Nested transaction

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With reference to a database transaction, a nested transaction occurs when a new transaction is started by an instruction that is already inside an existing transaction. The new transaction is said to be nested within the existing transaction, hence the term.

Changes made by the nested transaction are not seen by the 'host' transaction until the nested transaction is committed. This follows from the isolation property of transactions.

The capability to handle nested transactions properly is a prerequisite for true component based application architectures. In a component-based encapsulated architecture, nested transactions can occur without the programmer knowing it. A component function may or may not contain a database transaction (this is the encapsulated secret of the component. See Information hiding). If a call to such a component function is made inside a BEGIN - COMMIT bracket, nested transactions occur. Since popular databases like MySQL do not allow nesting BEGIN - COMMIT brackets, a framework or a transaction monitor is needed to handle this. When we speak about nested transactions, it should be made clear that this feature is DBMS dependent and is not available for all databases.

Theory for nested transactions is similar to the theory for flat transactions, and was introduced in the following paper:

Resende, R.F. & El Abbadi, A. (1994-05-25), “On the serializability theorem for nested transactions”, Information Processing Letters 50 (4): 177-183, DOI 10.1016/0020-0190(94)00033-6

The banking industry usually processes financial transactions using Open Nested Transactions, which is a looser variant of the nested transaction model that provides higher performance while accepting the accompanying trade-offs of inconsistency. Open Nested Transactions are discussed in the following paper:

Weikum, Gerhard; Hans-J. Schek (1992). "Concepts and Applications of Multilevel Transactions and Open Nested Transactions". Database Transaction Models for Advanced Applications: 515-553. Morgan Kaufmann.