Mistral Flow Control

When writing a Mistral workflow it is common that you want to add a condition to a specific task. For example, you only want to run a task if the user provided a specific input to the workflow. I found this counter intuitive at first and have since explained it to a few people, so I wanted to document it for the future.

As a contrived example, we want a workflow that returns "x is true" if we pass the x input or "x is not true" if it isn't passed (or false is passed).

As a Pythonista, I wanted to solve the problem with something equivalent to this short Python code. It's just a simple if/else after all.

def workflow(x=False):
    if x:
        return "x is true"
        return "x is not true"

In Mistral's Workflow DSL you need to think about it a little differently.

    - x: false

        - task_a: <% $.x = true %>
        - task_b: <% $.x != true %>
      action: std.echo output="x is true"
      action: std.echo output="x is not true"

In this workflow we have two tasks, task_a and task_b, they perform the different outcomes we want. The logic that switches between them becomes a task of it's own. The task task_switch doesn't specify an action or workflow to run, this means that it performs a std.noop. On the success of this, which should always happen, we then use a mapping of tasks and conditions (in the on-success attribute) to specify what happens next. task_a and task_b are the tasks and on the right of these we have the YAQL expression which is evaluated to determine if they run. If their result is truthy, that task will be scheduled for execution.

In our case, if I run the workflow with x=true, then it will evaluate to the following and only task_a will be executed.:

  - task_a: true
  - task_b: false

This then becomes something like a switch statement, however, unlike a switch statement in most languages, multiple tasks and workflow paths can be followed rather than the first that turns out to be true.

Here is a slightly more complicated workflow that shows multiple tasks being called depending on the input.

    - letter

        - letter_a: <% $.letter = 'a' %>
        - letter_a_or_b: <% $.letter in ['a', 'b']  %>
        - letter_other: <% not $.letter in ['a', 'b'] %>
      action: std.echo output="letter is a"
      action: std.echo output="letter a or b"
      action: std.echo output="letter is not a or b"

Input 'a' means tasks letter_a and letter_a_or_b are executed. Input 'b' means that only letter_a_or_b is executed and any other input would execute the letter_other command.

As mentioned before, these examples are contrived, but they can be quite useful when used in larger workflows to control the flow or provide some simple validation.

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