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3.1.1.1.2.2 Complex Dependencies

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A complex dependency is a dependency between a dependent resource and a non-empty set of provider resources with the following rules:

  • The dependent resource cannot be brought online until at least one resource in the set of provider resources is online.

  • If there is only one resource in the provider set that is online, then that resource cannot be brought offline until the dependent resource is offline.

  • The dependent resource cannot remain online if zero resources in the set of provider resources is online (for example, if all of the resources in the provider set are either offline or have failed).

Note that if the set of provider resources contains only one resource, then the complex dependency behaves identically to a simple dependency, where the dependent resource depends on the one resource in the provider set.

A resource can be simultaneously configured with zero or more simple dependencies and zero or more complex dependencies. However, a resource cannot have more than one dependency relationship with another resource, whether simple or complex. Thus, if the same provider resource is in two different sets of provider resources, then a dependent resource cannot have a complex dependency on both sets. For example, if a resource B is in two sets S1 and S2, then resource A cannot have a dependency on both S1 and S2.

Complex resource dependencies are maintained as nonvolatile configuration data in the cluster state.