Monitoring site errors using Server Density and Python

I recently stumbled upon the site Server Density while upgrading all my servers and signed up for the service. It as a great pretty interface, a customizable dashboard, and pretty extensive alert capabilities. They also have a free trial so I suggest signing up. The one thing I did not like was the inability to scan for a specific string on the website to confirm if the website is still operational, like with

The good news is Server Density allows you to create python plugins to run at the update interval on any/all of your servers. This gives you free reign over your reports. Using this tutorial, I will explain how to manually create and install this and any other plugin. Or you can just download or install it from the below links.

GitHub -

Server Density Plugin -

Create your plugin

First, log into your SD account and go to Plugins. Then add a new plugin and name it WebCheck.

Add your plugin to your server

Next you will need to ssh into your server and create the directory

mkdir /usr/bin/sd-agent/plugins/

In that directory, create a new file called This will be the main and only file for the plugin. Grab the code from and paste it into the file.

Configure your plugin

If this is your first plugin, you will need to add your plugin directory to your plugin config file. Open up /etc/sd-agent/config.cfg and replace the plugin_directory line with

plugin_directory: /usr/bin/sd-agent/plugins

WebCheck allows you to check the status of as many sites as you want. In order to check they you will need to enter a new config entry for each site, using the below fields:

  • name - The name and key you want to use on the SD interface. Remember that all "." will be replaced with "_" due to api limitations.
  • url -
  • find - The regex or string to search for. I typically just use an html tag but you can search for anything.
  • result - The result is the regex quantifier. Lets say you want to search from </html>, you would keep this True. Or lets say you want to search for Parse Error, you would then make this False.

Here is an example config entry

[WebCheck 1]
find: </html>
result: True

Restart the agent service to load the changes. Once it is restart it will begin sending data to SD

/etc/init.d/sd-agent restart

Settings up alerts

Because WebCheck only returns a boolean result, you probably wont be using it with graphs much. The alerts, however, are extremely useful. I personally recommend the iPhone push notifications, as they are uber fast.

  1. In your Server Density account, go to Alerts and click add new alert.
  2. Select your server that you installed it on, who is going to get it, and the alert type.
  3. For Check, select WebCheck.
  4. For Key, enter the name of your config entry. For me it is devin_la.
  5. For Trigger threshold, set it to Less than 1.
  6. Add it and you now know when your server is messed!

I love it!

One of my developers was working on a site and pushed some naughty changes to one of the sites I had just set up with this, and within seconds i got a push notification that it was broken. He fixed it pretty quick and it then alerted me when it was all fixed! Please share your cool plugins with me!