mistral/doc/source/dsl/dsl_v2.rst

Mistral DSL v2 specification

NOTE: DSL described in this document might slightly change within a short period of time (2-3 weeks) and should be now considered experimental. Mistral team is now actively working on stabilization.

Introduction

Current document fully describes Domain Specific Language (DSL) version 2 of Mistral Workflow Service. Since version 1 issued in May 2014 Mistral team completely reworked the language pursuing with the goal in mind to make it easier to understand while more consistent and flexible.

Unlike Mistral DSL v1 this second version of DSL assumes that all entities that Mistral works with like workflows, actions and triggers are completely independent in terms of how they're referenced and accessed through API (and also Python Client API and CLI). Workbooks, the entity that can combine combine workflows/actions/triggers still exist in the language but only for namespacing and convenience purposes. See Workbooks section for more details.

All DSL consists of the following main object(entity) types that will be described in details next:

• Workflows
• Actions
• Triggers

Prerequisites

Mistral DSL is fully based on YAML and knowledge of YAML is a plus for better understanding of the material in this specification. It also takes advantage of YAQL query language to define expressions in workflow, action and trigger definitions.

• Yet Another Markup Language (YAML): http://yaml.org
• Yet Another Query Language (YAQL): https://pypi.python.org/pypi/yaql/0.3

Workflows

Workflow is the main building block of Mistral DSL, the reason why the project exists. Workflow represents a process that can be described in a various number of ways and that can do some job interesting to the end user. Each workflow consists of tasks (at least one) describing what exact steps should be made during workflow execution.

YAML example

---
version: '2.0'
create_vm:
  description: Simple workflow sample
  type: direct
  input: # Input parameter declarations
    - vm_name
    - image_ref
    - flavor_ref
  output: # Output definition
    vm_id: <% $.vm_id %>
  tasks:
    create_server:
      action: nova.servers_create name=<% $.vm_name %> image=<% $.image_ref %> flavor=<% $.flavor_ref %>
      publish:
        vm_id: <% $.id %>
      on-success:
        - wait_for_instance
    wait_for_instance:
      action: nova.servers_find id=<% $.vm_id %> status='ACTIVE'
      policies:
        retry:
          delay: 5
          count: 15

Workflow Types

Mistral DSL v2 introduces different workflow types and the structure of each workflow type varies according to its semantics. Currently, Mistral provides two workflow types:

• Direct workflow
• Reverse workflow

See corresponding sections for details.

Common Workflow Attributes

• type - Workflow type. Either 'direct' or 'reverse'. Required.
• description - Arbitrary text containing workflow description. Optional.
• input - List defining required input parameter names. Optional.
• output - Any data structure arbitrarily containing YAQL expressions that defines workflow output. May be nested. Optional.
• task-defaults - Default settings for some of task attributes defined at workflow level. Optional. Corresponding attribute defined for a specific task always takes precedence. Specific task attributes that could be defined in task-defaults are the following:
  • on-error
  • on-success
  • on-complete
  • policies
• tasks - Dictionary containing workflow tasks. See below for more details. Required.

Tasks

Task is what a workflow consists of. It defines a specific computational step in the workflow. Each task can optionally take input data and produce output. In Mistral DSL v2 task can be associated with an action or with calling a workflow. In the example below there are two tasks of different types:

action_based_task:
   action: std.http url='openstack.org'
workflow_based_task:
   workflow: backup_vm_workflow vm_id=<% $.vm_id %>

Actions will be explained below in a individual paragraph but looking ahead it's worth saying that Mistral provides a lot of actions out of the box (including actions for most of the core OpenStack services) and it's also easy to plug new actions into Mistral.

Common Task Attributes

All Mistral tasks regardless of workflow type have the following common attributes:

• description - Arbitrary text containing task description. Optional.
• action - Name of the action associated with the task. Required but mutually exclusive with workflow.
• workflow - Name of the workflow associated with the task. Mutually exclusive with action.
• input - Actual input parameter values of the task. Optional. Value of each parameter is a JSON-compliant type such as number, string etc, dictionary or list. It can also be a YAQL expression to retrieve value from task context or any of the mentioned types containing inline YAQL expressions (for example, string "<% $.movie_name %> is a cool movie!")
• publish - Dictionary of variables to publish to the workflow context. Any JSON-compatible data structure optionally containing YAQL expression to select precisely what needs to be published. Published variables will be accessible for downstream tasks via using YAQL expressions. Optional.
• policies - Dictionary-like section defining task policies that influence how Mistral Engine runs tasks. Policies are explained in a separate paragraph. Optional.

