为 Amazon Aurora 的 DNS 数据设置 TTL

【问题标题】：Set TTL for DNS data for Amazon Aurora为 Amazon Aurora 的 DNS 数据设置 TTL
【发布时间】：2019-01-15 10:58:28
【问题描述】：

我们使用的是 Amazon Aurora (PostgreSQL)，在尝试连接到数据库时遇到了一些间歇性错误：

System.Net.Sockets.SocketException (0x80004005): This is usually a temporary error during hostname resolution and means that the local server did not receive a response from an authoritative server.

还有：

System.Exception: An error occurred while updating the entries. See the inner exception for details. ---> System.Exception: An error occured while retrieving the InformationSchemaTable information. See the inner exception for details. ---> System.Net.Sockets.SocketException: This is usually a temporary error during hostname resolution and means that the local server did not receive a response from an authoritative server

我们正在使用npgsql 3.2.5。我们从运行 Windows Server 2016 的专用服务器和托管在 Azure 应用服务上的代码间歇性地收到此类错误。数百个连接成功，但有些（每小时几个）失败并出现类似的错误消息。当它们发生时，数据库没有显着的负载，而不是异常的连接数量（远非限制）。

我在Amazon Aurora Best Practices找到这个：

如果您的客户端应用程序正在缓存数据库实例的域名服务 (DNS) 数据，请将生存时间 (TTL) 值设置为小于 30 秒。由于数据库实例的底层 IP 地址在故障转移后可能会发生变化，如果您的应用程序尝试连接到不再使用的 IP 地址，那么长时间缓存 DNS 数据可能会导致连接失败。

所以我的问题是：我如何知道我的客户端应用程序是否正在缓存 DNS 数据？有没有办法为 Npgsql 发出的请求设置 TTL，还是必须在 .NET 或服务器上的某个地方进行设置？

编辑

所以在谢斯回答之后，我又看了一遍。似乎我们会遇到两种不同类型的错误，具体取决于托管环境。

我们托管在本地专用服务器上的代码会出现以下类型的 (DNS) 错误：

System.Data.Entity.Core.ProviderIncompatibleException: An error occurred while getting provider information from the database. This can be caused by Entity Framework using an incorrect connection string. Check the inner exceptions for details and ensure that the connection string is correct. ---> System.Data.Entity.Core.ProviderIncompatibleException: The provider did not return a ProviderManifestToken string. ---> System.Net.Sockets.SocketException: This is usually a temporary error during hostname resolution and means that the local server did not receive a response from an authoritative server
   at System.Net.Dns.GetAddrInfo(String name)
   at System.Net.Dns.InternalGetHostByName(String hostName, Boolean includeIPv6)
   at System.Net.Dns.GetHostAddresses(String hostNameOrAddress)
   at Npgsql.NpgsqlConnector.Connect(NpgsqlTimeout timeout)
   at Npgsql.NpgsqlConnector.<RawOpen>d__139.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Npgsql.NpgsqlConnector.<Open>d__136.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Npgsql.NpgsqlConnection.<Open>d__28.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Npgsql.NpgsqlConnection.Open()
   at Npgsql.NpgsqlServices.UsingPostgresDBConnection(NpgsqlConnection connection, Action`1 action)
   at Npgsql.NpgsqlServices.GetDbProviderManifestToken(DbConnection connection)
   at System.Data.Entity.Core.Common.DbProviderServices.GetProviderManifestToken(DbConnection connection)
   --- End of inner exception stack trace ---
   at System.Data.Entity.Core.Common.DbProviderServices.GetProviderManifestToken(DbConnection connection)
   at System.Data.Entity.Utilities.DbProviderServicesExtensions.GetProviderManifestTokenChecked(DbProviderServices providerServices, DbConnection connection)
...

