【EF6学习笔记】（四）弹性连接及命令拦截调试

本章原文地址：Connection Resiliency and Command Interception

原文有些地方讲的比较细，个人根据实际理解做些缩减，或者加入一些个人理解；

第1部分 弹性连接

为什么要弹性连接？什么是弹性连接？

在实际的网络应用中，尤其是在Internet上的网络应用，就算Web服务器和数据库服务器在一个数据中心，也不能保证WEB服务器和数据库服务器没有任何延迟或者其他网络问题；

尤其如PaaS层的Azure的SQL或者阿里的SQL、MySQL数据库服务器，都是做了网络负载均衡的，在一定的条件下能提供的服务是有限的，在其SLA里都会有定义；

而超出其SLA的部分请求就会被取消响应，那么在WEB网页应用设计的时候，就需要考虑这一点，在出现一些异常情况的时候，需要能够在短时间内进行一次或多次Retry.

这就称为：弹性的连接

原文中对EF6实现弹性连接功能仅做了简单代码就实现了，至于具体在实际项目中是不是就这么简单，本人还需要进一步深入学习。

先看看原文如何做的：

在DAL文件夹定义 SchoolConfiguration类，继承自DbConfiguration类， 在这个类中设置SQL数据库的执行策略 （execution strategy ，这个名词是EF6 对于Retry Policy 的命名）

using System.Data.Entity;
using System.Data.Entity.SqlServer;

namespace EFTest.DAL
{
    public class SchoolConfiguration : DbConfiguration
    {
        public SchoolConfiguration()
        {
            SetExecutionStrategy("System.Data.SqlClient", () => new SqlAzureExecutionStrategy());
        }
    }
}

EF6会自动运行继承自DbConfiguration类的代码；当然也可以通过配置Web.config 来实现，可以参考 EntityFramework Code-Based Configuration
然后在 Student 控制器里增加申明：

using System.Data.Entity.Infrastructure;

最后就是将所有 Try-catch代码块的 catch 后面捕获的 Exception 类型转为 RetryLimitExceededException

catch (RetryLimitExceededException /* dex */)
{
    //Log the error (uncomment dex variable name and add a line here to write a log.
    ModelState.AddModelError("", "Unable to save changes. Try again, and if the problem persists see your system administrator.");
}

这样就会是出现数据库连接异常的时候，进行Retry，多次Retry达到Limit值还是错误，则抛出异常。

更多的说明，需要参考： Entity Framework Connection Resiliency / Retry Logic
 

第2部分 命令拦截调试

设置好了弹性连接后，如何能够看到到底是不是Retry了，则需要一些调试手段；

当然你可以使用Web请求Log等来查看，或者EF6 提供的Dedicated Logging API (dedicated logging API)

本次直接采用EF6的 Interception 功能来实现。

对于日志记录来说，最好的方式是做一个接口来定义，而不是直接每次用硬代码来Call System.Diagnostics.Trace

因为这样，对于以后如果日志记录机制有调整的话，就容易很多了。

实际操作体验：

1、新建一个Logging 的文件夹

2、在文件夹中新建一个ILogger 接口，接口中定义了不同等级的信息处理方式；

TraceApi 可以跟踪连接外部服务例如SQL服务的每一步延迟情况；

using System;

namespace EFTest.Logging
{
    public interface ILogger
    {
        void Information(string message);
        void Information(string fmt, params object[] vars);
        void Information(Exception exception, string fmt, params object[] vars);

        void Warning(string message);
        void Warning(string fmt, params object[] vars);
        void Warning(Exception exception, string fmt, params object[] vars);

        void Error(string message);
        void Error(string fmt, params object[] vars);
        void Error(Exception exception, string fmt, params object[] vars);

        void TraceApi(string componentName, string method, TimeSpan timespan);
        void TraceApi(string componentName, string method, TimeSpan timespan, string properties);
        void TraceApi(string componentName, string method, TimeSpan timespan, string fmt, params object[] vars);
    }
}

