I’ve been meaning to set up a network monitoring tool at work for a while. We have a couple of different applications using various technologies (currently mainly Ruby on Rails and PHP), running on various VPS servers. While we are using Monit to keep an eye on our Rails apps and restart them if necessary, as well as a couple of custom webpages to track vital and growth stats of our apps, we currently don’t use any monitoring or (perhaps more importantly) alerting tools beyond that. After one of our PHP / MySQL apps stopped responding (due to the fact that we ran out of disk space, as we later discovered), I figured it was about time to put some more sophisticated network monitoring in place.

The de-facto standard application seems to be Nagios, which is quite powerful and configurable, but has an extremely steep learning curve. It also does not offer a friendly UI for configuring services and relies on static configuration files instead. There is also a newer crop of network monitoring apps that I was hoping might be a bit less daunting to get up to speed on, such as Zenoss (here is a brief overview of open source network monitoring apps). I downloaded a few of these, but ultimately realized that I would not be able to get my head wrapped around any of these apps purely using intuition, and that I would actually have to invest a fair amount of time to master at least the basics. I’m simply not enough of an operations expert… It is clear that all of these apps are extremely powerful, and probably great for larger deployments, but I really just needed a simple tool to check some operating system level vital stats or ping some URLs, for a handful of machines.

That’s when I remembered Scout, a hosted network monitoring service that launched fairly recently and that sounded very interesting when I first came across it. Their subscription plans are pretty reasonable (the $29/month plan for 4 servers should suffice for us at this stage), particularly given that we would have had to pay for an additional VPS slice or EC2 instance to host Nagios or some other deployed solution anyway. Best of all, Scout offers a free plan, and even though this only supports a single server, this is a good way to evaluate how well it works for our purposes.

Scout uses an interesting approach at monitoring servers. Rather than using SNMP or an agent that is continuously running on each server, Scout uses a lightweight client (installed via a Ruby gem) that needs to be run periodically (10 minutes being the minimal reporting interval), generally via a cron job. Once the client app is installed on each server to be monitored, the servers don’t need to be touched for future configuration changes. Instead, everything is configured on the Scout website, and pulled down by the client the next time it checks in. The entire configuration consists of a number of plugins that can be installed for each client. Out of the box, Scout supports around 20 plugins that range from basic monitoring tasks for server load or disk space to more specific plugins for Ruby on Rails, Mongrel, or MySQL.

Even better, Scout offers a very simple Plugin API for integrating your own plugins. Plugins are written in Ruby and mainly consist of a single method that either returns a bunch of stats as a hash, which is exposed by Scout both in tabular report and graph form, or triggers an alert in case of a problem. Since plugins have the full Ruby stack at their disposal, it is easy to write a plugin that shells out to a Unix command, performs an HTTP request, hits a database, or anything else you can think of.

One minor downside is that (as far as I can tell), there is no way to simply upload a plugin. Instead, Scout relies on a pull mechanism, which means that we would need to expose any proprietary plugins via a publicly accessible URL. This might be an issue if the plugin itself contains sensitive information, although settings (such as passwords or paths) can be decoupled from the code and configured via the web interface. While not ideal, putting the plugin code in a publicly accessible but not automatically discoverable location and only making it available for the duration of the initial download or future updates should minimize this risk and turn it into a minor inconvenience.

Based on my initial impression, Scout looks very promising. The reporting functionality is fairly basic, and particularly the graphs could perhaps use a bit more polish, but everything is very easy to use. Scout is clearly geared towards developers rather than sysadmins, so perhaps that is why it appeals to me. If your monitoring needs are relatively straightforward and you don’t need all the functionality that a deployed solution like Nagios offers, Scout is definitely worth a look, at least for relatively small deployments. I am not sure how well it scales beyond 16 servers (both in terms of administration and pricing), so it is possible that a deployed application might make more sense at that point.