Recipe

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Intro

The Recipe module allows implementing multi-step, reversible workflows.

For example, you may wanna parse some incoming data, write to two different data stores and then push some notifications. If anything fails, you wanna rollback specific changes in different data stores. Recipe allows you to do that.

In addition, a recipe doesn’t enforce any constraint around which processes execute which step. You can assume that unless you explicitly involve other processes, all code that builds a recipe is executed by default by the calling process.

Ideal use cases are:

  • multi-step operations where you need basic transactional properties, e.g. saving data to Postgresql and Redis, rolling back the change in Postgresql if the Redis write fails
  • interaction with services that simply don’t support transactions
  • composing multiple workflows that can share steps (with the help of Kernel.defdelegate/2)
  • trace workflows execution via a correlation id

You can avoid using this library if:

  • A simple with macro will do
  • You don’t care about failure semantics and just want your operation to crash the calling process
  • Using Ecto, you can express your workflow with Ecto.Multi

Heavily inspired by the ktn_recipe module included in inaka/erlang-katana.

Core ideas

  • A workflow is a state machine
  • Each step can have a specific error handling scenario
  • Each step is a separate function that receives a state with the result of all previous steps
  • Each step should be easily testable in isolation

Installation

If available in Hex, the package can be installed by adding recipe to your list of dependencies in mix.exs:

def deps do
  [{:recipe, "~> 0.2.0"}]
end

Example

The example below outlines a possible workflow where a user creates a new conversation, passing an initial message.

Each step is named in steps/0. Each step definition uses data added to the workflow state and performs a specific task.

Any error shortcuts the workflow to handle_error/3, where a specialized clause for :create_initial_message deletes the conversation if the system failes to create the initial message (therefore simulating a transaction).

defmodule StartNewConversation do
  use Recipe

  ### Public API

  def run(user_id, initial_message_text) do
    state = Recipe.empty_state
            |> Recipe.assign(:user_id, user_id)
            |> Recipe.assign(:initial_message_text, initial_message_text)

    Recipe.run(__MODULE__, state)
  end

  ### Callbacks

  def steps, do: [:validate,
                  :create_conversation,
                  :create_initial_message,
                  :broadcast_new_conversation,
                  :broadcast_new_message]

  def handle_result(state) do
    state.assigns.conversation
  end

  def handle_error(:create_initial_message, _error, state) do
    Service.Conversation.delete(state.conversation.id)
  end
  def handle_error(_step, error, _state), do: error

  ### Steps

  def validate(state) do
    text = state.assigns.initial_message_text
    if MessageValidator.valid_text?(text) do
      {:ok, state}
    else
      {:error, :empty_message_text}
    end
  end

  def create_conversation(state) do
    case Service.Conversation.create(state.assigns.user_id) do
      {:ok, conversation} ->
        {:ok, Recipe.assign(state, :conversation, conversation)}
      error ->
        error
    end
  end

  def create_initial_message(state) do
    %{user_id: user_id,
      conversation: conversation,
      initial_message_text: text} = state.assigns
    case Service.Message.create(user_id, conversation.id, text) do
      {:ok, message} ->
        {:ok, Recipe.assign(state, :initial_message, message)}
      error ->
        error
    end
  end

  def broadcast_new_conversation(state) do
    Dispatcher.broadcast("conversation-created", state.assigns.conversation)
    {:ok, state}
  end

  def broadcast_new_message(state) do
    Dispatcher.broadcast("message-created", state.assigns.initial_message)
    {:ok, state}
  end
end

Development/Test

  • Initial setup can be done with mix deps.get
  • Run tests with mix test
  • Run dialyzer with mix dialyzer
  • Run credo with mix credo
  • Build docs with MIX_ENV=docs mix docs