Difference between Implicit and Explicit Transaction

Question

What is the difference between Implicit and Explicit transaction in Sql Server 2008?

What happens in TransactionScope background? I'm using TransactionScope but in Sql server profiler I don't see "Begin transaction..." statement.

How does it work?

Answer 1

• Implicit Transactions: http://msdn.microsoft.com/en-us/library/ms188317.aspx
• SET IMPLICIT_TRANSACTIONS { ON | OFF} http://msdn.microsoft.com/en-us/library/ms187807.aspx

Basically, in c# when you set the TransactionScope to Implicit, it calls the SQL Server SET command to put the connection in IMPLICIT_TRANSACTIONS mode. Anything that you do (using one of the commands listed in the 2nd link) starts a transaction that is kept open until a commit is issued. If no commit is issued at the end of a connection, an implicit ROLLBACK is performed.

This differs from the OFF setting, which also puts every statement into a transaction - the difference is that in the OFF mode (therefore transactions are explicit), each transaction (singular statement) is immediately committed.

Answer 2

In Explicit transaction mode, you will need to start a transaction explicitly. In Implicit transaction mode, a transaction is automatically started after each commit. So you will only have to commit.

Since the transaction is started 'implicitly', you will not see an explicit 'BEGIN' in the logs. :)

By default the database operates in explicit transaction mode with autocommiting transactions enabled. That actually meand that unless an explicit transaction is started using BEGIN TRANSACTION, every data modification is started in a separate transaction which is committed after the statement. That allows the database to rollback an entire statement when it fails (for instance a bulk insert, or an insert that modifies other data in a trigger).