Figure 
shows how the DD system is used in the E787 experiment.
Up to 16 data reducer ( dr) receive the individual events from FASTBUS over
the SGI VME port. They decode the trigger information of the event, request
empty events from the DD system (i.e. read the INPUT_FIFO), copy the
event into the free shared memory location and inject the event into the DD system.
Up to 10 data loggers share the events and write them to tape. These data
loggers are syncronized at the beginning and end of a run as well as the
end of every spill.
Several monitor programs are attached to the DD system on an optional basis.
Typical examples are online event display, processes that monitor the detector
performance and the DAQ, online calibration programs and various user programs. Some of these
programs are launched at the beginning of every run and exit at the end of a run.
Typically 50-60 processes are active at any given time.
The DD system is started and deleted by a control program ( dd_ctl) which accepts commands from its
standard input (pipe to the main DAQ user interface. (Note: The use of this
control program is optional. In general the first DD master (here ( dd_ctl) creates
the DD system, the last one (in this example also dd_ctl) deletes it.).
Figure:
Overview of the DD system in the E787 experiment.