Transactions (Database Engine)
A transaction is a sequence of operations performed as a single logical unit of work. A logical unit of work must exhibit four properties, called the atomicity, consistency, isolation, and durability (ACID) properties, to qualify as a transaction.
SQL programmers are responsible for starting and ending transactions at points that enforce the logical consistency of the data. The programmer must define the sequence of data modifications that leave the data in a consistent state relative to the organization's business rules. The programmer includes these modification statements in a single transaction so that the SQL Server Database Engine can enforce the physical integrity of the transaction.
It is the responsibility of an enterprise database system, such as an instance of the Database Engine, to provide mechanisms ensuring the physical integrity of each transaction. The Database Engine provides:
Locking facilities that preserve transaction isolation.
Logging facilities that ensure transaction durability. Even if the server hardware, operating system, or the instance of the Database Engine itself fails, the instance uses the transaction logs upon restart to automatically roll back any uncompleted transactions to the point of the system failure.
Transaction management features that enforce transaction atomicity and consistency. After a transaction has started, it must be successfully completed, or the instance of the Database Engine undoes all of the data modifications made since the transaction started.