AWS Lambda Cap a Function with Reserved Concurrency

arazzo: 1.0.1
info:
  title: AWS Lambda Cap a Function with Reserved Concurrency
  summary: Read a function's current concurrency reservation, set a ceiling on its simultaneous executions, and confirm the value Lambda stored.
  description: >-
    Reserved concurrency does two jobs at once, which is why it is worth setting
    deliberately. It guarantees a function can always reach the reserved level,
    and it caps that function so it can never consume more than its share of the
    account's regional concurrency pool and starve every other function in the
    account. It is the standard guardrail for a function that fans out onto a
    limited downstream resource such as a relational database. This workflow
    reads the existing reservation, applies the new ceiling, and reads it back.
    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: reserved-concurrency-guardrail
  summary: Set and verify the maximum simultaneous executions reserved for a function.
  description: >-
    Captures the prior reservation so the change is auditable and reversible,
    applies the new limit, and verifies the stored value.
  inputs:
    type: object
    required:
    - functionName
    - reservedConcurrentExecutions
    properties:
      functionName:
        type: string
        description: The name or ARN of the function to cap.
      reservedConcurrentExecutions:
        type: integer
        description: >-
          Maximum simultaneous executions for this function. Setting 0 stops the
          function from being invoked at all, which is the standard way to
          quarantine a misbehaving function without deleting it.
  steps:
  - stepId: readCurrentReservation
    description: >-
      Read the existing reservation before changing it, so the prior value is
      captured for audit and rollback. A function with no reservation returns no
      ReservedConcurrentExecutions value and draws from the shared account pool.
    operationId: getReservedConcurrency
    parameters:
    - name: FunctionName
      in: path
      value: $inputs.functionName
    successCriteria:
    - condition: $statusCode == 200
    outputs:
      previousReservation: $response.body#/ReservedConcurrentExecutions
  - stepId: applyReservation
    description: >-
      Set the ceiling. This carves the reserved amount out of the account's
      regional concurrency pool, so the capacity is no longer available to any
      other function in the account.
    operationId: putFunctionConcurrency
    parameters:
    - name: FunctionName
      in: path
      value: $inputs.functionName
    requestBody:
      contentType: application/json
      payload:
        ReservedConcurrentExecutions: $inputs.reservedConcurrentExecutions
    successCriteria:
    - condition: $statusCode == 200
    outputs:
      appliedReservation: $response.body#/ReservedConcurrentExecutions
  - stepId: verifyReservation
    description: >-
      Read the reservation back to confirm the ceiling Lambda actually stored
      matches what was requested.
    operationId: getReservedConcurrency
    parameters:
    - name: FunctionName
      in: path
      value: $inputs.functionName
    successCriteria:
    - condition: $statusCode == 200
    outputs:
      verifiedReservation: $response.body#/ReservedConcurrentExecutions
  outputs:
    previousReservation: $steps.readCurrentReservation.outputs.previousReservation
    reservedConcurrentExecutions: $steps.verifyReservation.outputs.verifiedReservation