Vitrage Template Loading

Overview

Vitrage templates are defined in yaml with specific format. During startup, templates are loaded into TemplateData. After that , scenarios in loaded templates will be added into scenario repository.

This document explains the implementation details of template data to help developer understand how scenario_evaluator works.

Example

Let’s take a basic template as example

metadata:
 name: basic_template
 description: basic template for general tests
definitions:
 entities:
  - entity:
     category: ALARM
     type: nagios
     name: HOST_HIGH_CPU_LOAD
     template_id: alarm
  - entity:
     category: RESOURCE
     type: nova.host
     template_id: resource
 relationships:
  - relationship:
     source: alarm
     target: resource
     relationship_type: on
     template_id : alarm_on_host
scenarios:
 - scenario:
    condition: alarm_on_host
    actions:
     - action:
        action_type: set_state
        properties:
         state: SUBOPTIMAL
        action_target:
         target: resource

TemplateData will build entites, relationships and most importantly scenarios out from the definition.

expected_entities = {
  'alarm': Vertex(vertex_id='alarm',
                  properties={'vitrage_category': 'ALARM',
                              'vitrage_type': 'nagios',
                              'name': 'HOST_HIGH_CPU_LOAD',
                              }),
  'resource': Vertex(vertex_id='resource',
                     properties={'vitrage_category': 'RESOURCE',
                                 'vitrage_type': 'nova.host',
                                 })
}

expected_relationships = {
  'alarm_on_host': EdgeDescription(
    edge=Edge(source_id='alarm',
              target_id='resource',
              label='on',
              properties={'relationship_type': 'on'}),
    source=expected_entities['alarm'],
    target=expected_entities['resource']
  )
}

expected_scenario = Scenario(
  id='basic_template-scenario0',
  condition=[
    [ConditionVar(symbol_name='alarm_on_host',
                  positive=True)]],
  actions=[
    ActionSpecs(
      type='set_state',
      targets={'target': 'resource'},
      properties={'state': 'SUBOPTIMAL'})],
  subgraphs=template_data.scenarios[0].subgraphs,  # ignore subgraphs
  entities=expected_entities,
  relationships=expected_relationships
)

Entities and relationships

Entities and relationships are loaded into dicts keyed by template_id so that the references in scenarios can be resolved quickly.

Note that entities and relationships dicts are NOT added to scenario repository. This implies the scope of template_id is restricted to one template file. It is NOT global.

It is considered invalid to have duplicated template_id in one template, but it is possible that two or more entities have exactly the same properties except template_id. There is an example in vitrage/tests/templates/evaluator/high_availability.yaml:

- entity:
   category: RESOURCE
   type: nova.instance
   template_id: instance1
- entity:
   category: RESOURCE
   type: nova.instance
   template_id: instance2

It is used to model scenario contains two or more entities of same type, such as high availability condition.

Scenarios

Scenario are defined as a namedtuple

Scenario = namedtuple('Scenario', ['id', 'condition', 'actions', 'subgraphs'])

id

Formatted from template name and scenario index

condition

Condition strings in template are expressions composed of template id and operators. As explained in embedded comment:

The condition string will be converted here into DNF (Disjunctive Normal Form), e.g., (X and Y) or (X and Z) or (X and V and not W)… where X, Y, Z, V, W are either entities or relationships more details: https://en.wikipedia.org/wiki/Disjunctive_normal_form

The condition variable lists is then extracted from the DNF object. It is a list of lists. Each inner list represents an AND expression compound condition variables. The outer list presents the OR expression

[[and_var1, and_var2, …], or_list_2, …]

param condition_str:  the string as it written in the template itself return:condition_vars_lists

actions

actions is a list of ActionSpecs.

The action targets in the spec must be referenced in the condition definition. They are either linked to vertex_id of entity condition variables or source_id and target_id in relationship condition variable extracted.

In each matched subgraph in the entity graph, the targets will be resolved as concrete vertices or edges.

subgraphs

Sub graphs are built from conditions for pattern matching in the entity graph. Each sub-list in condition variables list is compiled into one sub graph. The actions will be triggered if any of the subgraph is matched.

entities & relationships

Dicts of touched entities and relationships during subgraph building are saved in scenario.

This makes creation of the scenarios repository index on related entities and relationships easier and more efficient. You don’t need to traverse the condition object again, which is already done once during subgraphs building. It also eliminate the necessity of duplication check because there is no duplicate entities or relationships in these dicts compared to the condition variables lists.