3、在文件夹内新建Logger这个类，集成自ILogger接口，并实现接口内方法；

using System;
using System.Diagnostics;
using System.Text;

namespace EFTest.Logging
{
    public class Logger : ILogger
    {
        public void Information(string message)
        {
            Trace.TraceInformation(message);
        }

        public void Information(string fmt, params object[] vars)
        {
            Trace.TraceInformation(fmt, vars);
        }

        public void Information(Exception exception, string fmt, params object[] vars)
        {
            Trace.TraceInformation(FormatExceptionMessage(exception, fmt, vars));
        }

        public void Warning(string message)
        {
            Trace.TraceWarning(message);
        }

        public void Warning(string fmt, params object[] vars)
        {
            Trace.TraceWarning(fmt, vars);
        }

        public void Warning(Exception exception, string fmt, params object[] vars)
        {
            Trace.TraceWarning(FormatExceptionMessage(exception, fmt, vars));
        }

        public void Error(string message)
        {
            Trace.TraceError(message);
        }

        public void Error(string fmt, params object[] vars)
        {
            Trace.TraceError(fmt, vars);
        }

        public void Error(Exception exception, string fmt, params object[] vars)
        {
            Trace.TraceError(FormatExceptionMessage(exception, fmt, vars));
        }

        public void TraceApi(string componentName, string method, TimeSpan timespan)
        {
            TraceApi(componentName, method, timespan, ""); 
        }

        public void TraceApi(string componentName, string method, TimeSpan timespan, string fmt, params object[] vars)
        {
            TraceApi(componentName, method, timespan, string.Format(fmt, vars));
        }
        public void TraceApi(string componentName, string method, TimeSpan timespan, string properties)
        {
            string message = String.Concat("Component:", componentName, ";Method:", method, ";Timespan:", timespan.ToString(), ";Properties:", properties);
            Trace.TraceInformation(message);
        }

        private static string FormatExceptionMessage(Exception exception, string fmt, object[] vars)
        {
            // Simple exception formatting: for a more comprehensive version see 
            // http://code.msdn.microsoft.com/windowsazure/Fix-It-app-for-Building-cdd80df4
            var sb = new StringBuilder();
            sb.Append(string.Format(fmt, vars));
            sb.Append(" Exception: ");
            sb.Append(exception.ToString());
            return  sb.ToString();
        }
    }
}

看代码可以知道，实际是通过.NET 的 System.Diagnostics 来跟踪记录日志的；日志信息可以写到很多其他位置，例如Azure 的blob storage ；

本次，只是把日志输出到VS 的Output窗口；

在实际项目中，如果用其他记录日志机制来代替 System.Diagnostics的话，ILogging接口方式比较容易来实现这个切换。

下一步需要建一个拦截类来进行对EF6每一次向数据库发SQL请求的时候进行拦截记录日志或者发一个模拟的短暂错误；

这个类必须继承自 DbCommandInterceptor

在DAL文件夹新建 SchoolInterceptorLogging类，集成自DbCommandInterceptor

using System;
using System.Data.Common;
using System.Data.Entity;
using System.Data.Entity.Infrastructure.Interception;
using System.Data.Entity.SqlServer;
using System.Data.SqlClient;
using System.Diagnostics;
using System.Reflection;
using System.Linq;
using EFTest.Logging;

namespace EFTest.DAL
{
    public class SchoolInterceptorLogging : DbCommandInterceptor
    {
        private ILogger _logger = new Logger();
        private readonly Stopwatch _stopwatch = new Stopwatch();

        public override void ScalarExecuting(DbCommand command, DbCommandInterceptionContext<object> interceptionContext)
        {
            base.ScalarExecuting(command, interceptionContext);
            _stopwatch.Restart();
        }

        public override void ScalarExecuted(DbCommand command, DbCommandInterceptionContext<object> interceptionContext)
        {
            _stopwatch.Stop();
            if (interceptionContext.Exception != null)
            {
                _logger.Error(interceptionContext.Exception, "Error executing command: {0}", command.CommandText);
            }
            else
            {
                _logger.TraceApi("SQL Database", "SchoolInterceptor.ScalarExecuted", _stopwatch.Elapsed, "Command: {0}: ", command.CommandText);
            }
            base.ScalarExecuted(command, interceptionContext);
        }

        public override void NonQueryExecuting(DbCommand command, DbCommandInterceptionContext<int> interceptionContext)
        {
            base.NonQueryExecuting(command, interceptionContext);
            _stopwatch.Restart();
        }

