Multi-threaded message and event routing for the DO online system

The DO collider experiment at Fermilab will run with the data acquisition system operating at rates up to 50 Hz with 250 kbyte/event size within a multi-platform environment: Tru64-UNIX (OSF1), Linux, NT and VxWorks (and an IRIX development platform). The design is based on a distributed system using a messaging paradigm. Communication between processes is accomplished through the use of a multi-threaded client/server package, written in C++ using ACE, but also providing wrappers used in Python scripts for GUIs and high-level control. This paper describes the client/server package and the applications responsible for the data path from Level 3 to the mass storage system and to the event monitoring clients. The "Collector/Routers" are responsible for communicating with the Level 3 nodes and routing events to the appropriate "Data Logger" and "Data Distributor" processes. The Data Logger writes events to local buffer disks and interfaces to the mass storage framework in the Feynman Computing Center. The Data Distributor is a server process that per client maintains a queue of events matching a list of prescaled triggers and makes them available for monitoring purposes.