Prescriptive vs declarative OSS

TOSCA models are ‘declarative’ in that they describe WHAT it is you’re trying to provision (as opposed to ‘prescriptive’ models that describe HOW you’re going to get there). ONF uses the term ‘intent’ to describe the same concept, and yet others refer to these types of models as ‘desired-state’ models. While there are subtle nuances between these terms, they all effectively describe the same idea. As far as I know, TOSCA is currently the only standard for declarative models which is why it is gaining so much traction. One of the main benefits of TOSCA is that it provides abstraction, which reduces complexity tremendously for the service designer. TOSCA also use the same declarative approach to model services/applications as well as infrastructure, which eliminates the types of artificial boundaries one often sees in various industry ‘reference models‘.”
Chris Lauwers
(as referred to in Tom Nolle’s blog which then states, “The modeling distinction between prescriptive and declarative is an attribute of the first DevOps tools, and today’s most popular DevOps tools (Chef, which is prescriptive versus Puppet, which is declarative) still reflect the division. When you apply the terms to SDN and NFV, there are factors other than DevOps roots that come into play, and these explain the industry shift (IMHO) toward intent modeling.”).

This is an important distinction to make for the OSS of the future. I share Tom’s belief that not only will intent / declarative be the right approach for SDN / NFV but also a necessary approach for OSS to abstract the complexities of managing virtualised network platforms.

The looser coupling of the intent / declarative approach seems, to me, to be the only way to realistically handle the complexity of service orchestration across networks that contain legacy networks that are moving to increased virtualisation (and are likely to be in this transition phase for years to come).

As the TOSCA standard states, “The combination of topology and orchestration in a Service Template describes what is needed to be preserved across deployments in different environments to enable interoperable deployment of cloud services and their management throughout the complete lifecycle (e.g. scaling, patching, monitoring, etc.) when the applications are ported over alternative cloud environments.”

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