在命令中尝试对佐贺模式做出反应之前应用状态

Question

在我看来，传奇模式在很多方面都是聚合模式的倒置。首先将命令处理成域事件，然后在聚合的情况下将此域事件应用到状态中是很有用的，但我认为在saga的情况下，域事件是已经发生的事情的文档，应该在试图对状态做出反应之前应用到状态中。

现在我的问题是：对于Commanded.processmanagers.processmanager模块，有没有一种方法可以将命令配置为首先apply，然后handle？或者这是一个严重的bug，需要在一般情况下进行修复？

Answer 1

按照设计，在处理/2之后调用apply/2回调，不可能将命令配置为具有不同的行为。

我同意您的推理，即在尝试处理事件以产生任何命令之前，将事件应用于流程管理器的状态更有意义。这似乎是一个值得对命令进行的更改，可以通过您已经提出的问题(#176)进行跟踪。

同时，您可以按照以下方式实现流程管理器（saga）：

defmodule InvoicingProcessManager do
  use Commanded.ProcessManagers.ProcessManager,
    name: __MODULE__,
    router: InvoicingRouter

  defstruct [
    :batch_uuid,
    pending_invoice_ids: MapSet.new()
  ]

  def interested?(%InvoiceBatchStarted{batch_uuid: batch_uuid}), do: {:start, batch_uuid}
  def interested?(%InvoiceCreated{batch_uuid: batch_uuid}), do: {:continue, batch_uuid}
  def interested?(%InvoiceFailed{batch_uuid: batch_uuid}), do: {:continue, batch_uuid}
  def interested?(%InvoiceBatchStopped{batch_uuid: batch_uuid}), do: {:stop, batch_uuid}
  def interested?(_event), do: false

  # Event handlers

  def handle(%InvoicingSaga{}, %InvoiceBatchStarted{} = started) do
    %InvoiceBatchStarted{batch_uuid: batch_uuid, invoice_ids: invoice_ids} = started

    Enum.map(invoice_ids, fn invoice_id ->
      %CreateInvoice{
        invoice_id: invoice_id,
        batch_uuid: batch_uuid
      }
    end)
  end

  def handle(%InvoicingSaga{}, %InvoiceCreated{invoice_id: invoice_id}),
    do: attempt_stop_batch(pm, invoice_id)

  def handle(%InvoicingSaga{}, %InvoiceFailed{invoice_id: invoice_id}),
    do: attempt_stop_batch(pm, invoice_id)

  ## State mutators

  def apply(%InvoicingSaga{} = pm, %InvoiceBatchStarted{} = started) do
    %InvoiceBatchStarted{batch_uuid: batch_uuid, invoice_ids: invoice_ids} = started

    %InvoicingSaga{
      transfer
      | batch_uuid: batch_uuid,
        pending_invoice_ids: MapSet.new(invoice_ids)
    }
  end

  def apply(%InvoicingSaga{} = pm, %InvoiceCreated{invoice_id: invoice_id}) do
    %InvoicingSaga{pm | pending_invoice_ids: invoice_completed(pm, invoice_id)}
  end

  def apply(%InvoicingSaga{} = pm, %InvoiceFailed{invoice_id: invoice_id}) do
    %InvoicingSaga{pm | pending_invoice_ids: invoice_completed(pm, invoice_id)}
  end

  ## Private helpers

  def attempt_stop_batch(%InvoicingSaga{batch_uuid: batch_uuid} = pm, invoice_id) do
    pending_invoices = invoice_completed(pm, invoice_id)

    case empty?(pending_invoices) do
      true -> %StopInvoiceBatch{batch_uuid: batch_uuid}
      false -> []
    end
  end

  defp invoice_completed(%InvoicingSaga{pending_invoice_ids: pending_invoice_ids}, invoice_id) do
    MapSet.delete(pending_invoice_ids, invoice_id)
  end

  defp empty?(map_set, empty \\ MapSet.new())
  defp empty?(%MapSet{} = empty, %MapSet{} = empty), do: true
  defp empty?(%MapSet{}, %MapSet{}), do: false
end