Transaction Promotion
Applies to: 
SQL Server
Transaction promotion describes a lightweight, local transaction that can be automatically promoted to a fully distributable transaction as needed. When a managed stored procedure is invoked within a database transaction on the server, the common language runtime (CLR) code is run in the context of a local transaction. If a connection to a remote server is opened within a database transaction, the connection to the remote server is enlisted into the distributed transaction and the local transaction is automatically promoted to a distributed transaction. So, transaction promotion minimizes the overhead of distributed transactions by deferring the creation of a distributed transaction until it is needed. Transaction promotion is automatic, if it has been enabled using the Enlist keyword, and does not require intervention from the developer. The .NET Framework Data Provider for SQL Server provides support for transaction promotion, handled through the classes in the .NET Framework System.Data.SqlClient namespace.
The Enlist Keyword
The ConnectionString property of a SqlConnection object supports the Enlist keyword, which indicates whether System.Data.SqlClient detects transactional contexts and automatically enlists the connection in a distributed transaction. If this keyword is set to true (the default), the connection is automatically enlisted in the current transaction context of the opening thread. If this keyword is set to false, the SqlClient connection does not interact with a distributed transaction. If Enlist is not specified in the connection string, the connection is automatically enlisted in a distributed transaction if one is detected at the time the connection is opened.
Distributed Transactions
Distributed transactions typically consume significant system resources. Microsoft Distributed Transaction Coordinator (MS DTC) manages such transactions, and integrates all of the resource managers accessed in these transactions. Transaction promotion, on the other hand, is a special form of a System.Transactions transaction that effectively delegates the work to a simple SQL Server transaction. System.Transactions, System.Data.SqlClient, and SQL Server coordinate the work involved in handling the transaction, promoting it to a full distributed transaction as needed.
The benefit of using transaction promotion is that when a connection is opened with an active TransactionScope transaction, and no other connections are opened, the transaction commits as a lightweight transaction, rather than incurring the additional overhead of a full distributed transaction. For more information about TransactionScope, see Using System.Transactions.