subtransaction

Subtransaction is a nested transactional context that operates within a parent transaction. It provides a mechanism to group a portion of work so that it can be rolled back independently from the rest of the transaction, while sharing the same transactional resources and isolation level.

In many database systems, subtransactions are implemented using savepoints. Creating a subtransaction marks a point to which the system can revert. If the subtransaction is rolled back, only changes since its start are undone; if the outer transaction eventually commits, the subtransaction changes become durable as part of the global commit. Some systems support explicit nested transactions with their own commit and rollback boundaries; others treat subtransactions as savepoints where commits are not independent and only a rollback to a savepoint or release of the savepoint is meaningful.

Usage scenarios include multi-step operations that affect multiple resources. Subtransactions allow error handling to be more granular: if a later step fails, the system can undo only the work within the subtransaction while leaving earlier steps intact, thereby preserving partial progress without aborting the entire transaction.

Limitations and considerations include variability in semantics across database systems, limited portability, and potential complexity in error handling. True independent durability of a subtransaction is not always available; many implementations only provide partial rollback capabilities within the outer transaction. Performance overhead and resource management can also arise from maintaining nested transactional state.

See also: savepoint, nested transaction, two-phase commit, distributed transactions.