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diff --git a/docs/architecture/tosca-policy-primer.rst b/docs/architecture/tosca-policy-primer.rst new file mode 100644 index 00000000..686d3564 --- /dev/null +++ b/docs/architecture/tosca-policy-primer.rst @@ -0,0 +1,79 @@ +.. This work is licensed under a +.. Creative Commons Attribution 4.0 International License. +.. http://creativecommons.org/licenses/by/4.0 + +.. _tosca-label: + +TOSCA Policy Primer +------------------- + +.. contents:: + :depth: 2 + +This page gives a short overview of how Policy is modelled in the +`TOSCA Simple Profile in YAML <http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/v1.1/TOSCA-Simple-Profile-YAML-v1.1.pdf>`__. + +TOSCA defines three concepts for Policy: Policy Type, Policy, and Trigger. + +.. image:: images/ToscaPolicyStructure.png + :width: 800 + +Policy Type +~~~~~~~~~~~ + +A Policy Type is used to specify the types of policies that may be used in a service. The parameter definitions +for a policy of this type, the entity types to which it applies, and what triggers policies of this type may be +specified. + + +The types of policies that are used in a service are defined in the policy_types section of the TOSCA service template +as a Policy Type. More formally, TOSCA defines a Policy Type as an artifact that "defines a type of requirement that +affects or governs an application or service’s topology at some stage of its life cycle, but is not explicitly part of +the topology itself". In the definition of a Policy Type in TOSCA, you specify: + +* its properties, which define the type of configuration parameters that the policy takes +* its targets, which define the node types and/or groups to which the policy type applies +* its triggers, which specify the conditions in which policies of this type are fired + +Policy +~~~~~~ + +A Policy is used to specify the actual instances of policies that are used in a service. The parameter values of the +policy and the actual entities to which it applies may be specified. + +The policies that are used in a service are defined in the policies section of the TOSCA topology template as a Policy. +More formally, TOSCA defines a Policy as an artifact that "defines a policy that can be associated with a TOSCA +topology or top-level entity definition". In the definition of a Policy in TOSCA, you specify: + +* its properties, which define the values of the configuration parameters that the policy takes +* its targets, which define the node types and/or group types to which the policy type applies + +Note that policy triggers are specified on the Policy Type definition and are not specified on the Policy itself. + +Trigger +~~~~~~~ + +A Trigger defines an event, condition, and action that is used to initiate execution of a policy associated with it. +The definition of the Trigger allows specification of the type of events to trigger on, the filters on those events, +conditions and constraints for trigger firing, the action to perform on triggering, and various other parameters. + +The triggers that are used in a service are defined as reusable modules in the TOSCA service template as a Trigger. +More formally, TOSCA defines a Trigger as an artifact that "defines the event, condition and action that is used to +“trigger” a policy it is associated with". In the definition of a Trigger in TOSCA, you specify: + +* its event_type, which defines the name of the event that fires the policy +* its schedule, which defines the time interval in which the trigger is active +* its target_filter, which defines specific filters for firing such as specific characteristics of the nodes or +relations for which the trigger should or should not fire +* its condition, which defines extra conditions on the incoming event for firing the trigger +* its constraint, which defines extra conditions on the incoming event for not firing the trigger +* its period, which defines the period to use for evaluating conditions and constraints +* its evaluations, which defines the number of evaluations that must be performed over the period to assert the +condition or constraint exists +* its method, the method to use for evaluation of conditions and constraints +* its action, the workflow or operation to invoke when the trigger fires + +Note that how a Trigger actually works with a Policy is not clear from the specification. + +End of Document + |