一個 controller 的 action 需要去戳外部 api 三次的邏輯需求，使用 fiber 整理邏輯

multiple_api_request_in_a_action_with_fiber_example.rb

require 'fiber'

Result = Struct.new :success, :payload, :status do
  def self.create(args = {})
    args = { success: true, payload: {}, status: :ok }.merge(args.symbolize_keys)
    new *args.values_at(:success, :payload, :status)
  end

  def success?
    !!success
  end

  def error
    { message: 'Bad thing happends', errors: {} }.merge(payload)
  end
end

class Webhook::CustomersController < ApplicationController
  # 使用 fiber 組織邏輯，把每個步驟都劃分成一次 yield ，並且檢查結果成不成功，成功的話再把傳回給自己當參數繼續下去，失敗的話就 render
  # 好處是 create 的邏輯變得很清晰、一目了然
  # 但三個步驟拆出來的方法所需的參數可能有相依性（可能會有共用的參數），所以目前採用滾雪球的方式，每個步驟把自己新產生的結果都 merge 回參數，然後拋給下一個方法，有點詭異的作法
  # 實作上也多了許多 code ，所成本跟好處比起來真的不太明顯...
  def create
    token = get_access_token(params[:merchant_id])
    fiber = Fiber.new do
      payload = Fiber.yield fetch_customer(token, params)
      payload = Fiber.yield fetch_studioa_vip(payload)
      create_or_update_customer(payload)
    end
    
    # Ruby 的 fiber 不支援 for 或是其他迭帶性質的方法使用，只能自己循環下去，有一些麻煩
    while(fiber.alive?)
      result = fiber.resume(result ||= nil)
      (render json: result.payload, status: result.status and return) unless result.success?
    end
    render json: result.payload
  end

  protected

  def query_studioa_vip(mobile_phone)
    uri = URI(Settings.api_endpoint.query_vip)
    json_parameters = {
      "SYSPARAM": {
        "DB_ID": "EPB",
        "CHAR_SET": "eng",
        "INCLUDE_NULL": "N"
      },
      "PARAMETER": {
        "VIP_PHONE": mobile_phone
      }
    }
    req = Net::HTTP::Post.new(uri, 'Content-Type' => 'application/json')
    req.body = json_parameters.to_json
    Net::HTTP.start(uri.hostname, uri.port) do |http|
      http.request(req)
    end
  end

  def fetch_customer(token, params)
    client = OpenApiClient.set(access_token: token).customers
    customer = client.get("/#{params[:resource][:_id]}").json_body
    mobile_phone, confirmed_at = customer.symbolize_keys.values_at(:mobile_phone, :confirmed_at)
    if [mobile_phone, confirmed_at].all?(&:present?)
      Result.create payload: { customer: customer, mobile_phone: mobile_phone, token: token }
    else
      Result.create payload: { message: "customer: #{customer['id']} is not signed up." }, status: :no_content, success: false
    end
  end

  def fetch_studioa_vip(args = {})
    args = args.symbolize_keys
    res = query_studioa_vip(args[:mobile_phone])
    if res['RESULT']['STATUS'].to_s != 'S'
      Result.create(payload: args.merge(raw_customer: res.json_body['DATA']))
    else
      Result.create(payload: { errors: res.json_body }, status: :bad_request, success: false)
    end
  end

  def create_or_update_customer(args = {})
    args = args.symbolize_keys
    service_result = (args[:raw_customer].present? ? UpdateCustomerService : CreateCustomerService).new(args).execute
    if service_result.success?
      Result.create payload: service_result.payload
    else
      Result.create payload: service_result.error, status: :bad_request
    end
  end
end