Workflow Inputs

Introduction

Workflows and templates operate on a set of defined parameters and arguments that are supplied to the running container. The precise details of how to manage the inputs can be confusing; this article attempts to clarify concepts and provide simple working examples to illustrate the various configuration options.

The examples below are limited to DAGTemplates and mainly focused on parameters, but similar reasoning applies to the other types of templates.

Parameter Inputs

First, some clarification of terms is needed. For a glossary reference, see Argo Core Concepts.

A workflow provides arguments, which are passed in to the entry point template. A template defines inputs which are then provided by template callers (such as steps, dag, or even a workflow). The structure of both is identical.

For example, in a Workflow, one parameter would look like this:

arguments:
  parameters:
  - name: workflow-param-1

And in a template:

inputs:
  parameters:
  - name: template-param-1

Inputs to DAGTemplates use the arguments format:

dag:
  tasks:
  - name: step-A
    template: step-template-A
    arguments:
      parameters:
      - name: template-param-1
        value: abcd

Previous examples in context:

apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
  generateName: example-
spec:
  entrypoint: main
  arguments:
    parameters:
    - name: workflow-param-1
  templates:
  - name: main
    dag:
      tasks:
      - name: step-A 
        template: step-template-A
        arguments:
          parameters:
          - name: template-param-1
            value: "{{workflow.parameters.workflow-param-1}}"

  - name: step-template-A
    inputs:
      parameters:
        - name: template-param-1
    script:
      image: alpine
      command: [/bin/sh]
      source: |
          echo "{{inputs.parameters.template-param-1}}"

To run this example: argo submit -n argo example.yaml -p 'workflow-param-1="abcd"' --watch

Using Previous Step Outputs As Inputs

In DAGTemplates, it is common to want to take the output of one step and send it as the input to another step. However, there is a difference in how this works for artifacts vs parameters. Suppose our step-template-A defines some outputs:

outputs:
  parameters:
    - name: output-param-1
      valueFrom:
        path: /p1.txt
  artifacts:
    - name: output-artifact-1
      path: /some-directory

In my DAGTemplate, I can send these outputs to another template like this:

dag:
  tasks:
  - name: step-A 
    template: step-template-A
    arguments:
      parameters:
      - name: template-param-1
        value: "{{workflow.parameters.workflow-param-1}}"
  - name: step-B
    dependencies: [step-A]
    template: step-template-B
    arguments:
      parameters:
      - name: template-param-2
        value: "{{tasks.step-A.outputs.parameters.output-param-1}}"
      artifacts:
      - name: input-artifact-1
        from: "{{tasks.step-A.outputs.artifacts.output-artifact-1}}"

Note the important distinction between parameters and artifacts; they both share the name field, but one uses value and the other uses from.

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