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I have a cluster of N nodes which contains some distributed information. The query i want to monitor with nagios make sense to execute only on one node of the cluster (any). But i don't want to overload one particular node, so i want nagios to select every time random node from the list of nodes.

So, I would expect to see something like:

define service{
    service_description    Recent document time for the cluster
    hostgroup_name         random
    ...
}

where random hostgroup somehow defines random node from a predefined list of cluster nodes.

Is this possible to do with nagios3? Any suggestions?

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5 Answers 5

up vote 4 down vote accepted

Could you set up DNS to give you a random host for a certain name?

You could also write a custom service check.

If you REALLY want this feature in nagios...hack the source

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Just what I was going to suggest - the only issue being how you avoid hitting a host which is currently marked as failed in Nagios. –  symcbean Mar 5 '13 at 17:31
    
um ... very interesting. I gonna try this. The avoiding of failed host is not a big problem, since it's OK for me if this check also fails ... –  gakhov Mar 6 '13 at 9:08

I don't believe this is possible to do natively via nagios. But, it is possible to do via a custom check script, implemented in your language of choice. Just have the check script choose a random (or round-robin, etc.) node each time it is run.

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yeah, this is how we do it right now, but i thought there was another better solution do to it natively, since it requires to store cluster nodes' IPs in 2 places ... once in nagios for other queries and once in this script. –  gakhov Mar 5 '13 at 17:11

Not to my knowledge, if you're running a custom plugin that you've written, it should be pretty trivial to add an option that randomly pulls from a number of passed hosts, and only test one of them.

You do mention there are N nodes, I don't suppose these all sit behind a VIP of some kind, that only points at one node, and moves to a different box as needed?

A bit more of a description of the setup would be helpful, if we can't find anything, I'd be happy to help you come up with something even if it's custom.

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One other solution for this is to run the check on each node server and offsetting the times via a task scheduler/cron, then have the checks output the results to a central location. You can have the Nagios check look at the results in that file accordingly.

That way you wouldn't have to change anything in Nagios when adding a new node, and it could just be part of your node setup.

It is not the cleanest solution, but could help you deal with this.

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I have a check_nrpe wrapper that does this. I create a host object with the 'name' objective set to something like 'RANDOM-001' (also supports 'RNDRBN-001'). When the check_nrpe wrapper sees this, it uses the trailing digits to pick from a Perl list of server hostnames to send the check to. Then just add this as hostname to the Service Check description. A more recent version of the wrapper grabs the hostnames from the Nagios hostgroups.cfg file.

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1  
Can you show with some code how you're doing this? As it stands this answer isn't that useful. More details would make it much more helpful to others. –  slm Mar 6 '13 at 17:50
    
I will try. Don't want to upset the work/personnel code gap. –  Jim Black Mar 8 '13 at 4:46
    
Please try blackthorne.net/mycode/check_nrpe_random.pl.html –  Jim Black Mar 9 '13 at 23:20

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