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 ACID (Atomicity, Consistency, Isolation, and Durability) properties, to qualify as a transaction:
A transaction must be an atomic unit of work; either all of its data modifications are performed, or none of them is performed.
When completed, a transaction must leave all data in a consistent state. In a relational database, all rules must be applied to the transaction's modifications to maintain all data integrity. All internal data structures, such as B-tree indexes or doubly-linked lists, must be correct at the end of the transaction.
Modifications made by concurrent transactions must be isolated from the modifications made by any other concurrent transactions. A transaction either sees data in the state it was in before another concurrent transaction modified it, or it sees the data after the second transaction has completed, but it does not see an intermediate state. This is referred to as serializability because it results in the ability to reload the starting data and replay a series of transactions to end up with the data in the same state it was in after the original transactions were performed.
After a transaction has completed, its effects are permanently in place in the system. The modifications persist even in the event of a system failure.
Specifying and Enforcing Transactions
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 then includes these modification statements in a single transaction so that Microsoft® SQL Server™ can enforce the physical integrity of the transaction.
It is the responsibility of an enterprise database system, such as SQL Server, to provide mechanisms ensuring the physical integrity of each transaction. SQL Server provides:
- Locking facilities that preserve transaction isolation.
- Logging facilities that ensure transaction durability. Even if the server hardware, operating system, or SQL Server itself fails, SQL Server 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 SQL Server undoes all of the data modifications made since the transaction started.