托管在 Azure 应用服务中的相同代码出现以下（连接）错误：

System.InvalidOperationException: An exception has been raised that is likely due to a transient failure. ---> Npgsql.NpgsqlException: Exception while reading from stream ---> System.IO.IOException: Unable to read data from the transport connection: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond. ---> System.Net.Sockets.SocketException: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond
   at System.Net.Sockets.Socket.EndReceive(IAsyncResult asyncResult)
   at System.Net.Sockets.NetworkStream.EndRead(IAsyncResult asyncResult)
   --- End of inner exception stack trace ---
   at System.Net.Sockets.NetworkStream.EndRead(IAsyncResult asyncResult)
   at System.Threading.Tasks.TaskFactory`1.FromAsyncTrimPromise`1.Complete(TInstance thisRef, Func`3 endMethod, IAsyncResult asyncResult, Boolean requiresSynchronization)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Npgsql.ReadBuffer.<Ensure>d__27.MoveNext()
   --- End of inner exception stack trace ---
   at Npgsql.ReadBuffer.<Ensure>d__27.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Npgsql.NpgsqlConnector.<DoReadMessage>d__148.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Npgsql.NpgsqlConnector.<ReadPrependedMessages>d__150.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Npgsql.NpgsqlConnector.<ReadMessage>d__147.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Npgsql.NpgsqlConnector.<ReadExpecting>d__154`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Npgsql.NpgsqlDataReader.<NextResult>d__32.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Npgsql.NpgsqlDataReader.<<NextResultAsync>b__31_0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Npgsql.NpgsqlCommand.<Execute>d__71.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Npgsql.NpgsqlCommand.<ExecuteDbDataReader>d__92.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Npgsql.NpgsqlCommand.<>c__DisplayClass90_0.<<ExecuteDbDataReaderAsync>b__0>d.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.<ExecuteAsync>d__17.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncQueryingEnumerable`1.AsyncEnumerator.<BufferlessMoveNext>d__12.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Storage.Internal.NpgsqlExecutionStrategy.<ExecuteAsync>d__7`2.MoveNext()
   --- End of inner exception stack trace ---
   at Microsoft.EntityFrameworkCore.Storage.Internal.NpgsqlExecutionStrategy.<ExecuteAsync>d__7`2.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncQueryingEnumerable`1.AsyncEnumerator.<MoveNext>d__11.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Linq.AsyncEnumerable.<FirstOrDefault_>d__165`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.TaskResultAsyncEnumerable`1.Enumerator.<MoveNext>d__3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Linq.AsyncEnumerable.SelectEnumerableAsyncIterator`2.<MoveNextCore>d__7.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Linq.AsyncEnumerable.AsyncIterator`1.<MoveNext>d__10.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.AsyncLinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.<MoveNext>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<ExecuteSingletonAsyncQuery>d__21`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at OpenIddict.OpenIddictProvider`4.<HandleTokenRequest>d__25.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.GetResult()
   at AspNet.Security.OpenIdConnect.Server.OpenIdConnectServerHandler.<InvokeTokenEndpointAsync>d__20.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at AspNet.Security.OpenIdConnect.Server.OpenIdConnectServerHandler.<HandleRequestAsync>d__3.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.<Invoke>d__6.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Cors.Infrastructure.CorsMiddleware.<Invoke>d__8.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Localization.RequestLocalizationMiddleware.<Invoke>d__5.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at Microsoft.AspNetCore.Server.IISIntegration.IISMiddleware.<Invoke>d__13.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.<ProcessRequests>d__188`1.MoveNext()

所以也许这是同时出现的两个问题。在我们从连接字符串中删除Keepalive=30 之后，Azure 问题（连接问题）开始变得越来越频繁，也许我们应该把它放回去。

那么第二个问题可能是我们的服务器供应商和 DNS 服务器之间的连接问题？

这两个问题都是间歇性的，在数百个连接中只会出现几次。

【问题讨论】：

    标签： .net dns azure-web-app-service npgsql amazon-aurora


    【解决方案1】：

    Npgsql 不会以任何方式缓存 DNS 响应 - 它会在每次物理连接尝试时调用 Dns.GetHostAddresses()（或 Dns.GetHostAddressesAsync()）。操作系统通常会缓存，但基于它从 DNS 服务器接收到的响应。

    但是，您看到的错误消息似乎相反 - 它似乎表明您正在尝试执行 DNS 查找但超时，即权威 DNS 服务器在分配的时间内没有响应。这可能是您的服务器和权威 DNS 服务器之间的某种网络连接问题...

    【讨论】：

    • 好的，谢谢你的澄清！自从我们从连接字符串中删除了Keepalive=30 后，我们收到了更多此类错误。你认为这可能与此有关吗？问题是我们从专用服务器和 Azure 应用服务中都收到了连接错误。亚马逊支持声称他们没有问题。
    • Keepalive 不应该与这里的任何东西相关......它只是指示 Npgsql 在连接空闲时每 30 秒发送一次 SELECT NULL - 与初始物理连接尝试无关，这是 DNS 解析发生的地方...对不起，我不能提供更多帮助，但这似乎是某种晦涩难懂的网络/DNS 问题...
    • 请注意，您发布的第二个异常与 DNS 无关，而只是等待查询结果时的一般超时。当您的客户端等待答案时（例如，通过咨询 pg_stat_activity），您可以查看 PostgreSQL 中的查询发生了什么。这可能是一个非常慢的查询，也可能是网络问题。
    • 嗯好的。不过这很奇怪。自从将 Keepalive=30 放回连接字符串后，第二个异常已经消失，没有进行其他更改。但这不是连接错误吗？这就是我解释错误的方式，它在等待查询时不是超时。但我想你知道得更好:)
    • 我记得在某处读过一条评论，说像 Azure 这样的云提供商可能会关闭空闲时间较长的连接（当从池中获取该连接时会导致错误）。但也许这会带来另一个错误。
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