Hi Tal,
Thanks for putting this together. It will hopefully help us understand where the points of confusion/disagreement are and lead to some resolution.
Iâd like to just focus on your initial assumptions in this response:
From: tosca@lists.oasis-open.org <tosca@lists.oasis-open.org>
On Behalf Of Tal Liron
Sent: Tuesday, February 1, 2022 1:35 PM
To: tosca@lists.oasis-open.org
Subject: [tosca] Proposal for requirement "occurrences"
Hi all, as promised here is my attempt to add clarity to Calin's ad hoc proposal.
Let's start by pretending that the counting keywords do not exist and examine what the basic grammar means.
WHAT ARE REQUIREMENT ASSIGNMENTS?
- A node type does not in itself require anything. It does, however, allow us to declare and structurally constrain the
kinds of requirement assignments would be possible for the node template.
Right off the bat, I believe we have a fundamental disagreement:
-
Correct me if I'm wrong, but your statement seems to suggest that you look at node type definitions strictly as grammatical constructs: they are âschema definitionsâ against which node templates are validated. All the âservice semanticsâ are in the node templates.
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My perspective is very different. In my opinion, node types (and other TOSCA types, for that matter) are the fundamental constructs from which services are created. Node types define re-usable components that can be combined with other components to construct
service topologies. It is node typesânot node templatesâthat define (component) semantics.
-
As such, node types absolutely define what is required. A requirement definition is exactly what the name says: it specifies a capability in some other component that is required for nodes of this type to function correctly. A requirement is fulfilled by finding
a target node with the required capability, and then establishing a relationship to that node.
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In this context, the âoccurrencesâ keyword in a requirement definition is extremely clear: it specifies the minimum number of nodes with the required capability that must be found to fulfill the requirement, and the maximum allowed number of nodes with the
required capability to fulfill the requirement.
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Note that the âminimum requiredâ can be 0, in which case the node can function correctly even without the required capability (i.e. fulfilling the requirement is an optional task).
Letâs try to first agree on the concept of ârequirement definitionsâ before we turn to requirement assignments.
Thanks,
Chris
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