Amazon S3 Conditional Download of an Object

arazzo: 1.0.1
info:
  title: Amazon S3 Conditional Download of an Object
  summary: HEAD an object to read its ETag, then GET it only when it is present.
  description: >-
    A safe read flow for Amazon S3. The workflow issues a HEAD request to learn
    an object's ETag and size without transferring its bytes, and branches: when
    the object exists it proceeds to GET the full body, and when the object is
    missing it ends without downloading anything. Every step spells out its
    request inline so the flow can be read and executed without opening the
    underlying OpenAPI description.
  version: 1.0.0
sourceDescriptions:
- name: s3RestApi
  url: ../openapi/amazon-s3-rest-api-openapi.yml
  type: openapi
workflows:
- workflowId: conditional-download-object
  summary: HEAD then GET an object, downloading only when it exists.
  description: >-
    Probes an object with HeadObject and downloads it with GetObject only when
    the HEAD reports it is present.
  inputs:
    type: object
    required:
    - bucket
    - objectKey
    properties:
      bucket:
        type: string
        description: The bucket holding the object.
      objectKey:
        type: string
        description: The key of the object to download.
  steps:
  - stepId: headObject
    description: >-
      HEAD the object to learn its ETag and size before deciding to download.
    operationId: HeadObject
    parameters:
    - name: Bucket
      in: path
      value: $inputs.bucket
    - name: Key+
      in: path
      value: $inputs.objectKey
    successCriteria:
    - condition: $statusCode == 200
    outputs:
      etag: $response.header.ETag
    onSuccess:
    - name: present
      type: goto
      stepId: getObject
      criteria:
      - condition: $statusCode == 200
    onFailure:
    - name: missing
      type: end
      criteria:
      - condition: $statusCode == 404
  - stepId: getObject
    description: >-
      Download the full object body now that the HEAD confirmed it exists.
    operationId: GetObject
    parameters:
    - name: Bucket
      in: path
      value: $inputs.bucket
    - name: Key+
      in: path
      value: $inputs.objectKey
    successCriteria:
    - condition: $statusCode == 200
    outputs:
      etag: $response.header.ETag
      contentLength: $response.header.Content-Length
  outputs:
    headEtag: $steps.headObject.outputs.etag
    downloadedEtag: $steps.getObject.outputs.etag