        public override void NonQueryExecuted(DbCommand command, DbCommandInterceptionContext<int> interceptionContext)
        {
            _stopwatch.Stop();
            if (interceptionContext.Exception != null)
            {
                _logger.Error(interceptionContext.Exception, "Error executing command: {0}", command.CommandText);
            }
            else
            {
                _logger.TraceApi("SQL Database", "SchoolInterceptor.NonQueryExecuted", _stopwatch.Elapsed, "Command: {0}: ", command.CommandText);
            }
            base.NonQueryExecuted(command, interceptionContext);
        }

        public override void ReaderExecuting(DbCommand command, DbCommandInterceptionContext<DbDataReader> interceptionContext)
        {
            base.ReaderExecuting(command, interceptionContext);
            _stopwatch.Restart();
        }
        public override void ReaderExecuted(DbCommand command, DbCommandInterceptionContext<DbDataReader> interceptionContext)
        {
            _stopwatch.Stop();
            if (interceptionContext.Exception != null)
            {
                _logger.Error(interceptionContext.Exception, "Error executing command: {0}", command.CommandText);
            }
            else
            {
                _logger.TraceApi("SQL Database", "SchoolInterceptor.ReaderExecuted", _stopwatch.Elapsed, "Command: {0}: ", command.CommandText);
            }
            base.ReaderExecuted(command, interceptionContext);
        }
    }
}

 另外，为了模拟在筛选查询的时候产生一个短暂错误，写一个类让用户在输入框内输入 Throw 的触发这个错误，在DAL文件夹里建SchoolInterceptorTransientErrors类：

重载了ReaderExecuting方法，当用户输入的是Throw的时候，做一个特殊处理来进行测试；

如果希望测试其他数据库操作，则可以考虑重载 NonQueryExecuting 和ScalarExecuting这两个方法。

using System;
using System.Data.Common;
using System.Data.Entity.Infrastructure.Interception;
using System.Data.SqlClient;
using System.Reflection;
using System.Linq;
using EFTest.Logging;

namespace EFTest.DAL
{
    public class SchoolInterceptorTransientErrors : DbCommandInterceptor
    {
        private int _counter = 0;
        private ILogger _logger = new Logger();

        public override void ReaderExecuting(DbCommand command, DbCommandInterceptionContext<DbDataReader> interceptionContext)
        {
            bool throwTransientErrors = false;
            if (command.Parameters.Count > 0 && command.Parameters[0].Value.ToString() == "%Throw%")
            {
                throwTransientErrors = true;
                command.Parameters[0].Value = "%an%";
                command.Parameters[1].Value = "%an%";
            }

            if (throwTransientErrors && _counter < 4)
            {
                _logger.Information("Returning transient error for command: {0}", command.CommandText);
                _counter++;
                interceptionContext.Exception = CreateDummySqlException();
            }
        }

        private SqlException CreateDummySqlException()
        {
            // The instance of SQL Server you attempted to connect to does not support encryption
            var sqlErrorNumber = 20;

            var sqlErrorCtor = typeof(SqlError).GetConstructors(BindingFlags.Instance | BindingFlags.NonPublic).Where(c => c.GetParameters().Count() == 7).Single();
            var sqlError = sqlErrorCtor.Invoke(new object[] { sqlErrorNumber, (byte)0, (byte)0, "", "", "", 1 });

            var errorCollection = Activator.CreateInstance(typeof(SqlErrorCollection), true);
            var addMethod = typeof(SqlErrorCollection).GetMethod("Add", BindingFlags.Instance | BindingFlags.NonPublic);
            addMethod.Invoke(errorCollection, new[] { sqlError });

            var sqlExceptionCtor = typeof(SqlException).GetConstructors(BindingFlags.Instance | BindingFlags.NonPublic).Where(c => c.GetParameters().Count() == 4).Single();
            var sqlException = (SqlException)sqlExceptionCtor.Invoke(new object[] { "Dummy", errorCollection, null, Guid.NewGuid() });

            return sqlException;
        }
    }
}

在Global.asax中添加声明：
using System.Data.Entity.Infrastructure.Interception;

