AWS Lambda Release a Version Behind an Alias

arazzo: 1.0.1
info:
  title: AWS Lambda Release a Version Behind an Alias
  summary: Publish a version, then either move an existing alias onto it or create the alias if this is the first release.
  description: >-
    How production traffic actually moves on Lambda. Clients invoke a stable
    alias (e.g. "prod") rather than a version number, so a release is a change to
    what that alias points at. This workflow publishes an immutable version,
    probes for the alias, and branches: when the alias already exists it is moved
    onto the new version under a RevisionId guard so a concurrent release cannot
    be clobbered, and when the alias does not exist it is created pointing at the
    new version. 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: lambdaApi
  url: ../openapi/aws-lambda-api-openapi.yml
  type: openapi
workflows:
- workflowId: alias-release
  summary: Publish a version and point a named alias at it, creating the alias when absent.
  description: >-
    Cuts an immutable version from the current code, resolves the target alias,
    and either updates it onto the new version or creates it.
  inputs:
    type: object
    required:
    - functionName
    - aliasName
    properties:
      functionName:
        type: string
        description: The name or ARN of the function being released.
      aliasName:
        type: string
        description: The alias clients invoke (e.g. prod).
      versionDescription:
        type: string
        description: Description recorded against the published version (e.g. a git SHA or release tag).
  steps:
  - stepId: publishTheVersion
    description: >-
      Cut an immutable version from the function's current code and
      configuration. Lambda declines to publish a new version if neither the
      code nor the configuration has changed since the last one.
    operationId: publishVersion
    parameters:
    - name: FunctionName
      in: path
      value: $inputs.functionName
    requestBody:
      contentType: application/json
      payload:
        Description: $inputs.versionDescription
    successCriteria:
    - condition: $statusCode == 201
    outputs:
      newVersion: $response.body#/Version
      functionArn: $response.body#/FunctionArn
      codeSha256: $response.body#/CodeSha256
  - stepId: resolveAlias
    description: >-
      Probe for the alias to decide whether this is a move or a first release. A
      200 means an incumbent version exists and its RevisionId can be used as a
      concurrency guard; a 404 means the alias has never existed.
    operationId: getAlias
    parameters:
    - name: FunctionName
      in: path
      value: $inputs.functionName
    - name: AliasName
      in: path
      value: $inputs.aliasName
    successCriteria:
    - condition: $statusCode == 200
    outputs:
      incumbentVersion: $response.body#/FunctionVersion
      aliasRevisionId: $response.body#/RevisionId
    onSuccess:
    - name: aliasExists
      type: goto
      stepId: moveAliasToNewVersion
      criteria:
      - condition: $statusCode == 200
    onFailure:
    - name: aliasMissing
      type: goto
      stepId: createAliasOutright
      criteria:
      - condition: $statusCode == 404
  - stepId: moveAliasToNewVersion
    description: >-
      Point the existing alias at the newly published version. RevisionId is
      passed as a guard so Lambda rejects the update if another release moved the
      alias after it was read.
    operationId: updateAlias
    parameters:
    - name: FunctionName
      in: path
      value: $inputs.functionName
    - name: AliasName
      in: path
      value: $inputs.aliasName
    requestBody:
      contentType: application/json
      payload:
        FunctionVersion: $steps.publishTheVersion.outputs.newVersion
        Description: $inputs.versionDescription
        RevisionId: $steps.resolveAlias.outputs.aliasRevisionId
    successCriteria:
    - condition: $statusCode == 200
    outputs:
      aliasArn: $response.body#/AliasArn
      pointsAtVersion: $response.body#/FunctionVersion
      revisionId: $response.body#/RevisionId
    onSuccess:
    - name: released
      type: end
  - stepId: createAliasOutright
    description: >-
      Create the alias pointing at the new version. This is the first release for
      this alias, so there is no incumbent version and no RevisionId to guard
      against.
    operationId: createAlias
    parameters:
    - name: FunctionName
      in: path
      value: $inputs.functionName
    requestBody:
      contentType: application/json
      payload:
        Name: $inputs.aliasName
        FunctionVersion: $steps.publishTheVersion.outputs.newVersion
        Description: $inputs.versionDescription
    successCriteria:
    - condition: $statusCode == 201
    outputs:
      aliasArn: $response.body#/AliasArn
      pointsAtVersion: $response.body#/FunctionVersion
      revisionId: $response.body#/RevisionId
  outputs:
    newVersion: $steps.publishTheVersion.outputs.newVersion
    previousVersion: $steps.resolveAlias.outputs.incumbentVersion
    movedAliasArn: $steps.moveAliasToNewVersion.outputs.aliasArn
    createdAliasArn: $steps.createAliasOutright.outputs.aliasArn