I have done this by running the cronjob on all hosts that COULD be
running the service, but run it through a wrapper that just checks to
see if the service is active at the time. For example, below is a
script (sadly, from linux 2.4, but it would be just a regular expression
change away from a current kernel) that can be called as 'ismyname' and
then given a floating hostname and then a comnand. I have it in my
crontab as:
00 20 * * * root /usr/local/sbin/ismyname <floating-hostname> <Command
to run>
When this runs, checks to see if we have the hostname running on this
host at this time, and if so, runs the command. If we do not have the
hostname on this host, then we are a backup server for that service and
it exits out without running the command.
Here is my 'ismyname' script in case it's useful, but beware that it's
several years old and begins with spelling errors -- it is far more of a
quick tool than an example of elegant code :-) Still, it works fine
and is a good starting point if it helps.
-Ty!
=======================SNIP===========================
#!/usr/local/bin/perl -w
# Description:
#
# Generic wrapper for one command to run another, that
# preservs arguments in the process
#
# Check to see if we have a given network address on this host,
# and run the command only if we do have it. This is useful
# for our high availabilty servers with floating IP's, so that
# cron jobs (for example) can be run on both primary and backup
# machines, but only do real work on the active machine.
#
# THIS IS THE LINUX 2.4 VERSION. ifconfig parses different under
# other os's.
#
################################################################
#
# Configurables:
#
# Add /sbin to the path
$ENV{PATH}='/sbin:'.$ENV{PATH};
# End of Configurables
################################################################
#
# USE statements and associated controls
#
use strict;
use diagnostics;
use FileHandle;
STDOUT->autoflush();
# End of USE statements
################################################################
#
# Main
#
#Put any wrapper customizations in here:
my $IP;
my $HOSTNAME=shift;
if (!defined $HOSTNAME)
{
Useage();
}
else
{
my $addr=gethostbyname($HOSTNAME);
if (defined $addr)
{
my($a,$b,$c,$d)=unpack('C4', $addr);
$IP="$a.$b.$c.$d";
}
else
{
$IP="NOIP";
}
}
if ($IP !~ /^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/)
{
print "$HOSTNAME is not a valid host!\n\n";
Useage();
}
my $ifcfgline="";
open IFCONFIG, "ifconfig -a|" or die "Cannot run ifconfig\n";
while (my $line=<IFCONFIG>)
{
chomp $line;
$ifcfgline.=$line;
}
close IFCONFIG;
# Now, if we cannot find an "UP" interface with our address, bail out
if ($ifcfgline !~ /inet addr\:$IP.{1,100}UP/)
{
exit;
}
# Process the arguements into a string
my $arglist="";
my $cmd=shift;
for my $arg (@ARGV)
{
$arglist=$arglist.quotemeta($arg)." ";
}
#Run the desired command
exec("$cmd $arglist");
sub Useage
{
print "\nUseage: ismyname <HOSTNAME> <command> [[arg] ...]\n";
print "or ismyname <IPADDR> <command> [[arg] ...]\n\n";
print "This tool will check if we have the given ip address running\n";
print "on an interface on this host, and if so, runs the given
command.\n";
exit;
}
=======================SNIP===========================
Yan Seiner wrote:
I am trying to figure out how to set up a failover system that runs apps
from crontab.
In my current setup, the server starts a cron job every 2 hours that
reaches out to my clients and pulls data. This must be a server-pull;
there is no way to set it up as a client-push. The data that's pulled
will be replicated using rsync, coda, etc to the backup server. (I have
not gotten that far yet.)
I'd like to set this up as a failover system. Apache sounds easy, but how
do I failover a cron job?
Also, is it possible to have the two servers in different geographic
locations? Ie. to do the failover to another server, in another country?
--Yan
--
-===========================-
Ty! Boyack
NREL Unix Network Manager
[email protected]
(970) 491-1186
-===========================-
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