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Subject: [OASIS Issue Tracker] (TOSCA-177) Cardinality for node template

Moshe Elisha created TOSCA-177:

             Summary: Cardinality for node template
                 Key: TOSCA-177
                 URL: https://issues.oasis-open.org/browse/TOSCA-177
             Project: OASIS Topology and Orchestration Specification for Cloud Applications (TOSCA) TC
          Issue Type: New Feature
          Components: Profile-YAML
            Reporter: Moshe Elisha

The current YAML profile spec assumes 1:1 ratio between node templates.

The concept of node template cardinality (node instance count) was briefly raised in some discussions and is also addressed in the Plan Portability API

This issue wants to address the initial cardinality of a node template at application deployment phase.
Cardinality changes that are caused by scale operations are out of scope.

Simple use cases that require cardinality are:
1. Application with one instance of a BE compute and two instances of FE compute for HA.
2. One compute that has two instances of the same software component to better utilize CPU usage.

Adding cardinality raises questions about the usage of the get_property / get_attribute functions.

I think that for most common use cases - the current functions are good enough. The common use cases are:

N to 1 ratio
For example. two FE node instances and one BE node instance - the FE can get info from the BE node instance.
Using the current functions, the BE node will not be able to get info from the FE nodes (1 to N ratio) but this need is unlikely - such reference will probably be used in the relationship between them where the relationship functions are already working on a specific instance.

N to N ratio
For example, a SoftwareComponent node template is HostedOn a Compute.
The SoftwareComponent can use the get functions to get info from the Compute as every SoftwareComponent node instance will have a Compute node instance.
In an N to N ratio both nodes can get info from one another.

N to M ratio
For this case the current functions are insufficient but this use case is also unlikely as there will probably be another tier in between that will be used to aggregate the nodes.
For example, if the BE has two node instances (for HA) and the FE also has two node instances - the FE will probably access the BE via a LoadBalancer making this a "N to 1 to M" ratio which is supported by current functions.

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