This paper describes a transaction processing monitor that is the basis of a transaction processing system used for marine terminal applications. This system was developed to run under BSD-family UNIX. The applications supported by the system include distributed database and machine control. The "event"-based model offered by this system is described, as well as the underlying architecture that implements it. In this paper, the word system will refer to the transaction processing system. The term application is used to denote a user-level program that is known to the system.