Merge "scenario evaluator documentation"
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doc/source/scenario-evaluator.rst
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doc/source/scenario-evaluator.rst
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========================
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Vitrage Evaluator Design
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========================
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Overview
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========
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The Vitrage Scenario Evaluator is charged with the evaluation of the
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scenarios specified in the Vitrage templates, and executing the actions
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associated with these scenarios when they are triggered. The Scenario Evaluator
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is also responsible for resolving any inter-action dependencies, such as two
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contradicting actions (e.g., raise and disable the same alarm), two overlapping
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actions etc.
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Evaluator Design
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================
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::
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+-------------+
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| +-------------+
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| | |
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| | Templates |
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+-+ (YAML) |
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+-----------+-+
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|
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+------------------------------------------------------------------------------+
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|Template | |
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|Loading | load |
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| | |
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| +-----v-------------+ +---------------+ +---------------+ |
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| | Template | | | | | |
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| | |process | Template | add | Scenario | |
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| |(Python Dictionary)+--------> Object +-------> Repo | |
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| | | | | | | |
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| +-------------------+ +---------------+ +---+--------+--+ |
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| | ^ |
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| | | |
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| | | |
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+------------------------------------------------------------------------------+
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| |
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+-----------------------------------------------------------------------------------+
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| Event Processing return relevant| | get |
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| scenarios | | scenarios |
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| +--------------+ +----------------------------------+ |
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| | | perform | analyze | | | |
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| | Action | action | matches | | | |
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| | Executor <--------------------------------+ | | | |
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| | | | | | | | |
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| | | | | | | | |
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| +--------------+ +----------------------------------+ |
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| | | | |
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| return scenario | | | |
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| matches | | | |
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| +-----+------v--------+ |
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| | | |
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| | Entity Graph | |
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| | | |
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| | | |
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| +---------------------+ |
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+-----------------------------------------------------------------------------------+
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Flow
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====
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Concepts and Guidelines
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-----------------------
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- *Events:* The Scenario Evaluator is notified of each event in the Entity
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Graph *after* the event takes place. An event in this context is any change
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(create/update/delete) in a graph element (vertex/edge). The notification
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will consist of two copies of the element: the element *before* the change
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and the *current* element after the change took place.
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- *Execute and Undo Actions:* If the Entity Graph matches a scenario, the
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relevant actions will need to be executed. Conversely, if a previously
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matched scenario no longer holds, the action needs to be undone. Thus, for
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each action there must be a "do" and "undo" procedure defined. For example,
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the *raise_alarm* action will raise an alarm in the "do" procedure, and
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disable the alarm in the "undo" procedure. An important feature of this
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approach is that for a given scenario and a specific event, either the "do"
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or "undo" phases can be performed but not both simultaneously.
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Template Loading
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----------------
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Scenarios are written up in configuration files called *templates*. The format
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and specification of these can be seen here_.
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.. _here: https://github.com/openstack/vitrage/blob/master/doc/source/vitrage-template-format.rst
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Templates should all be located in the *<vitrage folder>/templates* folder.
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When Vitrage is started up, all the templates are loaded into a *Scenario*
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*Repository*. During this loading, template verification is done to
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ensure the correct format is used, references are valid, and more. Errors in
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each template should be written to the log. Invalid templates are skipped.
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The SR supports querying for scenarios based on a vertex or edge in the Entity
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Graph. Given such a graph element, the Scenario Repository will return all
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scenarios where this element appears in the scenario condition. This means that
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a corresponding element appears in the scenario condition, such that for each
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key-value in the template, the same key-value can be found in the element (but
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not always the reverse).
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Ongoing Operation
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-----------------
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1. The Scenario Evaluator is notified of an event on some element in the Entity
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Graph.
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2. The Scenario Evaluator queries the Scenario Repository for relevant
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scenarios for both *before* and *current* state of the element.
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3. The two sets of scenarios are analyzed and filtered, resulting in two
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disjoint sets, to avoid "do/undo" conflicts (See above in
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'Concepts and Guidelines'_).
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Currently, this filtering will be done by removing any scenario that appears
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in both from both sets.
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4. For each scenario related to the *before* element, the Scenario Evaluator
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queries the Entity Graph for all the matching patterns in the current
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system. For each match and each associated action, add a reference to the
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*undo* of the action to an *action collection*.
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5. For each scenario related to the *current* element,the Scenario Evaluator
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queries the Entity Graph for all the matching patterns in the current
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system. For each match and each associated action, add a reference to the
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*do* of the action to the same *action collection*.
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6. Given all the actions (do & undo) in the *action collection*, perform them.
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Using action executor.
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- Note that in Mitaka, the only action filtering planned is avoiding
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performing the same action twice.
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System Initialization
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---------------------
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During the initialization of Vitrage, the Scenario Evaluator will be
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de-activated until all the synchronizers complete their initial "get_all"
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process. After it is activated, the consistency flow will begin, which will
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trigger all the relevant scenarios for each element in the Entity Graph.
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This approach has several benefits:
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- During the initialization period, many events need to be processed into the
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Entity Graph. By postponing the evaluation till after this period, we avoid
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bottlenecks and other performance issues.
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- During the initialization period the Entity Graph is built step-by-step until
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it reflects the current status of the Cloud. Thus, during this interim period
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scenarios that contain "not" clauses might "fire" because a certain entity is
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not present in the graph, even though it is present in reality and just has
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not been processed into the graph (since the "get_all" is not finished).
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It is possible that this late activation of the evaluator will be removed or
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changed once we move to a persistent graph database for the Entity Graph in
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future version.
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Limitations / Next Steps
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========================
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- Need to check for contradictions between actions, i.e., setting different
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states for the same element.
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- Currently there is no support for ensuring that if an "undo" is to be
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activated, there is no "do" that contradicts it and maintains the current
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state (e.g., deduced alarm active).
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