Policies

Any Mistral task regardless of what its workflow type can optionally have configured policies.

YAML example

my_task:
  ...
  wait-before: 2
  wait-after: 4
pause-before: <% $.my_expr %>
  timeout: 30
  retry:
    count: 10
    delay: 20
    break-on: <% $.my_var = True %>

'wait-before'

Defines a delay in seconds that Mistral Engine should wait before starting a task.

'wait-after'

Defines a delay in seconds that Mistral Engine should wait after a task has completed before starting next tasks defined in 'on-success', 'on-error' or 'on-complete'.

'pause-before'

The Mistral Engine will pause the workflow and its task with the 'pause-before' policy before executing it. The workflow and task will be paused until a resume signal is received. This policy accepts a YAQL expression which will cause the policy to be applied only if the expression evaluates to 'True'.

'timeout'

Defines a period of time in seconds after which a task will be failed automatically by engine if hasn't completed.

'retry'

Defines a pattern how task should be repeated in case of an error.

• count - Defines a maximum number of times that a task can be repeated.
• delay - Defines a delay in seconds between subsequent task iterations.
• break-on - Defines a YAQL expression that will break iteration loop if it evaluates to 'True'. If it fires then the task is considered successful.

Simplified Input Syntax

When describing a workflow task it's possible to specify its input parameters in two ways:

Full syntax:

my_task:
  action: std.http
  input:
    url: `http://mywebsite.org <http://mywebsite.org>`__
    method: GET

Simplified syntax:

 my_task:
   action: std.http url="`http://mywebsite.org <http://mywebsite.org>__" method="GET"`

The same rules apply to tasks associated with workflows.

Full syntax:

my_task:
  workflow: some_nested_workflow
  input:
    param1: val1
    param2: val2

Simplified syntax:

 my_task:
   workflow: some_nested_workflow param1='val1' param2='val2'

Note: It's also possible to merge these two approaches and specify a part of parameters using simplified key-value pairs syntax and using keyword 'input'. In this case all the parameters will be effectively merged. If the same parameter is specified in both ways then the one under 'input' keyword takes precedence.

Direct Workflow

Direct workflow consists of tasks combined in a graph where every next task starts after another one depending on produced result. So direct workflow has a notion of transition. Direct workflow is considered to be completed if there aren't any transitions left that could be used to jump to next tasks.

Figure 1. Mistral Direct Workflow.

YAML example

---
version: '2.0'
create_vm_and_send_email:
  type: direct
  input:
    - vm_name
    - image_id
    - flavor_id
  output:
    result: <% $.vm_id %>
  tasks:
    create_vm:
      action: nova.servers_create name=<% $.vm_name %> image=<% $.image_id %> flavor=<% $.flavor_id %>
      publish:
        vm_id: <% $.id %>
      on-error:
        - send_error_email
      on-success:
        - send_success_email
    send_error_email:
      action: send_email to='admin@mysite.org' body='Failed to create a VM'
      on_complete:
        - fail
    send_success_email:
      action: send_email to='admin@mysite.org' body='Vm is successfully created and its id: <% $.vm_id %>'

Transitions with YAQL expressions

Task transitions can be determined by success/error/completeness of the previous tasks and also by additional YAQL guard expressions that can access any data produced by upstream tasks. So in the example above task 'create_vm' could also have a YAQL expression on transition to task 'send_success_email' as follows:

create_vm:
  ...
  on-success:
    - send_success_email: <% $.vm_id != null %>

And this would tell Mistral to run 'send_success_email' task only if 'vm_id' variable published by task 'create_vm' is not empty. YAQL expressions can also be applied to 'on-error' and 'on-complete'.

Direct Workflow Task Attributes

• on-success - List of tasks which will run after the task has completed successfully. Optional.
• on-error - List of tasks which will run after the task has completed with an error. Optional.
• on-complete - List of tasks which will run after the task has completed regardless of whether it is successful or not. Optional.

Reverse Workflow

In reverse workflow all relationships in workflow task graph are dependencies. In order to run this type of workflow we need to specify a task that needs to be completed, it can be conventionally called 'target task'. When Mistral Engine starts a workflow it recursively identifies all the dependencies that need to be completed first.

Figure 2. Mistral Reverse Workflow.

Figure 2 explains how reverse workflow works. In the example, task T1 is chosen a target task. So when the workflow starts Mistral will run only tasks T7, T8, T5, T6, T2 and T1 in the specified order (starting from tasks that have no dependencies). Tasks T3 and T4 won't be a part of this workflow because there's no route in the directed graph from T1 to T3 or T4.

