How-to Guide: Scattering on Input Parameters¶

This guide explains how to scatter workflow steps based on input parameters using ScatterFeatureRequirement and MultipleInputFeatureRequirement.

The focus is on creating workflows where multiple input parameters are processed in parallel.

Objective¶

Scatter the step_curl task to process multiple input bands (red, green, blue) in parallel and combine their results in subsequent steps.

Key Features¶

ScatterFeatureRequirement

Enables scattering, allowing parallel execution of workflow steps for array-like inputs.

requirements:

  ScatterFeatureRequirement: {}

MultipleInputFeatureRequirement

Allows using multiple input fields in a workflow.

requirements:

  MultipleInputFeatureRequirement: {}

Scatter on common_band_name

Scatters the step_curl task based on the common_band_name parameter array.

in:

  stac_item: stac-item

  common_band_name: [red-band, green-band, blue-band]

out:

  - hrefs

run: "#stac"

scatter: common_band_name

scatterMethod: dotproduct

Steps¶

Define the Workflow

The workflow uses default input values for the band names (red, green, blue) and processes them using scattering.

Workflow Inputs

The workflow accepts:

stac-item: URL of a STAC item.
red-band: Band name for the red channel (default: "red").
green-band: Band name for the green channel (default: "green").
blue-band: Band name for the blue channel (default: "blue").

Workflow Outputs

The final output is the RGB composite TIFF file (rgb-tif).

Workflow Definition

cwlVersion: v1.2

$graph:

  - class: Workflow

    id: main

    requirements:

      InlineJavascriptRequirement: {}

      NetworkAccess:

        networkAccess: true

      ScatterFeatureRequirement: {}

      MultipleInputFeatureRequirement: {}

    inputs:

      stac-item:

        type: string

      red-band:

        type: string

        default: "red"

      green-band:

        type: string

        default: "green"

      blue-band:

        type: string

        default: "blue"

    outputs:

      rgb-tif:

        outputSource: step_color/rgb

        type: File

    steps:

      step_curl:

in:

          stac_item: stac-item

          common_band_name: [red-band, green-band, blue-band]

        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"

  - class: CommandLineTool

    id: stac

    requirements:

      DockerRequirement:

        dockerPull: docker.io/curlimages/curl:latest

    baseCommand: curl

    stdout: message

    arguments:

      - $( inputs.stac_item )

    inputs:

      stac_item:

        type: string

      common_band_name:

        type: string

    outputs:

      hrefs:

        type: string

        outputBinding:

          glob: message

          loadContents: true

          outputEval: |

${

              const assets = JSON.parse(self[0].contents).assets;

              const bandKey = Object.keys(assets).find(key =>

                assets[key]['eo:bands'] &&

                assets[key]['eo:bands'].length === 1 &&

                assets[key]['eo:bands'].some(band => band.common_name === inputs.common_band_name)

);

              if (!bandKey) {

                throw new Error(`No valid asset found for band: ${inputs.common_band_name}`);

              return assets[bandKey].href;

  - class: CommandLineTool

    id: rio_stack

    requirements:

      DockerRequirement:

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

      EnvVarRequirement:

        envDef:

          GDAL_TIFF_INTERNAL_MASK: YES

          GDAL_HTTP_MERGE_CONSECUTIVE_RANGES: YES

          CPL_VSIL_CURL_ALLOWED_EXTENSIONS: ".tif"

      InitialWorkDirRequirement:

        listing:

          - entryname: run.sh

            entry: |-

              #!/bin/bash

              rio stack $@

    baseCommand: ["/bin/bash", "run.sh"]

    arguments:

      - valueFrom: "${  \n  var arr = [];\n  for(var i=0; i<inputs.tiffs.length; i++) {\n      arr.push(inputs.tiffs[i]); \n  }\n  return arr; \n  }\n"

      - stacked.tif

    inputs:

      tiffs:

        type: string[]

    outputs:

      stacked:

        type: File

        outputBinding:

          glob: stacked.tif

  - class: CommandLineTool

    id: rio_color

    requirements:

      DockerRequirement:

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

      InitialWorkDirRequirement:

        listing:

          - entryname: run.sh

            entry: |-

              #!/bin/bash

              rio color -j -1 --out-dtype uint8 $1 rgb.tif "gamma 3 0.95, sigmoidal rgb 35 0.13"

    baseCommand: ["/bin/bash", "run.sh"]

    arguments:

      - $( inputs.stacked.path )

    inputs:

      stacked:

        type: File

    outputs:

      rgb:

        type: File

        outputBinding:

          glob: rgb.tif

Its graphical representation:

No description has been provided for this image

Scatter Configuration

Step 1: Scatter on common_band_name

The step_curl step scatters over the common_band_name parameter array ([red-band, green-band, blue-band]).

in:

  stac_item: stac-item

  common_band_name: [red-band, green-band, blue-band]

out:

  - hrefs

run: "#stac"

scatter: common_band_name

scatterMethod: dotproduct

scatter: Scatters the step over the values of common_band_name.
scatterMethod: dotproduct: Matches corresponding elements in the array for parallel execution.

Step 2: Combine Results

The step_stack step combines the TIFF files fetched by step_curl.

in:

  tiffs:

    source: step_curl/hrefs

out:

  - stacked

run: "#rio_stack"

Substeps and Tools

stac Tool

Fetches URLs of the TIFF files corresponding to each band.

rio_stack Tool

Stacks the TIFF files into a single composite file.

