Think of a transaction processing application as making a series of database updates. These updates can be made online or in a batch. An online update is a message that comes from a client. This is a general model that includes applications where users login to a host via terminal emulation sessions.

A batch update can be internally driven from the database, such as end-of-day processing, or externally driven such as when processing checks from a clearinghouse. The processing model in each case is a transaction or a unit of work initiated by client input such as a request to transfer funds from one account to another, or as the next logical unit of work such as posting interest on the next account.