YAML example

---
version: '2.0'
create_vm_and_send_email:
  type: reverse
  input:
    - vm_name
    - image_id
    - flavor_id
  output:
    result: <% $.vm_id %>
  tasks:
    create_vm:
      action: nova.servers_create name=<% $.vm_name %> image=<% $.image_id %> flavor=<% $.flavor_id %>
      publish:
        vm_id: <% $.id %>
    search_for_ip:
      action: nova.floating_ips_findall instance_id=null
      publish:
        vm_ip: <% $[0].ip %>
    associate_ip:
      action: nova.servers_add_floating_ip server=<% $.vm_id %> address=<% $.vm_ip %>
      requires: [search_for_ip]
    send_email:
      action: send_email to='admin@mysite.org' body='Vm is created and id <% $.vm_id %> and ip address <% $.vm_ip %>'
      requires: [create_vm, associate_ip]

Reverse Workflow Task Attributes

• requires - List of tasks which should be executed before this task. Optional.

Actions

Action defines what exactly needs to be done when task starts. Action is similar to a regular function in general purpose programming language like Python. It has a name and parameters. Mistral distinguishes 'system actions' and 'Ad-hoc actions'.

System Actions

System actions are provided by Mistral out of the box and can be used by anyone. It is also possible to add system actions for specific Mistral installation via a special plugin mechanism. Currently, built-in system actions are:

std.http

Sends an HTTP request.

Input parameters:

• url - URL for the HTTP request. Required.
• method - method for the HTTP request. Optional. Default is 'GET'.
• params - Dictionary or bytes to be sent in the query string for the HTTP request. Optional.
• body - Dictionary, bytes, or file-like object to send in the body of the HTTP request. Optional.
• headers - Dictionary of HTTP Headers to send with the HTTP request. Optional.
• cookies - Dictionary of HTTP Cookies to send with the HTTP request. Optional.
• auth - Auth to enable Basic/Digest/Custom HTTP Auth. Optional.
• timeout - Float describing the timeout of the request in seconds. Optional.
• allow_redirects - Boolean. Set to True if POST/PUT/DELETE redirect following is allowed. Optional.
• proxies - Dictionary mapping protocol to the URL of the proxy. Optional.

Example:

http_task:
  action: std.http url='google.com'

std.mistral_http

This actions works just like 'std.http' with the only exception: when sending a request it inserts the following HTTP headers:

• Mistral-Execution-Id - Identifier of the workflow execution this action is associated with.
• Mistral-Task-Id - Identifier of the task instance this action is associated with.

Using this action makes it possible to do any work in asynchronous manner triggered via HTTP protocol. That means that Mistral can send a request using 'std.mistral_http' and then any time later whatever system that received this request can notify Mistral back (using its public API) with the result of this action. Header Mistral-Task-Id is required for this operation because it is used a key to find corresponding task in Mistral to attach the result to.

std.email

Sends an email message via SMTP protocol.

• params - Dictionary containing the following keys:
  • to - Comma separated list of recipients. Required.
  • subject - Subject of the message. Required.
  • body - Text containing message body. Required.
• settings - Dictionary containing the following keys:
  • from - Sender email address. Required.
  • smtp_server - SMTP server host name. Required.
  • password - SMTP server password. Required.

Example:

http_task:
  action: std.email
  input:
    params:
      to: admin@mywebsite.org
      subject: Hello from Mistral :)
      body: |
        Cheers! (:_:)
        -- Thanks, Mistral Team.
    settings:
      from: mistral@openstack.org
      smtp_server: smtp.google.com
      password: SECRET 

The syntax of 'std.emal' action is pretty verbose. However, it can be significantly simplified using Ad-hoc actions. More about them below.

std.ssh

Runs Secure Shell command.

Input parameters:

• cmd - String containing a shell command that needs to be executed. Required.
• host - Host name that the command needs to be executed on. Required.
• username - User name to authenticate on the host.
• password - User password to to authenticate on the host.

Note: Authentication using key pairs is currently not supported.

std.echo

Simple action mostly needed for testing purposes that returns a predefined result.

Input parameters:

• output - Value of any type that needs to be returned as a result of the action. Required.

Ad-hoc Actions

Ad-hoc action is a special type of action that can be created by user. Ad-hoc action is always created as a wrapper around any other existing system action and its main goal is to simplify using same actions many times with similar pattern.

Note: Nested ad-hoc actions currently are not supported (i.e. ad-hoc action around another ad-hoc action).

