SQL Server[转]Working with Transactions (EF6 Onwards)

正文转自:http://www.cnblogs.com/xiepeixing/p/5275999.html

Working with Transactions (EF6 Onwards)

This document will describe using transactions in EF6 including the
enhancements we have added since EF5 to make working with transactions
easy.

  • What EF does by
    default
  • How the APIs
    work

    • Combining several operations into one transaction within the
      same
      context
    • Passing an existing transaction to the
      context
  • Using transactions with other
    features

    • Connection
      resiliency
    • Asynchronous
      methods
    • TransactionScope
      transactions

What EF does by default

In all versions of Entity Framework, whenever you execute
SaveChanges() to insert, update or delete on the database the
framework will wrap that operation in a transaction. This transaction
lasts only long enough to execute the operation and then completes. When
you execute another such operation a new transaction is started.

Starting with EF6 Database.ExecuteSqlCommand() by default will wrap
the command in a transaction if one was not already present. There are
overloads of this method that allow you to override this behavior if you
wish. Also in EF6 execution of stored procedures included in the model
through APIs such as ObjectContext.ExecuteFunction() does the same
(except that the default behavior cannot at the moment be overridden).

In either case, the isolation level of the transaction is whatever
isolation level the database provider considers its default setting. By
default, for instance, on SQL Server this is READ COMMITTED.

Entity Framework does not wrap queries in a transaction.

This default functionality is suitable for a lot of users and if so
there is no need to do anything different in EF6; just write the code as
you always did.

However some users require greater control over their transactions –
this is covered in the following sections.

 

How the APIs work

Prior to EF6 Entity Framework insisted on opening the database
connection itself (it threw an exception if it was passed a connection
that was already open). Since a transaction can only be started on an
open connection, this meant that the only way a user could wrap several
operations into one transaction was either to use a
TransactionScope
or use the ObjectContext.Connection property and start calling
Open() and BeginTransaction() directly on the returned
EntityConnection object. In addition, API calls which contacted the
database would fail if you had started a transaction on the underlying
database connection on your own.

*Note: The limitation of only accepting closed connections was
removed in Entity Framework 6. For details, see
Connection Management
(EF6 Onwards).*

Starting with EF6 the framework now provides:

  1. Database.BeginTransaction() : An easier method for a user to
    start and complete transactions themselves within an existing
    DbContext – allowing several operations to be combined within the
    same transaction and hence either all committed or all rolled back
    as one. It also allows the user to more easily specify the isolation
    level for the transaction.
  2. Database.UseTransaction() : which allows the DbContext to use a
    transaction which was started outside of the Entity Framework.

Combining several operations into one transaction within the same context

Database.BeginTransaction() has two overrides – one which takes an
explicit
IsolationLevel
and one which takes no arguments and uses the default IsolationLevel
from the underlying database provider. Both overrides return a
DbContextTransaction object which provides Commit() and
Rollback() methods which perform commit and rollback on the
underlying store transaction.

The DbContextTransaction is meant to be disposed once it has been
committed or rolled back. One easy way to accomplish this is the
using(…) {…} syntax which will automatically call Dispose() when
the using block completes:

using System;  using System.Collections.Generic;  using System.Data.Entity;  using System.Data.SqlClient;  using System.Linq;  using System.Transactions;    namespace TransactionsExamples  {      class TransactionsExample      {          static void StartOwnTransactionWithinContext()          {              using (var context = new BloggingContext())              {                  using (var dbContextTransaction = context.Database.BeginTransaction())                  {                      try                      {                          context.Database.ExecuteSqlCommand(                              @"UPDATE Blogs SET Rating = 5" +                                  " WHERE Name LIKE '%Entity Framework%'"                              );                            var query = context.Posts.Where(p => p.Blog.Rating >= 5);                          foreach (var post in query)                          {                              post.Title += "[Cool Blog]";                          }                            context.SaveChanges();                            dbContextTransaction.Commit();                      }                      catch (Exception)                      {                          dbContextTransaction.Rollback();                      }                  }              }          }      }  }

