empty_events.fcl

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#
# File:    empty_events.fcl
# Purpose: Generates empty events with a timestamp.
# Date:    March 15, 2017
# Author:  Gianluca Petrillo (petrillo@fnal.gov)
# Description:
# 
# An empty event with a timestamp is a container whose purpose is to "fix" the
# random numbers generated for this event. This is achieved on jobs which use
# the "perEvent" policy of NuRandomService, which bases its random seeds on
# event timestamp.
# 
# Note that if the timestamp plugin takes the timestamp from the system clock,
# that clock must have a very good resolution, since art will take little time
# from an event to the next, and their tiem stamps must be different.
# This should not be a problem with `GeneratedEventTimestamp` plugin, whose
# clock resolution is supposed to be at the nanosecond level.
# Also note that if files are produced in different jobs, there is a faint
# chance that two empty events have the same time stamp. If the events have
# same time stamp *and* event ID, they'll get the same random seed and will be
# duplicate of each other.
#
# Dependencies: none
#
# Example:
# The command:
#     
#     lar -c empty_events.fcl -n 100 -o 'emptyevents-%tc.root'
#     
# will create 100 empty events in a file named 'emptyevents-%tc.root', where %tc
# is replaced by `RootOutput` module with the usual creation time of the file.
# Note that the creation time is in seconds, so for batch jobs you have to make
# sure the output file names are actually different.
# 

source: {
  module_type: EmptyEvent
  timestampPlugin: { plugin_type: "GeneratedEventTimestamp" }
}