How-to Guide: Using Conditional Workflows¶

This guide explains how to create workflows with conditional execution using when and pickValue.

These features allow tasks to run conditionally based on inputs and dynamically select outputs.

Objective¶

1. Process raster files using rio_stack if the EPSG code is "native".
2. Use rio_warp_stack if the EPSG code is not "native".
3. Dynamically choose the appropriate output using pickValue: the_only_non_null.

Key Features¶

1. Conditional Execution with when

The when field controls whether a step is executed based on an input condition.

• Executes the step only if the condition evaluates to true.

2. Dynamic Output Selection with pickValue

The pickValue field determines how to handle multiple output sources.

• Selects the only non-null value from the list of outputs.
• Ensures only one output is passed downstream.

Steps¶

1. Define the Workflow

Workflow Inputs and Outputs

The workflow accepts:

• stac-item: A STAC item URL.
• epsg_code: EPSG code for projection (default: "native").
• bands: An array of band names (e.g., ["red", "green", "blue"]).

Workflow Definition

Its graphical representation:

2. Conditional Steps

Step 1: Use rio_stack for Native EPSG

• Executes only if epsg_code == "native".

Step 2: Use rio_warp_stack for Reprojection

• Executes only if epsg_code != "native".

3. Dynamic Output Selection

Stack Selection

• Dynamically selects the output from step_stack or step_warp_stack.
• Ensures the downstream step (step_color) receives the correct output.

4. Run the Workflow

Run the workflow with the following commands based on the desired EPSG code:

Native EPSG Code:

cwltool conditional-workflows.cwl \
    --stac-item https://earth-search.aws.element84.com/v0/collections/sentinel-s2-l2a-cogs/items/S2B_53HPA_20210723_0_L2A  \
    --bands red \
    --bands green \
    --bands blue \
    --epsg_code native

Reprojected EPSG Code

cwltool conditional-workflows.cwl \
    --stac-item https://earth-search.aws.element84.com/v0/collections/sentinel-s2-l2a-cogs/items/S2B_53HPA_20210723_0_L2A  \
    --bands red \
    --bands green \
    --bands blue \
    --epsg_code "EPSG:4326"

5. Expected Output

Intermediate Outputs:

• stacked file from either step_stack or step_warp_stack.

Final Output:

• rgb-tif: RGB composite file produced by step_color.

Key Takeaways¶

Conditional Steps:

• Use when to execute steps based on input conditions.
• Allows workflows to adapt to varying requirements dynamically.

Dynamic Output Handling:

• Use pickValue to select the appropriate output among multiple sources.

Flexible Workflows:

• This approach creates workflows that handle both native and reprojected data seamlessly.

This guide demonstrates how to build adaptable workflows in CWL, leveraging conditional execution and dynamic output selection.