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Mistral

Mistral is an OpenStack project that manages and executes workflows as a service. Mistral is automatically installed as a separate service named “mistral” along with StackStorm. A Mistral workflow can be defined as a StackStorm action in a Mistral workbook using the v2 DSL.

Expression languages such as YAQL are used for formatting variables and condition evaluations. Starting with StackStorm v2.2, Jinja2 is also supported where YAQL expressions are accepted. Both workbook and workflow definitions are supported.

On action execution, StackStorm writes the definition to Mistral and executes the workflow. A workflow can invoke other StackStorm actions natively as subtasks. StackStorm handles the translations and calls transparently in Mistral and actively polls Mistral for execution results. StackStorm actions in the workflow can be traced back to the original parent action that invoked the workflow.

Essential Mistral Links:

Note

Workflow examples in this documentation use YAQL expressions unless otherwise stated.

Basic Workflow

Similarly to ActionChains, Mistral workflows have an action metadata file in /opt/stackstorm/packs/<mypack>/actions, and the workflow definition itself in /opt/stackstorm/packs/<mypack>/actions/workflows.

Let’s start with a very basic workflow that calls a StackStorm action and notifies StackStorm when the workflow is done. The files used in this example are also located under /usr/share/doc/st2/examples if StackStorm is already installed (see also deploy examples).

The first task is named run-cmd. It executes a shell command on the server where StackStorm is installed. A task can reference any registered StackStorm action directly. In this example, the run-cmd task calls core.local and passing the cmd as input. core.local is an action that comes installed with StackStorm. When the workflow is invoked, StackStorm will translate the workflow definition appropriately before sending it to Mistral. Let’s save this as /opt/stackstorm/packs/examples/actions/workflows/mistral-basic.yaml on our StackStorm server.

version: '2.0'

examples.mistral-basic:
    description: A basic workflow that runs an arbitrary linux command.
    type: direct
    input:
        - cmd
        - timeout
    output:
        stdout: <% $.stdout %>
    tasks:
        task1:
            action: core.local cmd=<% $.cmd %> timeout=<% $.timeout %>
            publish:
                stdout: <% task(task1).result.stdout %>
                stderr: <% task(task1).result.stderr %>

This is the corresponding StackStorm action metadata for the example above. The StackStorm pack for this workflow action is named “examples”. Note that the workflow is named fully qualified as <pack>.<action in the definition above. The StackStorm action runner is mistral-v2. The entry point for the StackStorm action refers to the YAML file of the workflow definition. Let’s save this metadata as /opt/stackstorm/packs/examples/actions/mistral-basic.yaml:

---
description: Run a local linux command
enabled: true
runner_type: mistral-v2
entry_point: workflows/mistral-basic.yaml
name: mistral-basic
pack: examples
parameters:
  cmd:
    required: true
    type: string
  timeout:
    type: integer
    default: 60

The following table list optional parameters that can be defined in the workflow action. In the example, these optional parameters are set to immutable. It is good practice to set them to immutable even if they are empty since these are Mistral-specific parameters for the workflow author.

options description
workflow If definition is a workbook containing many workflows, this specifies the main workflow to execute.
task If the type of workflow is “reverse”, this specifies the task to invoke.
context A dictionary containing additional workflow start up parameters.

Next, run st2 action create /opt/stackstorm/packs/examples/actions/mistral-basic.yaml to create this workflow action. This will register the workflow as examples.mistral-basic in StackStorm. To execute the workflow, run st2 run examples.mistral-basic cmd=date -a where -a tells the command to return and not wait for the workflow to complete.

If the workflow completed successfully, both the workflow examples.mistral-basic and the action core.local should have a succeeded status in the StackStorm action execution list. By default, st2 execution list only returns top level executions. This means subtasks are not displayed.

+--------------------------+--------------+--------------+-----------+-----------------+---------------+
| id                       | action.ref   | context.user | status    | start_timestamp | end_timestamp |
+--------------------------+--------------+--------------+-----------+-----------------+---------------+
| 54ee54c61e2e24152b769a47 | examples     | stanley      | succeeded | Wed, 25 Feb     | Wed, 25 Feb   |
|                          | .mistral-    |              |           | 2015 23:03:34   | 2015 23:03:34 |
|                          | basic        |              |           | UTC             | UTC           |
+--------------------------+--------------+--------------+-----------+-----------------+---------------+

To display subtasks, run st2 execution get <execution-id> --show-tasks:

+--------------------------+------------+--------------+-----------+------------------------------+------------------------------+
| id                       | action.ref | context.user | status    | start_timestamp              | end_timestamp                |
+--------------------------+------------+--------------+-----------+------------------------------+------------------------------+
| 54ee54c91e2e24152b769a49 | core.local | stanley      | succeeded | Wed, 25 Feb 2015 23:03:37    | Wed, 25 Feb 2015 23:03:37    |
|                          |            |              |           | UTC                          | UTC                          |
+--------------------------+------------+--------------+-----------+------------------------------+------------------------------+