并在Application_Start()方法中增加后面两行代码：

        protected void Application_Start()
        {
            AreaRegistration.RegisterAllAreas();
            RouteConfig.RegisterRoutes(RouteTable.Routes);
            //Database.SetInitializer<SchoolContext>(new SchoolInitializer());
            DbInterception.Add(new SchoolInterceptorTransientErrors());
            DbInterception.Add(new SchoolInterceptorLogging());            
        }

当然，这两行代码可以加在SchoolConfiguration类里，初始化的时候直接加载执行：（注：上面和下面两段代码只能选其中一种）

using System.Data.Entity;
using System.Data.Entity.Infrastructure.Interception;
using System.Data.Entity.SqlServer;

namespace EFTest.DAL
{
    public class SchoolConfiguration : DbConfiguration
    {
        public SchoolConfiguration()
        {
            SetExecutionStrategy("System.Data.SqlClient", () => new SqlAzureExecutionStrategy());
            DbInterception.Add(new SchoolInterceptorTransientErrors());
            DbInterception.Add(new SchoolInterceptorLogging());
        }
    }
}

下面就可以进行测试了，进Home/Index后点Students，进入Student/Index 就会发现VS 的Output 窗口有一些Debug信息输出：

SELECT Count(*)
FROM INFORMATION_SCHEMA.TABLES AS t
WHERE t.TABLE_SCHEMA + '.' + t.TABLE_NAME IN ('dbo.Course','dbo.Enrollment','dbo.Student')
    OR t.TABLE_NAME = 'EdmMetadata': 
Application Insights Telemetry (unconfigured): {"name":"Microsoft.ApplicationInsights.Dev.RemoteDependency","time":"2017-08-25T07:04:11.0689220Z","tags":{"ai.internal.sdkVersion":"rddf: 2.0.0.25000","ai.device.roleInstance":"chnsbopc02.dc01.fujixerox.net","ai.operation.name":"GET Student/Index","ai.operation.parentId":"yqYB5YWz4YY=","ai.operation.id":"yqYB5YWz4YY="},"data":{"baseType":"RemoteDependencyData","baseData":{"ver":2,"name":"(localdb)\ProjectsV13 | master","id":"+xoBYBiNrPM=","value":0.3565,"dependencyKind":0,"success":true,"properties":{"DeveloperMode":"true"}}}}
iisexpress.exe Information: 0 : Component:SQL Database;Method:SchoolInterceptor.ScalarExecuted;Timespan:00:00:00.0233412;Properties:Command: IF db_id(N'EFTest') IS NOT NULL SELECT 1 ELSE SELECT Count(*) FROM sys.databases WHERE [name]=N'EFTest': 
Application Insights Telemetry (unconfigured): {"name":"Microsoft.ApplicationInsights.Dev.RemoteDependency","time":"2017-08-25T07:04:11.2577658Z","tags":{"ai.internal.sdkVersion":"rddf: 2.0.0.25000","ai.device.roleInstance":"chnsbopc02.dc01.fujixerox.net","ai.operation.name":"GET Student/Index","ai.operation.parentId":"yqYB5YWz4YY=","ai.operation.id":"yqYB5YWz4YY="},"data":{"baseType":"RemoteDependencyData","baseData":{"ver":2,"name":"(localdb)\ProjectsV13 | EFTest","id":"hGEE5js+frc=","value":1.2592,"dependencyKind":0,"success":true,"properties":{"DeveloperMode":"true"}}}}
iisexpress.exe Information: 0 : Component:SQL Database;Method:SchoolInterceptor.ReaderExecuted;Timespan:00:00:00.0260725;Properties:Command: SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[__MigrationHistory] AS [Extent1]
        WHERE [Extent1].[ContextKey] = @p__linq__0
    )  AS [GroupBy1]: 
Application Insights Telemetry (unconfigured): {"name":"Microsoft.ApplicationInsights.Dev.RemoteDependency","time":"2017-08-25T07:04:11.3081972Z","tags":{"ai.internal.sdkVersion":"rddf: 2.0.0.25000","ai.device.roleInstance":"chnsbopc02.dc01.fujixerox.net","ai.operation.name":"GET Student/Index","ai.operation.parentId":"yqYB5YWz4YY=","ai.operation.id":"yqYB5YWz4YY="},"data":{"baseType":"RemoteDependencyData","baseData":{"ver":2,"name":"(localdb)\ProjectsV13 | EFTest","id":"ULROylXtnGQ=","value":1.6166,"dependencyKind":0,"success":true,"properties":{"DeveloperMode":"true"}}}}
