如何处理 poolboy 中的超时？

Question

我有一个耗时的迁移问题，我希望并行运行（它可以并行运行）。实际上，迁移就是获取数据库中的所有记录，并对每条记录执行耗时和耗费资源的操作。

有时个人记录迁移会挂起，所以我给了 10 分钟的时间来完成。如果迁移未完成，我希望它正常关闭，没有任何异常（见下文）

我还使用poolboy erlang 包来并行化实现，因为迁移不仅消耗时间，还消耗资源。问题是当超时发生并且代码将要中断时，我不知道如何处理错误。我的监督树是：

defmodule MyReelty.Repo.Migrations.MoveVideosFromVimeoToB2 do
  use Ecto.Migration

  alias MyReelty.Repo
  alias MyReelty.Repo.Migrations.MoveVideosFromVimeoToB2.Migrator

  # parallel nature of migration force us to disable transaction
  @disable_ddl_transaction true

  @migrator_waiting_time 10 * 60 * 1000 # timeout
  @poolboy_waiting_time @migrator_waiting_time + 10 * 1000 # give a time for graceful shutdown

  @pool_name :migrator
  @pool_size 3
  @pool_config [
    { :name, { :local, @pool_name }},
    { :worker_module, Migrator },
    { :size, @pool_size },
    { :max_overflow, 0 },
    { :strategy, :fifo }
  ]

  def up do
    children = [
      :poolboy.child_spec(@pool_name, @pool_config)
    ]
    opts = [strategy: :one_for_one, name: MyReelty.Supervisor]
    Supervisor.start_link(children, opts)

    rows = Review |> Repo.all

    IO.puts "Total amount of reviews is: #{length(rows)}"

    parallel_migrations(rows)
  end

  def parallel_migrations(rows) do
    Enum.map(rows, fn(row) ->
      pooled_migration(@pool_name, row)
    end)
  end

  def pooled_migration(pool, x) do
    :poolboy.transaction(
      pool,
      (fn(pid) -> Migrator.move(pid, { x, @migrator_waiting_time }) end),
      @poolboy_waiting_time
    )
  end

  defmodule Migrator do
    alias MyReelty.Repo
    alias MyReelty.Review

    use GenServer

    def start_link(_) do
      GenServer.start_link(__MODULE__, nil, [])
    end

    def move(server, { params, waiting_time }) do
      GenServer.call(server, { :move, params }, waiting_time)
    end

    def handle_call({ :move, result }, _from, state) do
      big_time_and_resource_consuming_task_here    
      {:reply, %{}, state}
    end
  end
end

如果迁移数据库中的某些记录需要超过 10 分钟的问题，我有这种异常：

20:18:16.917 [error] Task #PID<0.282.0> started from #PID<0.70.0> terminating
** (stop) exited in: GenServer.call(#PID<0.278.0>, {:move, [2, "/videos/164064419", "w 35th st Springfield United States Illinois 60020"]}, 60000)
    ** (EXIT) time out
    (elixir) lib/gen_server.ex:604: GenServer.call/3
    (poolboy) src/poolboy.erl:76: :poolboy.transaction/3
    (elixir) lib/task/supervised.ex:94: Task.Supervised.do_apply/2
    (elixir) lib/task/supervised.ex:45: Task.Supervised.reply/5
    (stdlib) proc_lib.erl:247: :proc_lib.init_p_do_apply/3
Function: #Function<5.53617785/0 in MyReelty.Repo.Migrations.MoveVideosFromVimeoToB2.parallel_migrations/1>
    Args: []

20:18:16.918 [error] GenServer MyReelty.Repo terminating
** (stop) exited in: GenServer.call(#PID<0.278.0>, {:move, [2, "/videos/164064419", "w 35th st Springfield United States Illinois 60020"]}, 60000)
    ** (EXIT) time out
Last message: {:EXIT, #PID<0.70.0>, {:timeout, {GenServer, :call, [#PID<0.278.0>, {:move, [2, "/videos/164064419", "w 35th st Springfield United States Illinois 60020"]}, 60000]}}}
State: {:state, {:local, MyReelty.Repo}, :one_for_one, [{:child, #PID<0.231.0>, DBConnection.Poolboy, {:poolboy, :start_link, [[name: {:local, MyReelty.Repo.Pool}, strategy: :fifo, size: 1, max_overflow: 0, worker_module: DBConnection.Poolboy.Worker], {Postgrex.Protocol, [types: true, username: "adik", types: true, name: MyReelty.Repo.Pool, otp_app: :my_reelty, repo: MyReelty.Repo, adapter: Ecto.Adapters.Postgres, database: "my_reelty_dev", hostname: "localhost", extensions: [{Geo.PostGIS.Extension, [library: Geo]}, {Ecto.Adapters.Postgres.DateTime, []}, {Postgrex.Extensions.JSON, [library: Poison]}], pool_size: 1, pool_timeout: 5000, timeout: 15000, adapter: Ecto.Adapters.Postgres, database: "my_dev", hostname: "localhost", pool_size: 10, pool: DBConnection.Poolboy, port: 5432]}]}, :permanent, 5000, :worker, [:poolboy]}], :undefined, 3, 5, [], 0, Ecto.Repo.Supervisor, {MyReelty.Repo, :my_reelty, Ecto.Adapters.Postgres, [otp_app: :my_reelty, repo: MyReelty.Repo, adapter: Ecto.Adapters.Postgres, database: "my_reelty_dev", hostname: "localhost", extensions: [{Geo.PostGIS.Extension, [library: Geo]}], pool_size: 1]}}

我尝试将terminate/2 或handle_info/2 插入Migrator 并使用它，但我什至还没有达到要调用的函数。如何处理超时并防止它们中断我的迁移？

更新

我使用了@johlo 的提示，但我仍然有时间。我的功能是：

def init(_) do
 Process.flag(:trap_exit, true)
 {:ok, %{}}
end

Answer 1

当Migrator.move/2（即GenServer.call）函数超时时，它将使整个MoveVideosFromVimeoToB2进程崩溃，因为这是进行GenServer调用的实际进程。

这里的解决方案是在pooled_migration 的匿名函数中捕获超时，类似于（我对 Elixir 语法不是很熟悉，所以它可能无法编译，但你应该明白）：

def pooled_migration(pool, x) do
:poolboy.transaction(
  pool,
  (fn(pid) ->
      try do 
          Migrator.move(pid, { x, @migrator_waiting_time })
      catch
          :exit, reason ->
             # Ignore error, log it or something else
             :ok
      end
   end),
  @poolboy_waiting_time
)
end

不是Migrator 进程超时，而是GenServer 调用Migrator 超时，我们需要try-catch。

还要注意Migrator 进程并未被杀死，它仍在运行，请参阅GenServer call documentation 中的timeouts 部分。

更新： 正如@asiniy 在 cmets 中提到的那样，@poolboy_waiting_time 应设置为 :infinity，因此在等待空闲的 Migrator 工作进程时，poolboy.transaction 函数不会引发超时错误。因为Migrator 最终会退出，所以这是安全的。