3

我错过了什么？

我正在尝试使用这样的 TransactionScope 使用 NOLOCK 阅读：

var scopeOptions = new TransactionOptions { IsolationLevel = IsolationLevel.ReadUncommitted };
using (var scope = new TransactionScope(TransactionScopeOption.Required, scopeOptions))
{
   using (var db = new MyDbContext(ConnectionStringEntities))
   {
      // Simple read with a try catch block...
   }
   scope.Complete();
}

我希望看到 NOLOCK 添加到 SQL 查询（查看 SQL Profiler 和自定义 DbCommandInterceptor - 但它不存在......

更新：经过更多研究，我想知道是否最终使用了所选游标，只是没有 NOLOCK“提示”（特定于 SQL Server - 也特定于一个表），我发现了一些获取当前事务的代码和它似乎显示了正确选择的事务隔离（ReadUncommitted / Serializable 等）我仍然想测试它，但如果您有任何想法，请告诉我

获取当前的 .net TransactionScope IsolationLevel

Transaction trans = Transaction.Current;
System.Transactions.IsolationLevel level = trans.IsolationLevel;
LogService.Instance.Debug($"Transaction IsolationLevel = {level.ToString()}");
4

3 回答 3

5

所以看起来实体框架确实尊重 IsolationLevel，只是它不使用 NOLOCK 提示（可能是因为它过于特定于数据库），顺便说一下，我主要抱怨 EF - 它没有针对不同的数据库类型进行非常优化，另一个例子是新身份将 AspNetUsers 的 GUID 主键保存为字符串（同样因为缺乏优化），除此之外（以及其他一些事情）EF 很棒！

我在任何地方都找不到解决问题的方法，我绝对不想让我的所有查询都使用 NOLOCK - 只是未提交的查询，所以我最终结合了两个解决方案（有一些更改）：

  1. NoLockInterceptor - 用于动态添加 NOLOCK（带有 NOLOCK 的实体框架）：

    /// <summary>
/// Add "WITH (NOLOCK)" hint to SQL queries, SQL Server specifc - may break queries on different databases.
/// (conditionally turn off with NoLockInterceptor.AddNoLockHintToSqlQueries = false to change on runtime)
/// <para>
/// https://stackoverflow.com/questions/926656/entity-framework-with-nolock
/// </para>
/// </summary>
public class NoLockInterceptor : DbCommandInterceptor
{
    private static readonly Regex TableAliasRegex = new Regex(
        @"(?<tableAlias>AS \[Extent\d+\](?! WITH \(NOLOCK\)))",
        RegexOptions.Multiline | RegexOptions.IgnoreCase);

    /// <summary>
    /// Add "WITH (NOLOCK)" hint to SQL queries - unique to each thread 
    /// (set to true only when needed and then back to false)
    /// </summary>
    [ThreadStatic]
    public static bool AddNoLockHintToSqlQueries;

    public NoLockInterceptor()
    {
        // Do not use by default for all queries
        AddNoLockHintToSqlQueries = false;
    }

    public override void ScalarExecuting(DbCommand command, DbCommandInterceptionContext<object> interceptionContext)
    {
        if (AddNoLockHintToSqlQueries)
        {
            command.CommandText = TableAliasRegex.Replace(command.CommandText, "${tableAlias} WITH (NOLOCK)");
        }
    }

    public override void ReaderExecuting(DbCommand command, DbCommandInterceptionContext<DbDataReader> interceptionContext)
    {
        if (AddNoLockHintToSqlQueries)
        {
            command.CommandText = TableAliasRegex.Replace(command.CommandText, "${tableAlias} WITH (NOLOCK)");
        }
    }
}

  2. TransactionWrapper - 调用 NoLockInterceptor 行为并且对于重复使用事务也很有用（http://haacked.com/archive/2009/08/18/simpler-transactions.aspx/）：

    /// <summary>
/// Transaction wrapper for setting pre-defined transaction scopes
/// <para>
/// http://haacked.com/archive/2009/08/18/simpler-transactions.aspx/
/// </para>
/// </summary>
public static class TransactionWrapper
{
    /// <summary>
    /// Set transaction scope and using NoLockInterceptor for adding SQL Server specific "WITH (NOLOCK)" 
    /// to ReadUncommitted isolation level transactions (not supported by Entity Framework)
    /// </summary>
    /// <param name="isolationLevel"></param>
    /// <param name="transactionScopeOption"></param>
    /// <param name="timeout"></param>
    /// <param name="action"></param>
    public static void SetScope(IsolationLevel isolationLevel, TransactionScopeOption transactionScopeOption,
        TimeSpan timeout, Action action)
    {
        var transactionOptions = new TransactionOptions { IsolationLevel = isolationLevel, Timeout = timeout };
        using (var transactionScope = new TransactionScope(transactionScopeOption, transactionOptions))
        {
            if (isolationLevel == IsolationLevel.ReadUncommitted)
                NoLockInterceptor.AddNoLockHintToSqlQueries = true;

            action();
            transactionScope.Complete();

            if (isolationLevel == IsolationLevel.ReadUncommitted)
                NoLockInterceptor.AddNoLockHintToSqlQueries = false;
        }
    }
}

像这样使用它：

var timeout = TimeSpan.FromSeconds(ConfigVariables.Instance.Timeout_Transaction_Default_In_Seconds);
TransactionWrapper.SetScope(IsolationLevel.ReadUncommitted, TransactionScopeOption.Required, timeout, () =>
{
    using (var db = new MyDbContext(MyDbContextConnectionStringEntities))
    {
       // Do stuff...
    }
});

NOLOCK 现在仅添加到具有 ReadUncommitted 事务隔离级别范围的查询。

于 2016-04-04T13:03:18.643 回答
2

您无法让实体框架呈现 NOLOCK 提示。如果你想读取未提交的数据，你必须做一些不同的事情，比如将带有 IsolationLevel.ReadUncommited 的 TransactionScope 添加到 TransactionOptions。

编写您自己的命令拦截器或您自己的 EF 提供程序也可以。

https://msdn.microsoft.com/en-us/data/dn469464.aspx

于 2016-04-03T14:54:33.427 回答
1

我已经尝试过事务范围，然后它会分析对数据库的调用。EF 开始和结束事务，但从不更改已提交读的隔离级别。

            using (var scope = new TransactionScope(
            TransactionScopeOption.Required,
            new TransactionOptions()
            {
                IsolationLevel = System.Transactions.IsolationLevel.ReadUncommitted
            }))
        {
            List<T> toReturn = query.ToList();
            scope.Complete();
            return toReturn;
        }
于 2017-02-01T18:30:55.020 回答