.NET TransactionScopes and SQL 2005 Lightweight Transaction Manager - Multiple Connections same SPID? - sql-server-2005

Question

Can someone shed light on what is happening behind the scenes with the SQL Lightweight transaction manager when multiple connections are opened to the same DB, using the Microsoft Data Access Application Block (DAAB)?
With the below code, we verified that MSDTC is indeed not required when opening 'multiple connections' to the same database.
This was the first scenario I tested: (where Txn1 and Txn2 use EntLib 4.1 to open a connection to the same DB and call different SPROCS)
using (var ts = new TransactionScope(TransactionScopeOption.Required))
{
DAL1.Txn1();
DAL2.Txn2();
ts.Complete();
}
Tracing this from profiler revealed that the same connection SPID was used for Txn1 and Txn2. After Txn1() was called, the Sql SPID would have been released back into the pool and Txn2() was able to re-use it.
However, when repeating this experiment and this time holding the connections open:
using (var ts = new TransactionScope(TransactionScopeOption.Required))
{
Database db1 = DatabaseFactory.CreateDatabase("db1");
DAL1.Txn1OnCon(db1);
Database db2 = DatabaseFactory.CreateDatabase("db1");
DAL2.Txn2OnCon(db2);
ts.Complete();
}
Viewing this from Profiler indicated that the 2 transactions were STILL using the same SPID. I was expecting the TransactionScope to have escalated to DTC as a distributed transaction should be required to control 2 concurrent connections. What have I missed?

Answer 1

Quoting from MSDN http://msdn.microsoft.com/en-us/library/8xx3tyca(VS.80).aspx
Connection pooling reduces the number
of times that new connections need to
be opened. The pooler maintains
ownership of the physical connection.
It manages connections by keeping
alive a set of active connections for
each given connection configuration.
Whenever a user calls Open on a
connection, the pooler looks to see if
there is an available connection in
the pool. If a pooled connection is
available, it returns it to the caller
instead of opening a new connection.
When the application calls Close on
the connection, the pooler returns it
to the pooled set of active
connections instead of actually
closing it. Once the connection is
returned to the pool, it is ready to
be reused on the next Open call.
Just because a connection was used in a transaction doesn't mean it cannot be available for the next call. I found that If the connection string varied by the slightest thing, such as capitalization of a hostname, then you'd get a new physical connection to the db.

Answer 2

Sql 2005 or Sql 2008?
If you use sql 2008, a sequence of open+close connections are not escalated to a distributed transaction. But all the connection must use exactly the same connection string.
(pseudo-code)
string connstring = "...."
using (TransactionScope ts=...)
{
c1 = new connection(connstring );
c1.open
...use c1
c1.close
c2 = new connection(connstring );
c2.open
...use c2
c2.close
ts.complete()
}
The same code with sql2005 escalates to distributed transaction --> yuo need MSDTC

Answer 3

OK, my misunderstanding was with DAAB.
The DAAB Database opens and closes connections as needed (or obtains / releases them from the pool), i.e. connections aren't held for the lifespan of the DAAB Database object.
It is possible to manually control the database connections in DAAB as per below - by holding the actual connections open, they cannot be reused. This then requires MSDTC to be running as soon as 2 physical connections are open, as I had expected in the original question.
using (TransactionScope ts = new TransactionScope(TransactionScopeOption.Required))
{
using (DbConnection dbConn1 = DatabaseFactory.CreateDatabase("myDb").CreateConnection())
using (DbConnection dbConn2 = DatabaseFactory.CreateDatabase("myDb").CreateConnection())
{
dbConn1.Open();
DAL1.Txn1OnCon(dbConn1);
dbConn2.Open();
DAL2.Txn2OnCon(dbConn2);
DAL1.Txn1OnCon(dbConn1);
ts.Complete();
}
}