 

*Note: Beginning a transaction requires that the underlying store
connection is open. So calling Database.BeginTransaction() will open the
connection  if it is not already opened. If DbContextTransaction opened
the connection then it will close it when Dispose() is called.*

Passing an existing transaction to the context

Sometimes you would like a transaction which is even broader in scope
and which includes operations on the same database but outside of EF
completely. To accomplish this you must open the connection and start
the transaction yourself and then tell EF a) to use the already-opened
database connection, and b) to use the existing transaction on that
connection.

To do this you must define and use a constructor on your context class
which inherits from one of the DbContext constructors which take i) an
existing connection parameter and ii) the contextOwnsConnection boolean.

*Note: The contextOwnsConnection flag must be set to false when
called in this scenario. This is important as it informs Entity
Framework that it should not close the connection when it is done with
it (e.g. see line 4 below):*

using (var conn = new SqlConnection("..."))  {      conn.Open();      using (var context = new BloggingContext(conn, contextOwnsConnection: false))      {      }  }

 

Furthermore, you must start the transaction yourself (including the
IsolationLevel if you want to avoid the default setting) and let the
Entity Framework know that there is an existing transaction already
started on the connection (see line 33 below).

Then you are free to execute database operations either directly on the
SqlConnection itself, or on the DbContext. All such operations are
executed within one transaction. You take responsibility for committing
or rolling back the transaction and for calling Dispose() on it, as well
as for closing and disposing the database connection. E.g.:

using System;  using System.Collections.Generic;  using System.Data.Entity;  using System.Data.SqlClient;  using System.Linq;  sing System.Transactions;    namespace TransactionsExamples  {       class TransactionsExample       {          static void UsingExternalTransaction()          {              using (var conn = new SqlConnection("..."))              {                 conn.Open();                   using (var sqlTxn = conn.BeginTransaction(System.Data.IsolationLevel.Snapshot))                 {                     try                     {                         var sqlCommand = new SqlCommand();                         sqlCommand.Connection = conn;                         sqlCommand.Transaction = sqlTxn;                         sqlCommand.CommandText =                             @"UPDATE Blogs SET Rating = 5" +                              " WHERE Name LIKE '%Entity Framework%'";                         sqlCommand.ExecuteNonQuery();                           using (var context =                            new BloggingContext(conn, contextOwnsConnection: false))                          {                              context.Database.UseTransaction(sqlTxn);                                var query =  context.Posts.Where(p => p.Blog.Rating >= 5);                              foreach (var post in query)                              {                                  post.Title += "[Cool Blog]";                              }                             context.SaveChanges();                          }                            sqlTxn.Commit();                      }                      catch (Exception)                      {                          sqlTxn.Rollback();                      }                  }              }          }      }  }

 

Notes:

  • You can pass null to Database.UseTransaction() to clear Entity
    Framework’s knowledge of the current transaction. Entity Framework
    will neither commit nor rollback the existing transaction when you
    do this, so use with care and only if you’re sure this is what you
    want to do.
  • You will see an exception from Database.UseTransaction() if you pass
    a transaction:

    • When the Entity Framework already has an existing transaction
    • When Entity Framework is already operating within a
      TransactionScope
    • Whose connection object is null (i.e. one which has no
      connection – usually this is a sign that that transaction has
      already completed)
    • Whose connection object does not match the Entity Framework’s
      connection.

 

Using transactions with other features

This section details how the above transactions interact with:

  • Connection resiliency
  • Asynchronous methods
  • TransactionScope transactions

Connection Resiliency

The new Connection Resiliency feature does not work with
user-initiated transactions. For details, see Limitations with Retrying
Execution Strategies.