YAML example

---
version: '2.0'
error_email:
  input:
    - execution_id
  base: std.email
  base-input:
    params:
      to: admin@mywebsite.org
      subject: Something went wrong with your Mistral workflow :(
      body: |
          Please take a look at Mistral Dashboard to find out what's wrong
          with your workflow execution <% $.execution_id %>.
          Everything's going to be alright!
          -- Sincerely, Mistral Team.
      settings:
        from: mistral@openstack.org
        smtp_server: smtp.google.com
        password: SECRET 

Once this action is uploaded to Mistral any workflow will be able to use it as follows:

my_workflow:
  tasks:
    ...
    send_error_email
      action: error_email execution_id=<% $.__execution.id %>

Attributes

• base - Name of base action that this action is built on top of. Required.
• base-input - Actual input parameters provided to base action. Look at the example above. Optional.
• input - List of declared action parameters which should be specified as corresponding task input. This attribute is optional and used only for documenting purposes. Mistral now does not enforce actual input parameters to exactly correspond to this list. Based parameters will be calculated based on provided actual parameters with using YAQL expressions so what's used in expressions implicitly define real input parameters. Dictionary of actual input parameters is referenced in YAQL as '$.'. Redundant parameters will be simply ignored.
• output - Any data structure defining how to calculate output of this action based on output of base action. It can optionally have YAQL expressions to access properties of base action output referenced in YAQL as '$.'.

Triggers [coming soon...]

NOTE: Triggers are not yet implemented as part of version 0.1, they will go into in one of the next builds, likely 0.2

Using triggers it is possible to run workflows according to specific rules: periodically setting a cron (http://en.wikipedia.org/wiki/Cron) pattern or on external events like ceilometer alarm.

Below are two options picturing what Mistral team is currently discussing as a candidate for implementation:

Option 1:

---
version: '2.0'
cron_trigger:
  type: periodic
  parameters:
    cron-pattern: "*/1 * * * *"
  workflows:
    - wf1:
      parameters:
        # Regular dictionary (heavy syntax)
      ...
    - wf2 param1=val1 param2=val2 task_name='task1' # Short syntax
  actions:
    # The same for actions

Option 2:

---
version: '2.0'
cron_trigger:
  type: periodic
  parameters:
    cron-pattern: "*/1 * * * *"
  workflows: ["wf2 param1=val1 param2=val2 task_name='task1'", ...] # List of workflows with using simplified syntax.
  actions: # same for actions

If you are interested in this functionality you can participate in mailing list openstack-dev@lists.openstack.org.

Workbooks

As mentioned before, workbooks still exist in Mistral DSL version 2 but purely for convenience. Using workbooks users can combine multiple entities of any type (workflows, actions and triggers) into one document and upload to Mistral service. When uploading a workbook Mistral will parse it and save its workflows, actions and triggers as independent objects which will be accessible via their own API endpoints (/workflows, /actions and /triggers/). Once it's done the workbook comes out of the game. User can just start workflows and use references to workflows/actions/triggers as if they were uploaded without workbook in the first place. However, if we want to modify these individual objects we can modify the same workbook definition and re-upload it to Mistral (or, of course, we can do it independently).

Namespacing

One thing that's worth noting is that when using a workbook Mistral uses its name as a prefix for generating final names of workflows, actions and triggers included into the workbook. To illustrate this principle let's take a look at the figure below.

 So after a workbook has been uploaded its workflows, actions and triggers become independent objects but with slightly different names.

YAML example

---
version: '2.0'
name: my_workbook
description: My set of workflows and ad-hoc actions
workflows:
  local_workflow1:
    type: direct
    
    tasks:
      task1:
        action: local_action str1='Hi' str2=' Mistral!'
        on-complete:
          - task2
    task2:
      action: global_action
      ...
    
  local_workflow2:
    type: reverse
    tasks:
      task1:
        workflow: local_workflow1
        on-complete:
          - task2
      
      task2:
        workflow: global_workflow param1='val1' param2='val2'
        ...
actions:
  local_action:
    input:
      - str1
      - str2
    base: std.echo output="<% $.str1 %><% $.str2 %>"

Note: Even though names of objects inside workbooks change upon uploading Mistral allows referencing between those objects using local names declared in the original workbook.

Attributes

• name - Workbook name. Required.
• description - Workbook description. Optional.
• tags - String with arbitrary comma-separated values. Optional.
• workflows - Dictionary containing workflow definitions. Optional.
• actions - Dictionary containing ad-hoc action definitions. Optional.
• triggers - Dictionary containing trigger definitions. Optional. (Currently not supported)