Thursday 11 April 2013

Cron scheduling for first Sunday of the month

For everyone who uses cron, you are familiar with the job schedule form:

min hr day-of-month month day-of-week <command>

A problem with cron job scheduling is if you want to schedule something, like backups or updates, for "the first Sunday of the month".  The job spec "0 0 1-7 * Sun" will run every Sunday and every day on the 1st to the 7th of the month.

The way to work around this is schedule the job for the possible days to run and then as part of the command, check the date before running the command.  I've just seen what is The Best format for this:

0 9 1-7 * * [ "$(date '+%a')" == "Sun" ] && <path>/

This solution comes from user kakapo in the post here:

Up until now I used a slightly different form of this using the day of the week in the cron job and then testing  date %d to test the day of the month.  But the above form is far clearer and easier to schedule jobs with.

So props to kakapo for sharing that form and until cron changes how the day-of-the-month and the day-of-the-week fields are used, this will be the best way to schedule a job on the first Sunday of the month.

Friday 5 April 2013

Nagios and SensorProbe

Nagios rules all and has great agents and plugins and also checks clusters.  One of the things we have got recently at #dayjob is a generator for emergency power.  Well, how do you know if the generator is running, is healthy, etc?  Sure, there's routine checks that someone has to do, weekly, monthly, seasonally, etc but how do you know what's going on in real-time?

Like much machinery, generators don't always have a nice web-based monitoring system or even an SNMP interface - they have "dry contacts".  Dry contacts, for those of you like me who think a voltmeter is something used by parking enforcement, is a simple electrical circuit which is either "open" and not passing current or "closed" and passing usually 5 volts.  Usually it is something like a screw which you thread a sensor through.

Okay, so no IP interface.  What do you use?  Well, we're using these AKCP SensorProbe devices which come in a variety of shapes and sizes.  For our generator we used the SensorProbe2DC to which you can connect two sensors supporting 5 dry contacts each.  It's a little IP device you need to feed a data run and power to and then you screw in the leads from the sensor to the dry contacts on the generator - your installer can help with the latter.

The SensorProbe is a monitoring device so you can configure the alerts and view status from there.  But the other thing is that it has an SNMP interface so you can now monitor your generator status from your regular network monitoring system, by which I mean of course Nagios.  Good old check_snmp, tell it which OID is which, and you're off to the races!

So now we've got in Nagios a host (the sensorpobe) which has service checks telling us whether the transfer switch is on mains or generator power, if the generator is running, if the generator is set to "auto start", or having any other problems.  

AKCP makes various other SensorProbe devices.  The SensorProbe8 has 8 ports to which you can connect various sensors for temperature and humidity, airflow, water detectors, etc or single-port dry-contacts.  If you look hard, you'll find also that your AC units and other equipment also have dry contacts. Avid readers who crack their equipment manuals will also find that dry contacts can also be used as output triggers not just receiving status.  Once you have a lot of dry contacts, you can check out the SensorProbe8 x20 and x60 which come with a whole lot more dry contacts to check everything in your datacenter.

Mmm, generator power, yum.

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