The following is a simple extension of the previous workflow definition. In this example, we have a second task named “task2”. It might be natural to think that “task2” will be executed after “task1”, i.e, in sequential order. However, when no tasks attributes like on-complete, on-success and on-error are defined, tasks are run in parallel. This is possible with Mistral because it provides a join flow control which allows us to synchronize multiple parallel workflow branches and aggregate their data.

version: '2.0'

examples.mistral-basic-two-tasks-with-notifications:
    description: A basic workflow that runs two Linux commands (one in each task).
    type: direct
    output:
        stdout: <% $.stdout %>
    tasks:
        task1:
            action: core.local cmd="echo task1"
            publish:
                stdout: <% task(task1).result.stdout %>
                stderr: <% task(task1).result.stderr %>
        task2:
            action: core.local cmd="echo task2"
            publish:
                stdout: <% task(task2).result.stdout %>
                stderr: <% task(task2).result.stderr %>

Pausing and Resuming Workflow Execution

An execution of a Mistral workflow can be paused by running st2 execution pause <execution-id>. An execution must be in a running state in order for pause to be successful. The execution will initially go into a pausing state, and will go into a paused state when no more tasks are in an active state such as running, pausing, or canceling. When a workflow execution is paused, it can be resumed by running st2 execution resume <execution-id>.

The pause and resume operation will cascade down to subworkflows, whether it’s another workflow defined in a workbook or it’s another StackStorm action that is a Mistral workflow or Action Chain. If the pause operation is performed from a subworkflow or subchain, then the pause will cascade up to the parent workflow or parent chain. However, if the resume operation is performed from a subworkflow or subchain, the resume will not cascade up to the parent workflow or parent chain. This allows users to resume and troubleshoot branches individually.

Canceling Workflow Execution

An execution of a Mistral workflow can be cancelled by running st2 execution cancel <execution-id>. Workflow tasks that are still running will not be canceled and will run to completion. No new tasks for the workflow will be scheduled.

Re-running Workflow Execution

An execution of a Mistral workflow can be re-run on error. The execution either can be re-run from the beginning or from the task(s) that failed. The latter is useful for long running workflows with temporary service or network outages. Re-running the workflow execution from the beginning is exactly like re-running any StackStorm execution with the command st2 execution re-run <execution-id>.

The re-run is a completely separate execution with a new execution ID in both StackStorm and Mistral. Re-running the workflow from where it errored is slightly different. To retain context, the original workflow execution is reused in Mistral but a new StackStorm execution will be created to stay consistent in StackStorm. The re-run command has a new --tasks option that takes a list of task names to re-run.

For example, given a workflow that fails at task3 and task4 on separate parallel branches, the command st2 execution re-run <execution-id> --tasks task3 task4 will resume the Mistral workflow execution and re-run both task3 and task4 using original inputs. Both the workflow and task execution in Mistral have to be in an errored state for re-run.

If using a Mistral workbook, tasks of subworkflows can also be re-run. For example, if the main workflow has a task1 that calls subflow1, then to re-run subtask1 of subflow1, the syntax for the st2 execution re-run command would be st2 execution re-run <execution-id> --tasks task1.subtask1.

If the task to re-run is a “with-items” task, there is an option to re-run only failed iterations. For example, task1 is a with-items task with 5 items. Let’s say 2 of the items failed. By specifying the st2 execution re-run --tasks task1 task2 --no-reset task1 option, task1 will only re-run the 2 items that failed. If the --no-reset option is not provided, then all 5 items will be re-run.

Note

Re-running workflow execution from the task(s) that failed is currently an experimental feature and subject to bug(s) and change(s). Please also note that re-running a subtask nested in another StackStorm action is not currently supported.

Publishing Variables in Mistral Workflows

A Mistral task can publish results from a task as variables that can be consumed in other tasks:

tasks:
    get_hostname:
        action: core.local
        input:
            cmd: "hostname"
        publish:
            hostname: <% task(get_hostname).result.stdout %>

In the above example, get_hostname is a core.local action which runs the command hostname. The `core.local action produces output consisting of the fields stdout, stderr, exit_code etc.

We just want to publish the variable stdout from it, for the rest of tasks to consume. To reference the result of the task, use the task function, which returns a dictionary containing attributes for the task such as id, state, result, and additional info.

Another example is shown below:

tasks:
    create_new_node:
        action: rackspace.create_vm
        input:
          name: <% $.hostname %>
          flavor_id: <% $.vm_size_id %>
          image_id: <% $.vm_image_id %>
          key_material: <% $.ssh_pub_key %>
          metadata:
            asg: <% $.asg %>
        publish:
          ipv4_address: '<% task(create_new_node).result.result.public_ips[1] %>'
          ipv6_address: '<% task(create_new_node).result.result.public_ips[0] %>'

In the above example, the action rackspace.create_vm is a Python action that produces a result object. We just want to publish the IP addresses from the public_ips list field from the result object.

