Question

我在.NET Core中有一个mvc应用程序，它使用MySQL。我有一个下载功能，其中传递了id以下载文件。但是，错误不断发生，并且似乎在某种程度上，该应用程序在服务器上的响应速度似乎有所降低，就好像在被回收一样。以下代码显示了负责下载文件的部分。我正在使用TransactionScope来确保下载计数器已正确更新。我遇到的错误与死锁有关。有人可以指出问题出在哪里吗？

using (var context = new MyDbContext())
using (TransactionScope scope = new TransactionScope())
{
    ProjectDownload download = context.ProjectDownload.Include(pd => pd.Project).Where(d => d.Id == downloadId).FirstOrDefault();

    if (download == null) throw new InvalidOperationException("Cannot find ProjectDownload.Id");

   download.DownloadCounter++;
   download.Project.DownloadCounter++;

   try
   {
       context.SaveChanges();
       scope.Complete();
       return (ProjectDownloadModel)download;
   }
   catch (DbUpdateException ex){
       throw;
   }
}

还尝试了这种方式，将之前的代码更改为：

using (var context = new MyDbContext())
using (TransactionScope scope = new TransactionScope())
{
    ProjectDownload download = context.ProjectDownload.Where(d => d.Id == downloadId).FirstOrDefault();

   download.DownloadCounter++;

   Project proj = context.Project.Where(p => p.ProjectDownload.Any(pd => pd.Id == downloadId)).FirstOrDefault();
   proj.DownloadCounter++;
   ...

发生以下错误：

Exception has occurred: CLR/MySql.Data.MySqlClient.MySqlException
An exception of type 'MySql.Data.MySqlClient.MySqlException' occurred in Microsoft.EntityFrameworkCore.dll but was not handled in user code: 'Deadlock found when trying to get lock; try restarting transaction'
  at MySql.Data.MySqlClient.MySqlStream.ReadPacket()
  at MySql.Data.MySqlClient.NativeDriver.GetResult(Int32& affectedRow, Int64& insertedId)
  at MySql.Data.MySqlClient.Driver.NextResult(Int32 statementId, Boolean force)
  at MySql.Data.MySqlClient.MySqlDataReader.NextResult()
  at MySql.Data.MySqlClient.MySqlCommand.ExecuteReader(CommandBehavior behavior)
  at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.Execute(IRelationalConnection connection, DbCommandMethod executeMethod, IReadOnlyDictionary`2 parameterValues)
  at Microsoft.EntityFrameworkCore.Storage.Internal.RelationalCommand.ExecuteReader(IRelationalConnection connection, IReadOnlyDictionary`2 parameterValues)    
  at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.Enumerator.BufferlessMoveNext(DbContext _, Boolean buffer)    
  at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.Enumerator.MoveNext()
  at System.Linq.Enumerable.SelectEnumerableIterator`2.MoveNext()    
  at System.Linq.Enumerable.TryGetFirst[TSource](IEnumerable`1 source, Boolean& found)    
  at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ResultEnumerable`1.GetEnumerator()
  at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.<_TrackEntities>d__17`2.MoveNext()
  at Microsoft.EntityFrameworkCore.Query.Internal.LinqOperatorProvider.ExceptionInterceptor`1.EnumeratorExceptionInterceptor.MoveNext()
  at System.Linq.Enumerable.TryGetFirst[TSource](IEnumerable`1 source, Boolean& found)    
  at System.Linq.Enumerable.First[TSource](IEnumerable`1 source)    
  at Microsoft.EntityFrameworkCore.Query.Internal.QueryCompiler.<>c__DisplayClass15_1`1.<CompileQueryCore>b__0(QueryContext qc)    
  at System.Linq.Queryable.FirstOrDefault[TSource](IQueryable`1 source)    
  at ProjectProcess.DownloadProject(Int32 downloadId) in C:\Users\Projects\ProjectWebsite\BusinessLogic\ProjectProcess.cs:line 124    
  at ProjectController.DownloadProject(Int32 downloadID) in C:\Users\Projects\ProjectWebsite\Controllers\ProjectController.cs:line 33    
  at Microsoft.Extensions.Internal.ObjectMethodExecutor.Execute(Object target, Object[] parameters)    
  at Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker.<InvokeActionMethodAsync>d__12.MoveNext()

我不了解僵局是如何发生的以及如何解决

Answer 1

问题是您没有锁定某些记录。在mysql中，您可以像这样锁定行

SELECT * FROM users WHERE name = 'name' FOR UPDATE;

但是对于您的问题，您可以尝试一下。希望能解决您的问题

using (var scope = new TransactionScope(TransactionScopeOption.RequireNew,
new TransactionOptions { 
    IsolationLevel = IsolationLevel.ReadUncommitted 
}))
{
  // your select and increment should go here  
}

Answer 2

可以再尝试一件事吗？只是快速修复

//replace your line with this
if (download == null) {
    context.SaveChanges();
    scope.Complete();
    throw new InvalidOperationException("Cannot find ProjectDownload.Id");
}

Answer 3

尝试在使用“交易范围”之前将这些放入：

 
ProjectDownload download = context.ProjectDownload.Where(d => d.Id == downloadId).FirstOrDefault();
Project proj = context.Project.Where(p => p.ProjectDownload.Any(pd => pd.Id == downloadId)).FirstOrDefault();

MySQL-DBUpdateException（“尝试获取锁时发现死锁；尝试重新启动事务”）

3 个答案: