Elixir，Phoenix和Ecto :当数据库关闭时，缓存数据的请求变得非常慢(从100ms到3秒)

Question

我用--no-html和--database mysql创建了一个phoenix项目

我们在工作中使用的版本是：

Erlang/OTP 21 [erts-10.2] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [hipe]

Elixir 1.8.0 (compiled with Erlang/OTP 20)

我正在做一个项目，作为状态页工具(hund.io)和我们自己的API和报告工具之间的代理，所以我需要一个数据库来存储服务名称+请求的url，并且，当我需要一点与数据库Cachex (https://github.com/whitfin/cachex)分离的时候，将我的数据放到缓存中

当数据被缓存时，使用邮递员请求我的包装器的api需要大约100ms才能获得响应，菲尼克斯的日志显示类似[info] Sent 200 in 1ms之类的内容，但是，我曾经关闭过数据库，邮递员需要整整3秒才能获得响应，菲尼克斯日志仍然会显示[info] Sent 200 in 1ms之类的内容

我确信包装器在这两种情况下都使用缓存数据：

我的代码：

def show(conn, %{"service_name" => service_name}) do
    Logger.debug("Top of show func")

    case Cachex.get(:proxies, service_name) do
      {:ok, nil} ->
        Logger.debug("Service #{service_name} not found in cache")
        proxy = Health.get_proxy_by_name!(service_name)
        Logger.debug("Service #{service_name} found in db")
        Cachex.put(:proxies, service_name, proxy)
        proxy_with_health = Checker.call_api(proxy)

        render(conn, "show.json", proxy_health: proxy_with_health)

      {:ok, proxy} ->
        Logger.debug("Found service #{service_name} in cache")
        proxy_with_health = Checker.call_api(proxy)
        render(conn, "show.json", proxy_health: proxy_with_health)
    end
  end

日志：

[info] GET /api/proxies_health/docto
[debug] Processing with StatusWeb.ProxyHealthController.show/2
  Parameters: %{"service_name" => "docto"}
  Pipelines: [:api]
[debug] Top of show func
[debug] Found service docto in cache

对于路由器部分：

 pipeline :api do
    plug :accepts, ["json"]
  end

  scope "/api", StatusWeb do
    pipe_through :api

    resources "/proxies", ProxyController, except: [:new, :edit]
    get "/proxies_health", ProxyHealthController, :index
    get "/proxies_health/:service_name", ProxyHealthController, :show
  end

我还在日志中看到两个错误：

[error] MyXQL.Connection (#PID<0.373.0>) failed to connect: ** (DBConnection.ConnectionError) connection refused

这看起来“正常”，当应用程序想要重新连接到数据库和这个数据库时，就在处理请求之前(至少根据日志)。

[error] Could not create schema migrations table. This error usually happens due to the following:

  * The database does not exist
  * The "schema_migrations" table, which Ecto uses for managing
    migrations, was defined by another library
  * There is a deadlock while migrating (such as using concurrent
    indexes with a migration_lock)

To fix the first issue, run "mix ecto.create".

To address the second, you can run "mix ecto.drop" followed by
"mix ecto.create". Alternatively you may configure Ecto to use
another table and/or repository for managing migrations:

    config :status, Status.Repo,
      migration_source: "some_other_table_for_schema_migrations",
      migration_repo: AnotherRepoForSchemaMigrations

The full error report is shown below.

[error] GenServer #PID<0.620.0> terminating
** (DBConnection.ConnectionError) connection refused
    (db_connection) lib/db_connection/connection.ex:100: DBConnection.Connection.connect/2
    (connection) lib/connection.ex:622: Connection.enter_connect/5
    (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3
Last message: nil
State: MyXQL.Connection

在查看How do I detect database connection issues from Elixir Ecto?时，我尝试放入我的Status.Repo文件(repo.ex

backoff_type: :stop

但这并没有改变我的问题

Answer 1

由于我的代码中，在缓存中找到一个对象(根本)不涉及数据库，我们最终发现问题来自端点的一个插件，该插件应该只存在于开发环境(MyAppWeb/endpoint.ex)中。

   if code_reloading? do
      plug Phoenix.CodeReloader
      plug Phoenix.Ecto.CheckRepoStatus, otp_app: :status
  end

这是一个插件，它会在每次请求时检查数据库是否正常，迁移是否正在运行等，等待Ecto的答复，在genserver中的超时意识到调用导致了延迟！

修复方法是简单地注释这行

          plug Phoenix.Ecto.CheckRepoStatus, otp_app: :status