iisexpress.exe Information: 0 : Component:SQL Database;Method:SchoolInterceptor.ReaderExecuted;Timespan:00:00:00.0155956;Properties:Command: SELECT TOP (1) 
    [Project1].[C1] AS [C1], 
    [Project1].[MigrationId] AS [MigrationId], 
    [Project1].[Model] AS [Model], 
    [Project1].[ProductVersion] AS [ProductVersion]
    FROM ( SELECT 
        [Extent1].[MigrationId] AS [MigrationId], 
        [Extent1].[Model] AS [Model], 
        [Extent1].[ProductVersion] AS [ProductVersion], 
        1 AS [C1]
        FROM [dbo].[__MigrationHistory] AS [Extent1]
        WHERE [Extent1].[ContextKey] = @p__linq__0
    )  AS [Project1]
    ORDER BY [Project1].[MigrationId] DESC: 
Application Insights Telemetry (unconfigured): {"name":"Microsoft.ApplicationInsights.Dev.RemoteDependency","time":"2017-08-25T07:04:11.3703060Z","tags":{"ai.internal.sdkVersion":"rddf: 2.0.0.25000","ai.device.roleInstance":"chnsbopc02.dc01.fujixerox.net","ai.operation.name":"GET Student/Index","ai.operation.parentId":"yqYB5YWz4YY=","ai.operation.id":"yqYB5YWz4YY="},"data":{"baseType":"RemoteDependencyData","baseData":{"ver":2,"name":"(localdb)\ProjectsV13 | EFTest","id":"/xZUhdnSdM8=","value":1.7563,"dependencyKind":0,"success":true,"properties":{"DeveloperMode":"true"}}}}
iisexpress.exe Information: 0 : Component:SQL Database;Method:SchoolInterceptor.ReaderExecuted;Timespan:00:00:00.0176946;Properties:Command: SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Student] AS [Extent1]
    )  AS [GroupBy1]: 
'iisexpress.exe' (CLR v4.0.30319: /LM/W3SVC/3/ROOT-1-131481182423379125): Loaded 'EntityFrameworkDynamicProxies-EFTest'. 
Application Insights Telemetry (unconfigured): {"name":"Microsoft.ApplicationInsights.Dev.RemoteDependency","time":"2017-08-25T07:04:11.4280626Z","tags":{"ai.internal.sdkVersion":"rddf: 2.0.0.25000","ai.device.roleInstance":"chnsbopc02.dc01.fujixerox.net","ai.operation.name":"GET Student/Index","ai.operation.parentId":"yqYB5YWz4YY=","ai.operation.id":"yqYB5YWz4YY="},"data":{"baseType":"RemoteDependencyData","baseData":{"ver":2,"name":"(localdb)\ProjectsV13 | EFTest","id":"bDKxb2FBJ1g=","value":0.9676,"dependencyKind":0,"success":true,"properties":{"DeveloperMode":"true"}}}}
iisexpress.exe Information: 0 : Component:SQL Database;Method:SchoolInterceptor.ReaderExecuted;Timespan:00:00:00.0194244;Properties:Command: SELECT 
    [Extent1].[ID] AS [ID], 
    [Extent1].[LastName] AS [LastName], 
    [Extent1].[FirstMidName] AS [FirstMidName], 
    [Extent1].[EnrollmentDate] AS [EnrollmentDate]
    FROM [dbo].[Student] AS [Extent1]
    ORDER BY [Extent1].[LastName] ASC
    OFFSET 0 ROWS FETCH NEXT 3 ROWS ONLY : 
'iisexpress.exe' (CLR v4.0.30319: /LM/W3SVC/3/ROOT-1-131481182423379125): Loaded 'C:UsersjaczhangAppDataLocalTempTemporary ASP.NET Files
oot644be5f6480b064fApp_Web_jlggwjiw.dll'. 
Application Insights Telemetry (unconfigured): {"name":"Microsoft.ApplicationInsights.Dev.Request","time":"2017-08-25T07:04:10.5173746Z","tags":{"ai.operation.id":"yqYB5YWz4YY=","ai.internal.sdkVersion":"web: 2.0.0.25000","ai.device.roleInstance":"chnsbopc02.dc01.fujixerox.net","ai.operation.name":"GET Student/Index"},"data":{"baseType":"RequestData","baseData":{"ver":2,"id":"yqYB5YWz4YY=","name":"GET Student/Index","startTime":"2017-08-25T15:04:10.5173746+08:00","duration":"00:00:01.6326287","success":true,"responseCode":"200","url":"http://localhost:9910/Student","httpMethod":"GET","properties":{"DeveloperMode":"true"}}}}

