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Composition Sub-Controller

A composition sub-controller (or just "sub-controller") is a variation of the composition controller role that represents nested composition logic. Whereas a regular composition controller is at the top of a typical composition hierarchy, a sub-controller generally contains a capability that is composing other service capabilities while this capability itself is also being composed by the parent composition controller.

As with the composition controller role, sub-controller is a temporary runtime role assumed by a service when its capability is composed by another service. Note that a service does not enter this role when invoked by a composition initiator. Also note that although a form of controller, a sub-controller is also considered a composition member, as it represents one of the composition participants composed by a parent controller.

Sub-controllers therefore need to be designed as both effective composition controllers as well as effective composition members in support of the parent composition. As a result, both composition controller and member-related design considerations raised by the Service Composability design principle are taken into account.

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