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Joe Miller

Ops/Engineering. Continuous DevOping at Webscale

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In my previous article I mentioned a key strength of Sensu is the ability to re-use existing Nagios plugins. This is a powerful feature of Sensu. Nagios has been around for at least 1000 years according to most recent archaeological discoveries, which means a vast amount of human effort (and capital) has gone into creating Nagios plugins. Being able to leverage this prior effort is a huge win. In this article I’ll demonstrate creating a Sensu check with the check_http Nagios plugin.

Install the check_http nagios plugin

Following on the previous article, we’ll be doing this demo based off of CentOS 5 but it should not be difficult to find a build of check_http for your distribution.

First, Make sure you have the EPEL yum repo installed on your machine. Next, let’s install the nagios-plugin-http package onto our Sensu client node(s):

sudo yum -y install nagios-plugins-http

Let’s see where the check_http binary was installed (note the arch specific path):

$ rpm -ql nagios-plugins-http

Write the Sensu check definition

Ok, now we are ready to create a check definition for Sensu. We will create this file on the nodes running sensu-client as well as sensu-server (pro tip: this part is easier if you’re using a CM tool like Chef or Puppet.)


  "checks": {
    "check_google": {
      "notification": "Google HTTP failed",
      "command": "PATH=$PATH:/usr/lib64/nagios/plugins:/usr/lib/nagios/plugins check_http -R 'search'",
      "subscribers": ["webservers"],
      "interval": 60,
      "handlers": ["default", "pagerduty"]

Let’s look at each part of this check definition:

  • notification: This can be thought of as a “friendly message”. It’s most useful with handlers like Twitter or Pagerduty. For example, this will be what you hear when the creepy computer voice wakes you up at 3 AM. The full output from the check is also available to handlers in the output attribute.
  • command: This is where we specify the check_http command we want to run and relevant options. Notice that we are adding both /usr/lib64/... and /usr/lib/... to the PATH. This is not required, but it is a nice way to make this check work on both i386 and x86_64 platforms for the EPEL-supplied version of check_http.
  • subscribers: This is where we specify which sensu-client nodes should run this check. Recall that we defined "subscriptions": "webservers" in our /etc/sensu/conf.d/client.json in the previous article.
  • interval: How often this check should be executed.
  • handlers: The handlers that should receive the output from this plugin. The handlers will be executed on the sensu-server node.

Turn a profit

After we’ve created the json file on the clients and servers, we need to restart the sensu-client and sensu-server services so they pickup the new .json file.

In a couple minutes, you should see the following in /var/log/sensu/sensu-server.log:

I, [2012-01-23T19:14:22.218301 #7397] INFO -- : [publisher] -- publishing check request -- check_google -- webservers {"message":"[publisher] -- publishing check request -- check_google -- webservers","level":"info","timestamp":"2012-01-23T19:14:22. %6N-0700"}

And on the client, in /var/log/sensu/sensu-client.log:

I, [2012-01-23T19:15:22.224437 #18787] INFO -- : [subscribe] -- received check request -- check_google {"message":"[subscribe] -- received check request -- check_google","level":"info","timestamp":"2012-01-23T19:15:22. %6N-0700"}
I, [2012-01-23T19:15:22.348884 #18787] INFO -- : [result] -- publishing check result -- check_google -- 0 -- HTTP OK: HTTP/1.0 200 OK - 11294 bytes in 0.099 second response time |time=0.099463s;;;0.000000 size=11294B;;;0

There we go. We have implemented a Sensu check using a Nagios plugin. There are quite a few Nagios plugins out there and we should be able to use most (all?) of them with Sensu. Go forth and profit.

For questions on using Sensu, stop by the #sensu channel on Freenode.