Cacti operation may be divided into three different tasks:
First task is to retrieve data. Cacti will do so using its Poller. The Poller is executed from the operating system's scheduler, e.g. crontab for Unix flavored OSes.
In current IT installations, you're dealing with lots of devices of different kind, e.g. servers, network equipment, appliances and the like. To retrieve data from remote targets/hosts, cacti will mainly use the Simple Network Management Protocol SNMP. Thus, all devices capable of using SNMP will be eligible to be monitored by cacti.
Later on, we demonstrate how to extend cacti's capabilities of retrieving data to scripts, script queries and more.
There are lots of different approaches for this task. Some may use an (SQL) database, others flat files. Cacti uses RRDTool to store data.
RRD is the acronym for Round Robin Database. RRD is a system to store and display time-series data (i.e. network bandwidth, machine-room temperature, server load average). It stores the data in a very compact way that will not expand over time, and it can create beautiful graphs. This keeps storage requirements at bay.
Likewise, rrdtool will perform some specific tasks. It performs consolidation to combine raw data (a primary data point in rrdtool lingo) to consolidated data (a consolidated data point). This way, historical data is compressed to save space. rrdtool knows different consolidation functions: AVERAGE, MAXIMUM, MINIMUM and LAST.
One of the most appreciated features of RRDTool is the built-in graphing function. This comes in useful when combining this with some commonly used webserver. Such, it is possible to access the graphs from merely any browser on any plattform.
Graphing can be done in very different ways. It is possible, to graph one or many items in one graph. Autoscaling is supported and logarithmic y-axis as well. You may stack items onto another and print pretty legends denoting characteristics such as minimum, average, maximum and lots more.