The first most valuable feature are the notifications that can be customized and even received via WhatsApp.
Another valuable feature is the reporting. As far as I know, there's no way to cheat on the reporting, that is, there's no way to go into the system to change the results. This makes the reporting feature very reliable. The reports are also very easy to understand, which is good when I present them to my boss.
Lastly, Nagios is not a resource hog. I can set it up on a busy server and it will still function reliably. This allows sysadmins to keep server maintenance costs low.
Improvements to My Organization
I can give an example. It was during a seasonal festival and visitors to our e-commerce site increase several-fold. The log partition quickly filled up within two days. If it wasn't for Nagios' alerts every minute until we acknowledged the problem, our website would have stopped working. (I can't remember why the logrotate didn't work, though.)
Room for Improvement
I like to have the option to configure Nagios using the web interface. Although I agree that the CLI gives a lot of customization options, I'd like to take a break from looking at lines of words. Also, configuration via a web interface could be expanded to not-so-Linux-literate users.
There have been no issues with the deployment.
I did encounter stability issues when exploring plugins, but not with Nagios itself. Other than that, I never faced any issues on the production side.
There have been no issues scaling it for our needs.
Customer Service and Technical Support
Since Nagios is open source, I had to rely completely on forums and web articles. However, Nagios was set up before I joined the company, so my colleagues were able to give me ample support when trying to understand how it works.
I never used a different solution because this current position is my first. Nagios was already set up before I joined the company. Nagios was already good enough for us so we didn't allocate time to research other products.
The initial setup is easy if you just follow the basic guide. The complexity comes when you want to customize it to suit your environment. For example, different plugins require different configurations. There's also another challenge in that Nagios was originally designed to monitor Linux servers but has since expanded to Windows servers as well.
It was all done by us. We were given time to do our own research and through regular testing, trials and errors, we finally implemented it. My advice is to not be scared by the need to configure everything through the CLI. It's actually quite fun and rewarding when you see your monitoring system finally up and you know you can count on it to give you a heads up on alerts before something nasty happens to your server.
Nagios is able to minimize server downtime and this in turn helps to generate more revenue.
Pricing, Setup Cost and Licensing
Nagios is open sourced, therefore there's no need for licensing.
The product is robust and reliable. The notifications can be customized so that I can even configure it to send the notifications via WhatsApp! Last but not least, the reporting feature is very easy to understand, which is good when presenting to my boss.
Disclosure: I am a real user, and this review is based on my own experience and opinions.
May 02 2016