Process and data flow control in KLOE

The core of the KLOE distributed event building system is a switched network. The online processes are distributed over a large set of processors in this network. All processes have to change coherently their state of activity as a consequence of local or remote commands. A fast and reliable message system based on the SNMP protocol has been developed. A command server has been implemented as a non privileged daemon able to respond to "set" and "get" queries on private SNMP variables. This process is able to convert remote set operations into local commands and to map automatically an SNMP subtree on a user-defined set of process variables. Process activity can be continuously monitored by remotely accessing their variables by means of the command server. Only the command server is involved in these operations, without disturbing the process flow. Subevents coming from subdetectors are sent to different nodes of a computing farm for the last stage of event building. Based on features of the SNMP protocol and of the KLOE message system, the Data Flow Control System (DFC) is able to rapidly redirect network traffic, keeping in account the dynamics of the whole DAQ system in order to assure coherent subevent addressing in an asynchronous "push" architecture, without introducing dead time. The KLOE DFC is currently working in the KLOE DAQ system. Its main characteristics and performance are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.