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Difference between revisions of "Mistral/Blueprints/ActionsDesign"

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So that signature of action initializer defines all the parameters it needs. Respectively, when an action is declared in DSL it is expected to have the same set of parameters.
 
So that signature of action initializer defines all the parameters it needs. Respectively, when an action is declared in DSL it is expected to have the same set of parameters.
 +
 +
==== Plugin Architecture ====
 +
Mistral should provide actions extensibility so that anyone could write their own actions and plug them into the system. The essential idea is that actions form a class hierarchy with the root class Action described above. The main requirements for action plugin system are:
 +
* Ability to specify action namespace.
 +
* Ability to specify action shortcut.
 +
* Once an action is registered in Mistral it can be accessed in DSL using form 'namespace.action_shortcut'.
  
 
==== DSL ====
 
==== DSL ====
 +
In Mistral DSL actions can be used like shown below:
 +
 +
tasks:
 +
    task1:
 +
        action: std.http
 +
        parameters:
 +
            url: http://myhost.org/ping_me
 +
            method: GET
 +
 +
===== Namespaces =====
 +
Actions are split into groups called namespaces. Out of the box Mistral has a number of standard actions. The standard actions can be referred to in DSL via "std" namespace. For example, "std.http".

Revision as of 07:08, 12 March 2014

Mistral Actions

NOT FINISHED

This BP describes the main requirements to how actions should be designed in Mistral in order to address flexibility and clarity of their usage.

From implementation perspective we'll be referring to python class Action that represents Mistral action:

class Action(object):
    @abc.abstractmethod
    def run():
        pass

Method run() here implements action logic (issuing SSH command, sending a message over MQ etc.). All specific action classes must extend class Action and implement abstract method run().

Synchronous/Asynchronous

Mistral action can be synchronous and asynchronous. Synchronous actions act as a regular Python method: when it's called it returns a result upon completion so a caller can immediately use this result. Asynchronous actions don't return a result immediately but rather "launch" an independent activity (e.g. sending a request to an external system to do the heavy job) and when it completes a result becomes know to Mistral.

class Action(object):

    @abc.abstractmethod
    def run():
        pass

    def is_sync():
        return False

Depending on action implementation and its properties method is_sync() returns True or False. This part of Action contract is required because a caller subsystem must know how to handle action result.

Result

The notion of 'result' only makes sense for synchronous type of actions since in case of asynchronous action a result gets delivered (conveyed directly via Mistral public API).

Asynchronous Actions

Method run() always returns None.

Synchronous Actions

Method run() returns action result. In cases when action is based on using some protocols like HTTP action implementation itself is responsible for converting protocol specific result to a form meaningful for a particular workflow. So, for example, HTTP response that action receives during its work typically can't be treated as action result since a user may be interested only in part of information residing in the response.

Errors

During action work various errors may occur. To notify action callers action must throw ActionException. In this case corresponding task state will be set to ERROR.

def run():
    ...
    raise ActionException("Failed to send an SSH command. %s" % cause)
    ...

Dry-Run

Actions may optionally implement dry_run() method so that a user can test their workflows in 'dry-run' mode, the mode in which every action doesn't perform a real work but instead emulates useful behaviour.

def dry_run():
    LOG.info("Running action in dry-run mode [parameters=%s]" % self.parameters)
    
    return 'my_result'

Input

Action may optionally have parameters needed to alter its behaviour. Each action has different set of parameters depending on the underlying protocol/library/technology it uses. For example, in case of HTTP action parameters will be:

  • url
  • query string (params)
  • method
  • headers
  • body

When Action instance is created it gets initialized with required parameters:

class HTTPAction(object):
    def __init__(self, url, params, method, headers, body):
        self.url = url
        self.params = params
        self.method = method
        self.headers = headers
        self.body = body

So that signature of action initializer defines all the parameters it needs. Respectively, when an action is declared in DSL it is expected to have the same set of parameters.

Plugin Architecture

Mistral should provide actions extensibility so that anyone could write their own actions and plug them into the system. The essential idea is that actions form a class hierarchy with the root class Action described above. The main requirements for action plugin system are:

  • Ability to specify action namespace.
  • Ability to specify action shortcut.
  • Once an action is registered in Mistral it can be accessed in DSL using form 'namespace.action_shortcut'.

DSL

In Mistral DSL actions can be used like shown below:

tasks:
    task1:
        action: std.http
        parameters:
            url: http://myhost.org/ping_me
            method: GET
Namespaces

Actions are split into groups called namespaces. Out of the box Mistral has a number of standard actions. The standard actions can be referred to in DSL via "std" namespace. For example, "std.http".