transaction.ex

defmodule Raven.Util.Transaction do
  use GenServer, restart: :temporary

  @moduledoc """
  Claim a given transaction handle

  Blocks if other callers have already claimed the handle. This will return with
  `:ok` if the current process wins the claim, otherwise it will return `:already_taken`

  ### Example Usage

  This is would be more ergonomic by doing it Repo.transaction still with an wrapping
  function, but the split up explicit form we use here makes it easier to do
  commanded stuff.

  ```
  email = "[email protected]"

  lookup_fun = fn ->
    User.find_by_email(email)
  end

  with :ok <- Transaction.claim(email, lookup_fun),
  {:ok, user} <- create_user(email: email) do
    :ok = Transaction.release(email)
    {:ok, user}
  else
    error ->
    :ok = Transaction.release(email)
    error
  end
  ```

  The idea here is that we ask for a claim with a specific handle, usually the handle
  is the value we want to have unique access to. The lookup fun is run from within
  the GenServer, and if it returns a truthy value then the value is already present,
  and you get back `{:error, :already_taken}`. Running this from within the GenServer
  avoids the usual race condition around this check. If the value is does not exist
  then you successfully claim the handle and get back `:ok`.

  With the claim in hand we go ahead doing whatever we want with the value. It is critical
  that whatever action we take be strongly consistent with respect to the check done by
  the lookup fun. After we have done what we want we release the transaction handle. This is where the lookup
  function comes back in. The GenServer runs the lookup function again and if it returns a truthy
  value then the transaction is considered to have been successful and any other
  processes waiting on a claim will receive `{:error, :already_taken}`.

  If the lookup function returns a falsey value then the next process gets a shot.
  """

  def claim(handle, lookup_fun, timeout \\ 15_000) do
    case DynamicSupervisor.start_child(
           Raven.Util.Transactions,
           {__MODULE__, handle}
         ) do
      {:error, {:already_started, pid}} ->
        GenServer.call(pid, {:claim, lookup_fun}, timeout)

      {:ok, pid} ->
        GenServer.call(pid, {:claim, lookup_fun}, timeout)
    end
  end

  def release(handle, timeout \\ 5_000) do
    GenServer.call({:global, handle}, :release, timeout)
  end

  defstruct [
    :current,
    :lookup_fun,
    claimants: []
  ]

  def start_link(handle) do
    GenServer.start_link(__MODULE__, [], name: {:global, handle})
  end

  def init(_) do
    state = %__MODULE__{}
    # if nobody actually makes a claim, then exit in 5
    Process.send_after(self(), :timeout, 5_000)
    {:ok, state}
  end

  def handle_call({:claim, _fun}, {caller, _}, %{current: caller} = state) do
    {:reply, :ok, state}
  end

  def handle_call({:claim, fun}, {caller, _}, %{current: nil} = state) do
    if fun.() do
      {:reply, {:error, :already_taken}, state}
    else
      Process.monitor(caller)
      state = %{state | current: caller, lookup_fun: fun}
      {:reply, :ok, state}
    end
  end

  def handle_call({:claim, _fun}, {caller, _} = claim, state) do
    Process.monitor(caller)
    # ++ is not the most efficient but the number of callers will generally be low.
    # consider fixing later.
    state = %{state | claimants: state.claimants ++ [claim]}
    {:noreply, state}
  end

  def handle_call(:release, {caller, _}, %{current: caller} = state) do
    case next(state) do
      {:done, state} -> {:reply, :ok, state, 5_000}
      {:next, state} -> {:reply, :ok, state}
    end
  end

  def handle_call(:release, _, state) do
    {:reply, :ok, state}
  end

  def handle_info({:DOWN, _, :process, pid, _}, %{current: pid} = state) do
    case next(state) do
      {:done, state} -> {:noreply, state, 5_000}
      {:next, state} -> {:noreply, state}
    end
  end

  def handle_info(:timeout, %{current: nil} = state) do
    {:stop, :normal, state}
  end

  def handle_info(:timeout, state) do
    {:noreply, state}
  end

  def handle_info({:DOWN, _, :process, dead_pid, _}, state) do
    claimants = for {pid, _} = claim <- state.claimants, pid != dead_pid, do: claim
    {:noreply, %{state | claimants: claimants}}
  end

  defp next(state) do
    if state.lookup_fun.() do
      for claim <- state.claimants do
        GenServer.reply(claim, {:error, :already_taken})
      end

      state = %{state | current: nil, claimants: []}
      {:done, state}
    else
      case next_claimant(%{state | current: nil}) do
        {:none, state} ->
          {:done, state}

        {:ok, next, state} ->
          GenServer.reply(next, :ok)
          {:next, state}
      end
    end
  end

  defp next_claimant(%{claimants: []} = state) do
    {:none, state}
  end

  defp next_claimant(%{claimants: [{pid, _} = next | rest]} = state) do
    {:ok, next, %{state | current: pid, claimants: rest}}
  end
end

uniqueness.ex

defmodule Raven.Ordering.Uniqueness do
  @behaviour Commanded.Middleware

  alias Commanded.Middleware.Pipeline
  alias Raven.Ordering.Commands.{CreateStaff}
  alias Raven.Util.Transaction
  alias Raven.Ordering

  def before_dispatch(%Pipeline{command: %CreateStaff{email: email}} = pipeline) do
    transaction = Transaction.claim(email, fn ->
      Ordering.get_by_email(Ordering.Staff, email)
    end)

    case transaction do
      :ok ->
        Pipeline.assign(pipeline, :transaction_handle, email)
      {:error, :already_taken} ->
        pipeline
        |> Pipeline.respond({:error, email: :already_taken})
        |> Pipeline.halt
    end
  end

  def before_dispatch(pipeline) do
    pipeline
  end

  def after_dispatch(pipeline) do
    if handle = pipeline.assigns[:transaction_handle] do
      Transaction.release(handle)
    end

    pipeline
  end

  def after_failure(pipeline) do
    if handle = pipeline.assigns[:transaction_handle] do
      Transaction.release(handle)
    end

    pipeline
  end
end

Note the expectation for a dynamic supervisor on line 53, could be adjusted according to use case.