Asynchronous Programming

The approach outlined in the previous sections needs no further options
or settings to work with the asynchronous query and save
methods. But be aware
that, depending on what you do within the asynchronous methods, this may
result in long-running transactions – which can in turn cause deadlocks
or blocking which is bad for the performance of the overall application.

TransactionScope Transactions

Prior to EF6 the recommended way of providing larger scope transactions
was to use a TransactionScope object:

using System.Collections.Generic;  using System.Data.Entity;  using System.Data.SqlClient;  using System.Linq;  using System.Transactions;    namespace TransactionsExamples  {      class TransactionsExample      {          static void UsingTransactionScope()          {              using (var scope = new TransactionScope(TransactionScopeOption.Required))              {                  using (var conn = new SqlConnection("..."))                  {                      conn.Open();                        var sqlCommand = new SqlCommand();                      sqlCommand.Connection = conn;                      sqlCommand.CommandText =                          @"UPDATE Blogs SET Rating = 5" +                              " WHERE Name LIKE '%Entity Framework%'";                      sqlCommand.ExecuteNonQuery();                        using (var context =                          new BloggingContext(conn, contextOwnsConnection: false))                      {                          var query = context.Posts.Where(p => p.Blog.Rating > 5);                          foreach (var post in query)                          {                              post.Title += "[Cool Blog]";                          }                          context.SaveChanges();                      }                  }                    scope.Complete();              }          }      }  }

 

The SqlConnection and Entity Framework would both use the ambient
TransactionScope transaction and hence be committed together.

Starting with .NET 4.5.1 TransactionScope has been updated to also work
with asynchronous methods via the use of the
TransactionScopeAsyncFlowOption
enumeration:

using System.Collections.Generic;  using System.Data.Entity;  using System.Data.SqlClient;  using System.Linq;  using System.Transactions;    namespace TransactionsExamples  {      class TransactionsExample      {          public static void AsyncTransactionScope()          {              using (var scope = new TransactionScope(TransactionScopeAsyncFlowOption.Enabled))              {                  using (var conn = new SqlConnection("..."))                  {                      await conn.OpenAsync();                        var sqlCommand = new SqlCommand();                      sqlCommand.Connection = conn;                      sqlCommand.CommandText =                          @"UPDATE Blogs SET Rating = 5" +                              " WHERE Name LIKE '%Entity Framework%'";                      await sqlCommand.ExecuteNonQueryAsync();                        using (var context = new BloggingContext(conn, contextOwnsConnection: false))                      {                          var query = context.Posts.Where(p => p.Blog.Rating > 5);                          foreach (var post in query)                          {                              post.Title += "[Cool Blog]";                          }                            await context.SaveChangesAsync();                      }                  }              }          }      }  }

 

There are still some limitations to the TransactionScope approach:

  • Requires .NET 4.5.1 or greater to work with asynchronous methods.
  • It cannot be used in cloud scenarios unless you are sure you have
    one and only one connection (cloud scenarios do not support
    distributed transactions).
  • It cannot be combined with the Database.UseTransaction() approach of
    the previous sections.
  • It will throw exceptions if you issue any DDL (e.g. because of a
    Database
    Initializer)
    and have not enabled distributed transactions through the MSDTC
    Service.

Advantages of the TransactionScope approach:

  • It will automatically upgrade a local transaction to a distributed
    transaction if you make more than one connection to a given database
    or combine a connection to one database with a connection to a
    different database within the same transaction (note: you must have
    the MSDTC service configured to allow distributed transactions for
    this to work).
  • Ease of coding. If you prefer the transaction to be ambient and
    dealt with implicitly in the background rather than explicitly under
    you control then the TransactionScope approach may suit you better.

In summary, with the new Database.BeginTransaction() and
Database.UseTransaction() APIs above, the TransactionScope approach is
no longer necessary for most users. If you do continue to use
TransactionScope then be aware of the above limitations. We recommend
using the approach outlined in the previous sections instead where
possible.

 

 

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