然后在筛选输入框输入Throw 点确定： 可以看到里面触发一个 代码为20的短暂错误：

iisexpress.exe Information: 0 : Returning transient error for command: SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Student] AS [Extent1]
        WHERE ([Extent1].[LastName] LIKE @p__linq__0 ESCAPE N'~') OR ([Extent1].[FirstMidName] LIKE @p__linq__1 ESCAPE N'~')
    )  AS [GroupBy1]
iisexpress.exe Error: 0 : Error executing command: SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Student] AS [Extent1]
        WHERE ([Extent1].[LastName] LIKE @p__linq__0 ESCAPE N'~') OR ([Extent1].[FirstMidName] LIKE @p__linq__1 ESCAPE N'~')
    )  AS [GroupBy1] Exception: System.Data.SqlClient.SqlException (0x80131904): Dummy
ClientConnectionId:9b424f7d-306b-450b-8a3e-58198487b8bb
Error Number:20,State:0,Class:0
iisexpress.exe Information: 0 : Returning transient error for command: SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Student] AS [Extent1]
        WHERE ([Extent1].[LastName] LIKE @p__linq__0 ESCAPE N'~') OR ([Extent1].[FirstMidName] LIKE @p__linq__1 ESCAPE N'~')
    )  AS [GroupBy1]
iisexpress.exe Error: 0 : Error executing command: SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Student] AS [Extent1]
        WHERE ([Extent1].[LastName] LIKE @p__linq__0 ESCAPE N'~') OR ([Extent1].[FirstMidName] LIKE @p__linq__1 ESCAPE N'~')
    )  AS [GroupBy1] Exception: System.Data.SqlClient.SqlException (0x80131904): Dummy
ClientConnectionId:941ac661-66d3-4779-a690-188d13144150
Error Number:20,State:0,Class:0
iisexpress.exe Information: 0 : Returning transient error for command: SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Student] AS [Extent1]
        WHERE ([Extent1].[LastName] LIKE @p__linq__0 ESCAPE N'~') OR ([Extent1].[FirstMidName] LIKE @p__linq__1 ESCAPE N'~')
    )  AS [GroupBy1]
iisexpress.exe Error: 0 : Error executing command: SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Student] AS [Extent1]
        WHERE ([Extent1].[LastName] LIKE @p__linq__0 ESCAPE N'~') OR ([Extent1].[FirstMidName] LIKE @p__linq__1 ESCAPE N'~')
    )  AS [GroupBy1] Exception: System.Data.SqlClient.SqlException (0x80131904): Dummy
ClientConnectionId:47685ec4-e837-4e05-bbb3-948e3f3f87a8
Error Number:20,State:0,Class:0
iisexpress.exe Information: 0 : Returning transient error for command: SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Student] AS [Extent1]
        WHERE ([Extent1].[LastName] LIKE @p__linq__0 ESCAPE N'~') OR ([Extent1].[FirstMidName] LIKE @p__linq__1 ESCAPE N'~')
    )  AS [GroupBy1]
iisexpress.exe Error: 0 : Error executing command: SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Student] AS [Extent1]
        WHERE ([Extent1].[LastName] LIKE @p__linq__0 ESCAPE N'~') OR ([Extent1].[FirstMidName] LIKE @p__linq__1 ESCAPE N'~')
    )  AS [GroupBy1] Exception: System.Data.SqlClient.SqlException (0x80131904): Dummy
ClientConnectionId:35de62b7-cda6-4db7-88e0-48d708f42826
Error Number:20,State:0,Class:0