Run the Workflow

Execute the workflow with the default parameters:

cwltool scatter-input-parameters.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/scatter-input-parameters.cwl' to 'file:///home/runner/work/how-to/how-to/cwl-workflows/scatter-input-parameters.cwl'

INFO [workflow ] start

INFO [workflow ] starting step step_curl

INFO [step step_curl] start

INFO [job step_curl] /tmp/pjim30c_$ docker \

    run \

    -i \

    --mount=type=bind,source=/tmp/pjim30c_,target=/immwZn \

    --mount=type=bind,source=/tmp/w076lvqq,target=/tmp \

    --workdir=/immwZn \

    --read-only=true \

    --log-driver=none \

    --user=1001:118 \

    --rm \

    --cidfile=/tmp/x8kvg4tx/20250620072043-011732.cid \

    --env=TMPDIR=/tmp \

    --env=HOME=/immwZn \

    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/pjim30c_/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

100 10156  100 10156    0     0  28588      0 --:--:-- --:--:-- --:--:-- 28528

100 10156  100 10156    0     0  28582      0 --:--:-- --:--:-- --:--:-- 28528

INFO [job step_curl] completed success

INFO [step step_curl] start

INFO [job step_curl_2] /tmp/9u3r00yd$ docker \

    run \

    -i \

    --mount=type=bind,source=/tmp/9u3r00yd,target=/immwZn \

    --mount=type=bind,source=/tmp/pmd8at18,target=/tmp \

    --workdir=/immwZn \

    --read-only=true \

    --log-driver=none \

    --user=1001:118 \

    --rm \

    --cidfile=/tmp/bj_7ok79/20250620072044-019069.cid \

    --env=TMPDIR=/tmp \

    --env=HOME=/immwZn \

    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/9u3r00yd/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

100 10156  100 10156    0     0  46062      0 --:--:-- --:--:-- --:--:-- 46163

INFO [job step_curl_2] completed success

INFO [step step_curl] start

INFO [job step_curl_3] /tmp/slsb0sya$ docker \

    run \

    -i \

    --mount=type=bind,source=/tmp/slsb0sya,target=/immwZn \

    --mount=type=bind,source=/tmp/xf1wb7_f,target=/tmp \

    --workdir=/immwZn \

    --read-only=true \

    --log-driver=none \

    --user=1001:118 \

    --rm \

    --cidfile=/tmp/qv_0qywn/20250620072045-026504.cid \

    --env=TMPDIR=/tmp \

    --env=HOME=/immwZn \

    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/slsb0sya/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

100 10156  100 10156    0     0  58413      0 --:--:-- --:--:-- --:--:-- 58705

INFO [job step_curl_3] completed success

INFO [step step_curl] completed success

INFO [workflow ] starting step step_stack

INFO [step step_stack] start

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

    run \

    -i \

    --mount=type=bind,source=/tmp/eyszqcz_,target=/immwZn \

    --mount=type=bind,source=/tmp/5rrkbtid,target=/tmp \

    --workdir=/immwZn \

    --read-only=true \

    --user=1001:118 \

    --rm \

    --cidfile=/tmp/0xnzhw20/20250620072046-046081.cid \

    --env=TMPDIR=/tmp \

    --env=HOME=/immwZn \

    --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 \

    /bin/bash \

    run.sh \

    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: 1249MiB

INFO [job step_stack] completed success

INFO [step step_stack] completed success

INFO [workflow ] starting step step_color

INFO [step step_color] start

INFO [job step_color] /tmp/d8xs8_i9$ docker \

    run \

    -i \

    --mount=type=bind,source=/tmp/d8xs8_i9,target=/immwZn \

    --mount=type=bind,source=/tmp/tn80e284,target=/tmp \

    --mount=type=bind,source=/tmp/eyszqcz_/stacked.tif,target=/var/lib/cwl/stg8f936c6e-9afb-4620-b18a-09db34efa67d/stacked.tif,readonly \

    --workdir=/immwZn \

    --read-only=true \

    --user=1001:118 \

    --rm \

    --cidfile=/tmp/yq40jdiv/20250620072104-328838.cid \

    --env=TMPDIR=/tmp \

    --env=HOME=/immwZn \

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

    /bin/bash \

    run.sh \

    /var/lib/cwl/stg8f936c6e-9afb-4620-b18a-09db34efa67d/stacked.tif

INFO [job step_color] Max memory used: 746MiB

INFO [job step_color] completed success

INFO [step step_color] completed success

INFO [workflow ] completed success

INFO Final process status is success

Expected Output

Intermediate Outputs:

hrefs: URLs of the TIFF files for red, green, and blue bands.
stacked.tif: Composite TIFF file of all bands.

Final Output:

rgb-tif: RGB composite TIFF file.

  "rgb-tif": {

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

    "basename": "rgb.tif",

    "class": "File",

    "checksum": "sha1$87fea93a525287654a6e23e5b031fdb64e379094",

    "size": 361747464,

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

Key Takeaways¶

Scattering with Input Parameters:

The scatter field enables parallel execution over array inputs.
scatterMethod: dotproduct ensures corresponding elements in arrays are processed together.

Multiple Input Fields:

MultipleInputFeatureRequirement allows combining multiple input parameters in a single scatter operation.

Parallel and Modular Design:

Scattering simplifies workflows by enabling parallel processing of input parameters.

This guide demonstrates how to use scattering to process multiple input parameters in parallel within a CWL workflow.