Please note that result.result is not a typo. The Python action posts output to a key named result for the st2 action execution and the Mistral task function puts the result of the Python action in result of its output dictionary.

Such published variables are accessible as input parameters to other tasks in the workflow. An example of using ipv4_address from the above example in another task is shown below:

tasks:
    # ... <snap>

    setup_ipv4_dns:
        action: rackspace.create_dns_record
        wait-before: 1 # delay, in seconds
        input:
          name: '<% $.hostname %>.<% $.asg %>.<% $.domain %>'
          zone_id: <% $.dns_zone_id %>
          type: 'A'
          data: <% $.ipv4_address %>

    # .... </snap>

Stitching Together a More Complex Workflow

The following is a mock up of a more complex workflow. In this mock up running simple printf and sleep commands, the workflow demonstrates nested workflows, fork, and join:

version: "2.0"
name: examples.mistral-workbook-complex
description: A sample workflow that demonstrates nested workflows, forks, and join.

workflows:

    main:
        type: direct
        input:
            - vm_name
            - cpu_cores
            - memory_mb
        output:
            vm_id: <% $.vm_id %>
            ip: <% $.ip %>
        tasks:
            register_dns:
                action: core.local
                input:
                    cmd: "sleep 1; printf 'Registering <% $.vm_name %>...'"
                publish:
                    ip: "10.1.23.99"
                    status_message: "DNS for <% $.vm_name %> is registered."
                on-success:
                    - configure_vm
                    - notify
            create_vm:
                wait-before: 1
                workflow: create_vm
                input:
                    name: <% $.vm_name %>
                    cpu_cores: <% $.cpu_cores %>
                    memory_mb: <% $.memory_mb %>
                publish:
                    vm_id: <% task(create_vm).result.vm_id %>
                    status_message: "VM <% $.vm_name %> is created."
                on-success:
                    - configure_vm
                    - notify
            configure_vm:
                join: all
                workflow: configure_vm
                input:
                    vm_id: <% $.vm_id %>
                    ip: <% $.ip %>
                publish:
                    status_message: "VM <% $.vm_name %> is reconfigured."
                on-success:
                    - close_request
                    - notify
            close_request:
                action: std.noop
                publish:
                    status_message: "VM request is fulfilled."
                on-success:
                    - notify
            notify:
                action: core.local
                input:
                    cmd: "printf '<% $.status_message %>'"

    create_vm:
        type: direct
        input:
            - name
            - cpu_cores
            - memory_mb
        output:
            vm_id: <% $.vm_id %>
        tasks:
            create:
                action: core.local
                input:
                    cmd: "printf 'vm1234'; sleep 5"
                publish:
                    vm_id: <% task(create).result.stdout %>

    configure_vm:
        type: direct
        input:
            - vm_id
            - ip
        tasks:
            add_disks:
                action: core.local
                input:
                    cmd: "sleep 1; printf 'disks created'"
            add_nics:
                action: core.local
                input:
                    cmd: "sleep 1; printf 'nics created'"
            install_apps:
                action: core.local
                input:
                    cmd: "sleep 1; printf 'apps installed'"

Since there are multiple workflows defined in this workbook, the workflow author has to specify which workflow to execute in the metadata:

---
description: Run a series of simulated actions.
enabled: true
entry_point: workflows/mistral-workbook-complex.yaml
name: mistral-workbook-complex
pack: examples
parameters:
  cpu_cores:
    default: 1
    type: integer
  memory_mb:
    default: 1024
    type: integer
  vm_name:
    required: true
    type: string
  workflow:
    default: examples.mistral-workbook-complex.main
    immutable: true
    type: string
runner_type: mistral-v2

To test out this workflow, save the metadata file to /opt/stackstorm/packs/examples/actions/ and the workflow file to /opt/stackstorm/packs/examples/actions/workflows. Run st2 action create /opt/stackstorm/packs/examples/actions/mistral-workbook-complex.yaml to create the action and run st2 run examples.mistral-workbook-complex vm_name="vmtest1" -a to test.

Validation

The Mistral CLI includes tools for performing high-level sanity checks of Mistral workflow YAML files:

# Validate a workflow
mistral workflow-validate /path/to/workflow.yaml

# Validate a workbook
mistral workbook-validate /path/to/workbook.yaml

Note

These sanity checks simply provide a test against the Mistral DSL schema. They do NOT test YAQL or Jinja2 expressions.

More Examples

There are more workflow examples under /usr/share/doc/st2/examples. These include error handling, repeat, and retries.

Check out this step-by-step tutorial on building a workflow in StackStorm https://stackstorm.com/2015/07/08/automating-with-mistral-workflow/

More details about Mistral can be found at https://docs.openstack.org/mistral/latest/.