OPEN - CSD04 - Constraining the capabilities of multiple node templates

Let's consider a more general use case where we want to specify the requirements for a set of node templates. Note that the node templates could be of different types which polymorphically unrelated (they are not sub/super types of each other). Nevertheless, they will have to have some common capability for the constraints to make any sense. To concisely express these semantics we need: 1) A way to specify node template membership of the set of node templates we want to constrain 2) A method for specifying the schema of the constraints 3) A method to validate that a member can be constrained, i.e. to check if a member is a valid member or not node_template_requirements: # the name is not important, it's just its own section in the document tosca.caps.MyCapability: # capability type we want to constrain across all members targets: [ node1, node2 ] # targets are the subject node templates which must support the capability contraints: secure: true failure_alert: true Each of the above requirements are now satisfied: 1) Is satisfied by the membership list targets: [ m1, ... mn ] 2) The constraint schema is specified with a capability type 3) Validation is a matter of checking if each member provides the capability type This is one of many possible syntaxes. What's most critical is to be able to express the necessary semantics.