What is a transaction?

Jerry Smith

A transaction is an abstraction of an atomic and reliable execution sequence.

Transaction processing is important for almost all modern computing environments that support concurrent processing. In a distributed computing environment, if, for example, multiple clients were to interact with the same database table concurrently, it's possible that interleaved database operations could leave the table in an inconsistent state.

Transaction support is also important because systems can fail at any point in an execution sequence. If, for example, a database update operation involves several dependent tables, and the computer system fails when the database update is only partially completed, the database could be left in an inconsistent, even inoperable, state.

File and database processing were essential operations with early multiprocessing computing systems; hence, transaction processing was necessary to guarantee their integrity. In today's complex distributed environments, transactions are fundamental to many distributed operations, for example, guaranteed delivery and ordering of a series of messages exchanged between two distributed application components. In this scenario, the message exchange should take place within an atomic execution sequence.

During an atomic execution sequence a concurrency control algorithm manages interleaved operations to achieve the same effect as executing those operations in serial order. This algorithm includes, or calls on, a recovery algorithm that provides the logic necessary to undo and/or redo partially completed operations, depending on the execution context.

Concurrency control and recovery algorithms are based in part on serializability theory.