domain driven design - Transactions in the Repository Pattern

Question

How do I encapsulate the saving of more than one entity in a transactional manner using the repository pattern? For example, what if I wanted to add an order and update the customer status based on that order creation, but only do so if the order completed successfully? Keep in mind that for this example, orders are not a collection inside the customer. They are their own entity.

This is just a contrived example, so I don’t really care whether orders should or should not be inside the customer object or even in the same bounded context. I don’t really care what underlying technology will be used (nHibernate, EF, ADO.Net, Linq, etc.) I just want to see what some calling code might look like in this admittedly contrived example of an all or nothing operation.

Answer 1

Booting my computer this morning I faced the exact problem for a project I am working on. I had some ideas which lead to the following design - and comments would be more than awesome. Unfortunately the design suggested by Josh isn't possible, as I have to work with a remote SQL server and can't enable the Distribute Transaction Coordinator service it relies on.

My solution is based on a few yet simple changes to my existing code.

First, I have all my repositories implement a simple marker interface:

/// <summary>
/// A base interface for all repositories to implement.
/// </summary>
public interface IRepository
{ }

Secondly, I let all my transaction enabled repositories implement the following interface:

/// <summary>
/// Provides methods to enable transaction support.
/// </summary>
public interface IHasTransactions : IRepository
{
    /// <summary>
    /// Initiates a transaction scope.
    /// </summary>
    void BeginTransaction();

    /// <summary>
    /// Executes the transaction.
    /// </summary>
    void CommitTransaction();
}

The idea is that in all my repositories I implement this interface and add code which introduces transaction directly depending on the actual provider (for fake repositories I have made a list of delegates which gets executed on commit). For LINQ to SQL it would be easy to make implementations such as:

#region IHasTransactions Members

public void BeginTransaction()
{
    _db.Transaction = _db.Connection.BeginTransaction();
}

public void CommitTransaction()
{
    _db.Transaction.Commit();
}

#endregion

This of course requires that a new repository class is created for each thread, but this is reasonable for my project.

Each method using the repository needs to invoke the BeginTransaction() and the EndTransaction(), if the repository implements IHasTransactions. To make this call even easier, I came up with the following extensions:

/// <summary>
/// Extensions for spawning and subsequently executing a transaction.
/// </summary>
public static class TransactionExtensions
{
    /// <summary>
    /// Begins a transaction if the repository implements <see cref="IHasTransactions"/>.
    /// </summary>
    /// <param name="repository"></param>
    public static void BeginTransaction(this IRepository repository)
    {
        var transactionSupport = repository as IHasTransactions;
        if (transactionSupport != null)
        {
            transactionSupport.BeginTransaction();
        }
    }

    public static void CommitTransaction(this IRepository repository)
    {
        var transactionSupport = repository as IHasTransactions;
        if (transactionSupport != null)
        {
            transactionSupport.CommitTransaction();
        }
    }
}

Comments are appreciated!