SQL Server clients can control transaction-isolation levels for a connection. To control transaction-isolation level, the SQL Server Native Client OLE DB provider consumer uses:
DBPROPSET_SESSION property DBPROP_SESS_AUTOCOMMITISOLEVELS for the SQL Server Native Client OLE DB provider default autocommit mode.
The SQL Server Native Client OLE DB provider default for the level is DBPROPVAL_TI_READCOMMITTED.
The isoLevel parameter of the ITransactionLocal::StartTransaction method for local manual-commit transactions.
The isoLevel parameter of the ITransactionDispenser::BeginTransaction method for MS DTC-coordinated distributed transactions.
SQL Server allows read-only access at the dirty read isolation level. All other levels restrict concurrency by applying locks to SQL Server objects. As the client requires greater concurrency levels, SQL Server applies greater restrictions on concurrent access to data. To maintain the highest level of concurrent access to data, the SQL Server Native Client OLE DB provider consumer should intelligently control its requests for specific concurrency levels.
SQL Server 2005 introduced snapshot isolation level. For more information, see Working with Snapshot Isolation.