Monitoring generally means being alerted. However, monitoring can also be graphing, historical data, log centralization, log correlation altering, and so on. We're going to see some free and open source solutions here that can be useful, in addition to Nagios.
In the following sections, we won't dive deep into existing solutions, as that could cover several dedicated books; however, you will see what is the key to good monitoring and what solutions exist.
Using graphs to view history is very important. It helps to know what happened in the past and what the evolution of MariaDB was (or any other component).
For example, it can be interesting to have an idea of the number of threads during a year. It can help to understand if an application is usually updated, how is it evolving, and if modifications have to be made on the application side or on the MariaDB architecture side.
The most popular solution is called Cacti, which uses RRD to graph data. You can find...