Application Insights Telemetry (unconfigured): {"name":"Microsoft.ApplicationInsights.Dev.RemoteDependency","time":"2017-08-25T07:06:51.9163289Z","tags":{"ai.internal.sdkVersion":"rddf: 2.0.0.25000","ai.device.roleInstance":"chnsbopc02.dc01.fujixerox.net","ai.operation.name":"GET Student/Index","ai.operation.parentId":"kZm9/0YA8y4=","ai.operation.id":"kZm9/0YA8y4="},"data":{"baseType":"RemoteDependencyData","baseData":{"ver":2,"name":"(localdb)\ProjectsV13 | EFTest","id":"0iOb6Vj5zqI=","value":3.3859,"dependencyKind":0,"success":true,"properties":{"DeveloperMode":"true"}}}}
iisexpress.exe Information: 0 : Component:SQL Database;Method:SchoolInterceptor.ReaderExecuted;Timespan:00:00:00.0382038;Properties:Command: SELECT 
    [GroupBy1].[A1] AS [C1]
    FROM ( SELECT 
        COUNT(1) AS [A1]
        FROM [dbo].[Student] AS [Extent1]
        WHERE ([Extent1].[LastName] LIKE @p__linq__0 ESCAPE N'~') OR ([Extent1].[FirstMidName] LIKE @p__linq__1 ESCAPE N'~')
    )  AS [GroupBy1]: 
Application Insights Telemetry (unconfigured): {"name":"Microsoft.ApplicationInsights.Dev.RemoteDependency","time":"2017-08-25T07:06:51.9705260Z","tags":{"ai.internal.sdkVersion":"rddf: 2.0.0.25000","ai.device.roleInstance":"chnsbopc02.dc01.fujixerox.net","ai.operation.name":"GET Student/Index","ai.operation.parentId":"kZm9/0YA8y4=","ai.operation.id":"kZm9/0YA8y4="},"data":{"baseType":"RemoteDependencyData","baseData":{"ver":2,"name":"(localdb)\ProjectsV13 | EFTest","id":"b1S5ExJ8ugQ=","value":2.5604,"dependencyKind":0,"success":true,"properties":{"DeveloperMode":"true"}}}}
iisexpress.exe Information: 0 : Component:SQL Database;Method:SchoolInterceptor.ReaderExecuted;Timespan:00:00:00.0195543;Properties:Command: SELECT 
    [Project1].[ID] AS [ID], 
    [Project1].[LastName] AS [LastName], 
    [Project1].[FirstMidName] AS [FirstMidName], 
    [Project1].[EnrollmentDate] AS [EnrollmentDate]
    FROM ( SELECT 
        [Extent1].[ID] AS [ID], 
        [Extent1].[LastName] AS [LastName], 
        [Extent1].[FirstMidName] AS [FirstMidName], 
        [Extent1].[EnrollmentDate] AS [EnrollmentDate]
        FROM [dbo].[Student] AS [Extent1]
        WHERE ([Extent1].[LastName] LIKE @p__linq__0 ESCAPE N'~') OR ([Extent1].[FirstMidName] LIKE @p__linq__1 ESCAPE N'~')
    )  AS [Project1]
    ORDER BY [Project1].[LastName] ASC
    OFFSET 0 ROWS FETCH NEXT 3 ROWS ONLY : 
Application Insights Telemetry (unconfigured): {"name":"Microsoft.ApplicationInsights.Dev.Request","time":"2017-08-25T07:06:40.0512275Z","tags":{"ai.operation.id":"kZm9/0YA8y4=","ai.internal.sdkVersion":"web: 2.0.0.25000","ai.device.roleInstance":"chnsbopc02.dc01.fujixerox.net","ai.operation.name":"GET Student/Index"},"data":{"baseType":"RequestData","baseData":{"ver":2,"id":"kZm9/0YA8y4=","name":"GET Student/Index","startTime":"2017-08-25T15:06:40.0512275+08:00","duration":"00:00:11.9404844","success":true,"responseCode":"200","url":"http://localhost:9910/Student?SearchString=Throw","httpMethod":"GET","properties":{"DeveloperMode":"true"}}}}

为了看Retry是否其作用，可以先注释掉以下行：
　　public SchoolConfiguration()
        {
            //SetExecutionStrategy("System.Data.SqlClient", () => new SqlAzureExecutionStrategy());           
        }

然后再进入网页，输入Throw后点确定，就会发现出现了数据库错误：

看起来是起作用了；

最后记得把刚才注释掉的行取消注释。