如何使用 SqlAzureExecutionStrategy 和“Nolock”

Question

为了处理 SQL 超时，我尝试使用 SqlAzureExecutionStrategy (https://msdn.microsoft.com/en-us/data/dn456835.aspx)

我遇到的问题是它阻止了“用户启动的事务”，这似乎是在 EF（http://www.hanselman.com/blog/GettingLINQToSQLAndLINQToEntitiesToUseNOLOCK.aspx、NOLOCK with Linq to SQL）中实现“with (nolock)”的推荐方法。

示例代码

    public AspnetUser GetAspnetUserByUserName(string userName)
    {
        using (var tx = new TransactionScope(TransactionScopeOption.Required, new TransactionOptions() { IsolationLevel = IsolationLevel.ReadUncommitted }))
        {
            return context.AspnetUsers.Where(x => x.UserName == userName).FirstOrDefault();
        }
    }

抛出错误

配置的执行策略“SqlAzureExecutionStrategy”不支持用户发起的事务。有关更多信息，请参阅http://go.microsoft.com/fwlink/?LinkId=309381。

我已经看到了关于在每次调用的基础上关闭 SqlAzureExecutionStrategy 的答案，但是如果我的所有读取都忽略了该策略，那将违背使用它的目的。可以同时拥有“NoLock”和 SqlAzureExecutionStrategy

Answer 1

SqlAzureExecutionStrategy 不支持在要重试的操作之外发起的事务。要解决此限制，您需要暂停策略，创建事务范围并将工作作为您手动传递给执行策略以重试的操作：

public AspnetUser GetAspnetUserByUserName(string userName)
{
    new SuspendableSqlAzureExecutionStrategy().Execute(() =>
        {
            using (var tx = new TransactionScope(
                    TransactionScopeOption.Required,
                    new TransactionOptions() { IsolationLevel = IsolationLevel.ReadUncommitted }))
            {
                return context.AspnetUsers.Where(x => x.UserName == userName).FirstOrDefault();
            }
        });
}

这里我使用了 https://msdn.microsoft.com/en-us/data/dn307226 的可暂停策略的替代方案，它将自动暂停任何嵌套调用：

using System.Data.Entity.Infrastructure;
using System.Data.Entity.SqlServer;
using System.Data.Entity.Utilities;
using System.Runtime.Remoting.Messaging;
using System.Threading;
using System.Threading.Tasks;

public class SuspendableSqlAzureExecutionStrategy : IDbExecutionStrategy
{
    private readonly IDbExecutionStrategy _azureExecutionStrategy;

    public SuspendableSqlAzureExecutionStrategy()
    {
        _azureExecutionStrategy = new SqlAzureExecutionStrategy();
    }

    private static bool Suspend
    {
        get { return (bool?)CallContext.LogicalGetData("SuspendExecutionStrategy") ?? false; }
        set { CallContext.LogicalSetData("SuspendExecutionStrategy", value); }
    }

    public bool RetriesOnFailure
    {
        get { return !Suspend; }
    }

    public virtual void Execute(Action operation)
    {
        if (!RetriesOnFailure)
        {
            operation();
            return;
        }

        try
        {
            Suspend = true;
            _azureExecutionStrategy.Execute(operation);
        }
        finally
        {
            Suspend = false;
        }
    }

    public virtual TResult Execute<TResult>(Func<TResult> operation)
    {
        if (!RetriesOnFailure)
        {
            return operation();
        }

        try
        {
            Suspend = true;
            return _azureExecutionStrategy.Execute(operation);
        }
        finally
        {
            Suspend = false;
        }
    }

    public virtual async Task ExecuteAsync(Func<Task> operation, CancellationToken cancellationToken)
    {
        if (!RetriesOnFailure)
        {
            await operation();
            return;
        }

        try
        {
            Suspend = true;
            await _azureExecutionStrategy.ExecuteAsync(operation, cancellationToken);
        }
        finally
        {
            Suspend = false;
        }
    }

    public virtual async Task<TResult> ExecuteAsync<TResult>(Func<Task<TResult>> operation, CancellationToken cancellationToken)
    {
        if (!RetriesOnFailure)
        {
            return await operation();
        }

        try
        {
            Suspend = true;
            return await _azureExecutionStrategy.ExecuteAsync(operation, cancellationToken);
        }
        finally
        {
            Suspend = false;
        }
    }
}

public class MyConfiguration : DbConfiguration
{
    public MyConfiguration()
    {
        SetExecutionStrategy("System.Data.SqlClient", () => new SuspendableSqlAzureExecutionStrategy());
    }
}