whole demand a higher degree of robustness and reliability, then a distributed composition may be insufficient.

Figure 6.18 – In a highly distributed environment, a service compositionŐs overall autonomy will be collectively determined by the autonomy of its individual composition members. When services are designed in accordance with the Service Autonomy design principle, this collective autonomy will generally be sufficient. However, there are times when a higher degree of composition autonomy is required. As shown in this figure, Service D offers the lowest level of autonomy because its logic is accessed by external programs.

Solution

The members of the composition are deployed within an isolated environment so as to give the composition as a whole a high level of autonomy.

Figure 6.19 – By grouping the member services of a composition into a separate deployment environment, the collective autonomy is maximized because the implementation is dedicated to the compositionŐs business task and none of the services are otherwise shared. Services C and D in particular benefit from this new implementation as they are no longer subject to shared access.

Application

There are many variations of this design pattern that can be applied, several of which correspond to the typical autonomy levels described in Chapter 10 of SOA: Principles of Service Design.

To increase the autonomy of a composition the following steps can be taken:

•     Two or more composition members are deployed on the same physical server to avoid remote communication.

•     The composition member services are isolated to avoid shared access.

•     The agnostic composition members are redundantly implemented, as per the Redundant Implementation pattern.

•     Composition members requiring data access are supplied with dedicated or replicated data stores, as per the Service Data Replication pattern.

Impacts

Because the requirements for increasing the overall autonomy of a service composition usually involve upgrading or extending the existing enterprise infrastructure, there are obvious costs and impacts that need to be taken into consideration.

Redundant implementations of agnostic services further add governance effort and expense so that these independent implementations are kept in synch with their shared counterparts.

Relationships

Because of its narrow emphasis on supporting the goals of the Service Autonomy design principle, this pattern has just a handful of relationships. Fundamentally, Composition Autonomy is concerned with maximizing the reliability and availability of services based on the Agnostic Context pattern in full support of the Capability Recomposition pattern.

Service Data Replication and Redundant Implementation design patterns help establish an appropriate level of isolation for service compositions. Often, security reasons will dictate the need to isolate externally facing services as well as their composition members, which may lead to the need to apply Composition Autonomy in support of the Inventory Endpoint pattern.

Figure 6.20 – The Composition Autonomy pattern is implementation-centric and will generally require the application of more specialized patterns to realize required levels of composition isolation.

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