How to force only one transaction within multiple DbContext classes?

Question

Background:
From another question here at SO I have a Winforms solution (Finance) with many projects (fixed projects for the solution). Now one of my customers asked me to "upgrade" the solution and add projects/modules that will come from another Winforms solution (HR).

I really don't want to keep these projects as fixed projects on the existing finance solution. For that I'm trying to create plugins that will load GUI, business logic and the data layer all using MEF.

Question:
I have a context (DbContext built to implment the Generic Repository Pattern) with a list of external contexts (loaded using MEF - these contexts represent the contexts from each plugin, also with the Generic Repository Pattern).

Let's say I have this:

public class MainContext : DbContext
{
   public List<IPluginContext> ExternalContexts { get; set; }

   // other stuff here
}

and

public class PluginContext_A : DbContext, IPluginContext
{ /* Items from this context */ }

public class PluginContext_B : DbContext, IPluginContext
{ /* Items from this context */ }

and within the MainContext class, already loaded, I have both external contexts (from plugins).

With that in mind, let's say I have a transaction that will impact both the MainContext and the PluginContext_B.

How to perform update/insert/delete on both contexts within one transaction (unity of work)?

Using the IUnityOfWork I can set the SaveChanges() for the last item but as far as I know I must have a single context for it to work as a single transaction.

There's a way using the MSDTC (TransactionScope) but this approach is terrible and I'd reather not use this at all (also because I need to enable MSDTC on clients and server and I've had crashes and leaks all the time).

Update:
Systems are using SQL 2008 R2. Never bellow.
If it's possible to use TransactionScope in a way that won't scale to MSDTC it's fine, but I've never achieved that. All the time I've used TransactionScope it goes into MSDTC. According to another post on SO, there are some cases where TS will not go into MSDTC: check here. But I'd really prefer to go into some other way instead of TransactionScope...

Answer 1

If you are using multiple contexts each using separate connection and you want to save data to those context in single transaction you must use TransactionScope with distributed transaction (MSDTC).

Your linked question is not that case because in that scenario first connection do not modify data so it can be closed prior to starting the connection where data are modified. In your case data are concurrently modified on multiple connection which requires two-phase commit and MSDTC.

You can try to solve it with sharing single connection among multiple contexts but that can be quite tricky. I'm not sure how reliable the following sample is but you can give it a try:

using (var connection = new SqlConnection(connnectionString))
{
    var c1 = new Context(connection);
    var c2 = new Context(connection);

    c1.MyEntities.Add(new MyEntity() { Name = "A" });
    c2.MyEntities.Add(new MyEntity() { Name = "B" });

    connection.Open(); 

    using (var scope = new TransactionScope())
    {
        // This is necessary because DbContext doesnt't contain necessary methods
        ObjectContext obj1 = ((IObjectContextAdapter)c1).ObjectContext;
        obj1.SaveChanges(SaveOptions.DetectChangesBeforeSave);

        ObjectContext obj2 = ((IObjectContextAdapter)c2).ObjectContext;
        obj2.SaveChanges(SaveOptions.DetectChangesBeforeSave);

        scope.Complete();

        // Only after successful commit of both save operations we can accept changes
        // otherwise in rollback caused by second context the changes from the first
        // context will be already accepted = lost

        obj1.AcceptAllChanges();
        obj2.AcceptAllChanges();
    }
}

Context constructor is defined as:

public Context(DbConnection connection) : base(connection,false) { }

The sample itself worked for me but it has multiple problems:

• First usage of contexts must be done with closed connection. That is the reason why I'm adding entities prior to opening the connection.
• I rather open connection manually outside of the transaction but perhaps it is not needed.
• Both save changes successfully run and Transaction.Current has empty distributed transaction Id so it should be still local.
• The saving is much more complicated and you must use ObjectContext because DbContext doesn't have all necessary methods.
• It doesn't have to work in every scenario. Even MSDN claims this:

Promotion of a transaction to a DTC may occur when a connection is closed and reopened within a single transaction. Because the Entity Framework opens and closes the connection automatically, you should consider manually opening and closing the connection to avoid transaction promotion.

The problem with DbContext API is that it closes and reopens connection even if you open it manually so it is a opened question if API always correctly identifies if it runs in the context of transaction and do not close connection.

Answer 2

So, any chance this has changed by October 19th? All over the intertubes, people suggest the following code, and it doesn't work:

    (_contextA as IObjectContextAdapter).ObjectContext.Connection.Open();
    (_contextB as IObjectContextAdapter).ObjectContext.Connection.Open();

    using (var transaction = new TransactionScope(TransactionScopeOption.Required, new TransactionOptions{IsolationLevel = IsolationLevel.ReadUncommitted, Timeout = TimeSpan.MaxValue}))
{
    _contextA.SaveChanges();
    _contextB.SaveChanges();

    // Commit the transaction
    transaction.Complete();
}

    // Close open connections
    (_contextA as IObjectContextAdapter).ObjectContext.Connection.Close();
    (_contextB as IObjectContextAdapter).ObjectContext.Connection.Close();

This is a serious drag for implementing a single Unit of Work class across repositories. Any new way around this?

Answer 3

To avoid using MSDTC (distributed transaction):

This should force you to use one connection within the transaction as well as just one transaction. It should throw an exception otherwise.

Note: At least EF6 is required

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(); 
                } 
            } 
        } 
    } 
} 

Source: http://msdn.microsoft.com/en-us/data/dn456843.aspx#existing

Answer 4

@Ladislav Mrnka You were right from the start: I have to use MSDTC.

I've tried multiple things here including the sample code I've provided. I've tested it many times with changed hare and there but it won't work. The error goes deep into how EF and DbContext works and for that to change I'd finally find myself with my very own ORM tool. It's not the case.

I've also talked to a friend (MVP) that know a lot about EF too. We have tested some other things here but it won't work the way I want it to. I'll end up with multiple isolated transactions (I was trying to get them together with my sample code) and with this approach I don't have any way to enforce a full rollback automatically and I'll have to create a lot of generic/custom code to manually rollback changes and here comes another question: what if this sort of rollback fails (it's not a rollback, just an update)?

So, the only way we found here is to use the MSDTC and build some tools to help debug/test if DTC is enabled, if client/server firewalls are ok and all that stuff.

Thanks anyway. =)