llx is a parallel execution shell. It provides a simple way to process an input list using a command template, maintaining a constant number of processes running in parallel at any given time. llx originally was a script to parallelize the transfer of a large number of files and was later generalized. llx supports rerunning failed processes and timing out on processes that exceed a user-defined threshold.
Try this simulation in bash(1):
while true; do echo $((RANDOM%8)); done | llx -v -n 4 -c 'sleep $1'