How-to Guide: Creating Nested Workflows in CWL¶

This guide explains how to build and use nested workflows in CWL by leveraging the SubworkflowFeatureRequirement.

The focus is on the workflow composition and the integration of subworkflows to create reusable components.

Objective¶

Main Workflow: Accepts inputs and calls a subworkflow (rgb-composite) as a single step.
Subworkflow (rgb-composite): Performs a series of steps to process data and produce the desired output.

Key Blocks¶

SubworkflowFeatureRequirement

The SubworkflowFeatureRequirement allows workflows to include other workflows as steps.

requirements:

  SubworkflowFeatureRequirement: {}

Subworkflow Step Definition

The main workflow calls the subworkflow using:

step_rgb_composite:

in:

    stac-item: stac-item

    bands: bands

  out:

    - rgb-tif

  run: "#rgb-composite"

run: "#rgb-composite": Links to the rgb-composite subworkflow.
Inputs and Outputs: The subworkflow accepts inputs (stac-item, bands) and produces an output (rgb-tif).

Steps¶

Define the Subworkflow

The rgb-composite subworkflow performs the following:

Fetch band-specific asset URLs using the stac tool.
Stack the asset TIFFs into a single file using the rio_stack tool.
Apply color correction to generate the RGB composite using the rio_color tool.

Subworkflow Definition (rgb-composite)

class: Workflow

id: rgb-composite

requirements:

  InlineJavascriptRequirement: {}

  NetworkAccess:

    networkAccess: true

  ScatterFeatureRequirement: {}

inputs:

  stac-item:

    type: string

  bands:

    type: string[]

outputs:

  rgb-tif:

    outputSource: step_color/rgb

    type: File

steps:

  step_curl:

in:

      stac_item: stac-item

      common_band_name: bands

    out:

      - hrefs

    run: "#stac"

    scatter: common_band_name

    scatterMethod: dotproduct

  step_stack:

in:

      tiffs:

        source: step_curl/hrefs

    out:

      - stacked

    run: "#rio_stack"

  step_color:

in:

      stacked:

        source: step_stack/stacked

    out:

      - rgb

    run: "#rio_color"

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Define the Main Workflow

The main workflow invokes the rgb-composite subworkflow:

class: Workflow

id: main

requirements:

  SubworkflowFeatureRequirement: {}

  InlineJavascriptRequirement: {}

  NetworkAccess:

    networkAccess: true

  ScatterFeatureRequirement: {}

inputs:

  stac-item:

    type: string

  bands:

    type: string[]

    default: ["red", "green", "blue"]

outputs:

  rgb-tif:

    outputSource: step_rgb_composite/rgb-tif

    type: File

steps:

  step_rgb_composite:

in:

      stac-item: stac-item

      bands: bands

    out:

      - rgb-tif

    run: "#rgb-composite"

Requirements:
- SubworkflowFeatureRequirement: Enables the use of nested workflows.
- ScatterFeatureRequirement: Allows processing multiple bands simultaneously.
Inputs:
- stac-item: URL to a STAC item.
- bands: Array of band names (default: ["red", "green", "blue"]).
Outputs:
- rgb-tif: The RGB composite file produced by the subworkflow.

Run the Workflow

To execute the main workflow, use the following command:

cwltool nested-workflow.cwl \
  --stac-item https://earth-search.aws.element84.com/v0/collections/sentinel-s2-l2a-cogs/items/S2B_53HPA_20210723_0_L2A

INFO /opt/hostedtoolcache/Python/3.13.3/x64/bin/cwltool 3.1.20250110105449

INFO Resolved '../cwl-workflows/nested-workflow.cwl' to 'file:///home/runner/work/how-to/how-to/cwl-workflows/nested-workflow.cwl'

INFO [workflow ] start

INFO [workflow ] starting step step_rgb_composite

INFO [step step_rgb_composite] start

INFO [workflow step_rgb_composite] start

INFO [workflow step_rgb_composite] starting step step_curl

INFO [step step_curl] start

INFO [job step_curl] /tmp/pufx_v3d$ docker \

    run \

    -i \

    --mount=type=bind,source=/tmp/pufx_v3d,target=/yZUdzu \

    --mount=type=bind,source=/tmp/7eymyjv6,target=/tmp \

    --workdir=/yZUdzu \

    --read-only=true \

    --log-driver=none \

    --user=1001:118 \

    --rm \

    --cidfile=/tmp/cakv9aaf/20250620071829-260192.cid \

    --env=TMPDIR=/tmp \

    --env=HOME=/yZUdzu \

    docker.io/curlimages/curl:latest \

    curl \

    https://earth-search.aws.element84.com/v0/collections/sentinel-s2-l2a-cogs/items/S2B_53HPA_20210723_0_L2A > /tmp/pufx_v3d/message

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current

                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0

100 10156  100 10156    0     0  44020      0 --:--:-- --:--:-- --:--:-- 43965

INFO [job step_curl] completed success

INFO [step step_curl] start

INFO [job step_curl_2] /tmp/sxcka5p1$ docker \

    run \

    -i \

    --mount=type=bind,source=/tmp/sxcka5p1,target=/yZUdzu \

    --mount=type=bind,source=/tmp/hionhjgm,target=/tmp \

    --workdir=/yZUdzu \

    --read-only=true \

    --log-driver=none \

    --user=1001:118 \

    --rm \

    --cidfile=/tmp/3t4e8won/20250620071830-267272.cid \

    --env=TMPDIR=/tmp \

    --env=HOME=/yZUdzu \

    docker.io/curlimages/curl:latest \

    curl \

    https://earth-search.aws.element84.com/v0/collections/sentinel-s2-l2a-cogs/items/S2B_53HPA_20210723_0_L2A > /tmp/sxcka5p1/message

