cron file 000-delay.cron (it is on my FC5)

jtmoon · 03-07-2007, 06:12 PM

I found the file 000-delay.cron.
It introduces a per-machine static sleep before cron jobs.
My guess it that putting a random wait for cron jobs for all *nix machines reduces traffic jams.

For example, what if all machines in a network ran the cron job download-huge-file.cron at the same time? It would cause a big lag for the whole network!

Does anyone else have thoughts on why the file 000-delay.cron exists?

Code:

$ cat /etc/cron.daily/000-delay.cron
#!/bin/bash

# Generate per system static delay for cron.{daily,weekly,monthly}

# (P) & (C) 2006 by Peter Bieringer <pb@bieringer.de>
#
# 20060322/PB: initial release

# License: GPLv2

factor=1        # max. ~ 68 minutes
#factor=2       # max. ~ 34 minutes

# Create md5sum of hostname (static over system lifetime)
md5sum="`echo ${HOSTNAME} | md5sum`"

# Extract the first 3 hexdigits (12 Bit: 0-4095)
hexvalue="${md5sum:0:3}"

# Create decimal value
decvalue="`printf "%d" "0x${hexvalue}"`"

# Divide delay by factor
DELAY=$[ ${decvalue} / ${factor} ]

sleep $DELAY
exit 0

-JTomMoon79

This was also posted here:
http://lists.linuxcoding.com/rhl/2007q1/msg04528.html

macemoneta · 03-08-2007, 11:15 AM

It's function is to prevent unintentional denial of service attacks on servers. For example, you have 4000 machines, and daily (in the cron.daily directory) they run ntpdate. Without 000-delay.cron, all 4000 machines would hit the time server at exactly the same time (probably causing it grief, or even killing it).

The 000-delay.cron delay is only introduced at the begining of the sequential execution of all jobs in the cron.daily, cron.weekly and cron.monthly directories.