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To realize iPOJO, OSGi service specifications must be extended to contain other information. For example, services must be able to declare specificaionspecification-level dependencies on other services. Services with specification-level dependencies are referred to as composable services, since it is possible to compose them with other service. In the core OSGi framework, all service dependencies are at the implementation level, thus only component instances are composable.

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For example, in standard OSGi if component implementation A provides service Foo and requires service Bar, it is not possible to know whether component implementation B, which also provides Foo and requires Bar, uses Bar for the same purpose as A. Composable services makes this possible, since the purpose of some such service dependencies can be defined in the service specification. The resulting model obeys two levels of service dependencies:

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