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current

                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0

100 10156  100 10156    0     0  47808      0 --:--:-- --:--:-- --:--:-- 47680

INFO [job step_curl_2] completed success

INFO [step step_curl] start

INFO [job step_curl_3] /tmp/twxvo_8b$ docker \

    run \

    -i \

    --mount=type=bind,source=/tmp/twxvo_8b,target=/yZUdzu \

    --mount=type=bind,source=/tmp/rwhcvgjb,target=/tmp \

    --workdir=/yZUdzu \

    --read-only=true \

    --log-driver=none \

    --user=1001:118 \

    --rm \

    --cidfile=/tmp/vpt14fg_/20250620071831-274759.cid \

    --env=TMPDIR=/tmp \

    --env=HOME=/yZUdzu \

    docker.io/curlimages/curl:latest \

    curl \

    https://earth-search.aws.element84.com/v0/collections/sentinel-s2-l2a-cogs/items/S2B_53HPA_20210723_0_L2A > /tmp/twxvo_8b/message

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current

                                 Dload  Upload   Total   Spent    Left  Speed

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0

  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0

100 10156  100 10156    0     0  45382      0 --:--:-- --:--:-- --:--:-- 45339

INFO [job step_curl_3] completed success

INFO [step step_curl] completed success

INFO [workflow step_rgb_composite] starting step step_stack

INFO [step step_stack] start

INFO [job step_stack] /tmp/e53l9l2z$ docker \

    run \

    -i \

    --mount=type=bind,source=/tmp/e53l9l2z,target=/yZUdzu \

    --mount=type=bind,source=/tmp/rn2471xj,target=/tmp \

    --workdir=/yZUdzu \

    --read-only=true \

    --user=1001:118 \

    --rm \

    --cidfile=/tmp/29_fbisl/20250620071832-296183.cid \

    --env=TMPDIR=/tmp \

    --env=HOME=/yZUdzu \

    --env=CPL_VSIL_CURL_ALLOWED_EXTENSIONS=.tif \

    --env=GDAL_HTTP_MERGE_CONSECUTIVE_RANGES=YES \

    --env=GDAL_TIFF_INTERNAL_MASK=YES \

    ghcr.io/eoap/how-to/rio:1.0.0 \

    rio \

    stack \

    https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/53/H/PA/2021/7/S2B_53HPA_20210723_0_L2A/B04.tif \

    https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/53/H/PA/2021/7/S2B_53HPA_20210723_0_L2A/B03.tif \

    https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/53/H/PA/2021/7/S2B_53HPA_20210723_0_L2A/B02.tif \

    stacked.tif

INFO [job step_stack] Max memory used: 1199MiB

INFO [job step_stack] completed success

INFO [step step_stack] completed success

INFO [workflow step_rgb_composite] starting step step_color

INFO [step step_color] start

INFO [job step_color] /tmp/rri4c0zo$ docker \

    run \

    -i \

    --mount=type=bind,source=/tmp/rri4c0zo,target=/yZUdzu \

    --mount=type=bind,source=/tmp/m1rd6m6g,target=/tmp \

    --mount=type=bind,source=/tmp/e53l9l2z/stacked.tif,target=/var/lib/cwl/stgb368b707-6529-4134-9bbd-6700eb174bde/stacked.tif,readonly \

    --workdir=/yZUdzu \

    --read-only=true \

    --user=1001:118 \

    --rm \

    --cidfile=/tmp/thowdx3l/20250620071851-051982.cid \

    --env=TMPDIR=/tmp \

    --env=HOME=/yZUdzu \

    ghcr.io/eoap/how-to/rio:1.0.0 \

    rio \

    color \

    -j \

    -1 \

    --out-dtype \

    uint8 \

    /var/lib/cwl/stgb368b707-6529-4134-9bbd-6700eb174bde/stacked.tif \

    rgb.tif \

    'gamma 3 0.95, sigmoidal rgb 35 0.13'

INFO [job step_color] Max memory used: 786MiB

INFO [job step_color] completed success

INFO [step step_color] completed success

INFO [workflow step_rgb_composite] completed success

INFO [step step_rgb_composite] completed success

INFO [workflow ] completed success

INFO Final process status is success

Expected Output

Intermediate Outputs:

URLs of band-specific TIFFs (hrefs).
Stacked TIFF file (stacked.tif).

Final Output:

RGB composite TIFF file (rgb-tif).

  "rgb-tif": {

    "location": "file:///home/runner/work/how-to/how-to/docs/rgb.tif",

    "basename": "rgb.tif",

    "class": "File",

    "checksum": "sha1$dc1b292898d647116fdb31ec9c04be3a6ff9e5e9",

    "size": 361747464,

    "path": "/home/runner/work/how-to/how-to/docs/rgb.tif"

Key Takeaways¶

Modularity with Subworkflows:

Use SubworkflowFeatureRequirement to encapsulate reusable workflows.
Subworkflows simplify complex workflows by isolating specific logic.

Integration of Subworkflows:

Define subworkflow steps in the main workflow.
Use run to link the subworkflow.

Reusability:

Subworkflows can be reused in multiple workflows, promoting modularity and efficiency.

This approach makes it easy to manage and scale CWL workflows by leveraging nested subworkflows.