Reinventing the wheel
Simpler solutions aren't always obvious. When looking for a way to monitor a parameter, one is always tempted to re-invent the wheel, e.g. to create a custom script every time the exiting monitor types are not supported.Definitely, it's not the best idea. First of all, it takers time. Second, it might be an inefficient solution. Third, it might be non-portable: if you wish to cerate the same monitor type for another host, you might have to create a similar script from scratch.
When it comes to monitoring the simplest parameters of the server, such as CPU load, memory usage and so on, there is no need to create a complex scheme of running remote scripts/applications and reporting data back to the monitor.
The magic abbreviation is SNMP, Simple Network Management Protocols. Let's explain, briefly, how it can be used to monitor a number of system-level parameters of a server. A Linux-powered server is assumed, although in this given case most of other operating system can be monitored through the same facility.
SNMP
SNMP daemon isn't running by default; refer to longer how-tos such as Monitoring server performance for more details on installing the daemon.Take care when setting up the daemon: it can support a number of protocols; if you don't wish to handle security-relate issues when using version 3of SNMP, you may use v1 or v2c, but keep in mind their security level is but basic, and if you don't restrict, by other means, who is granted access to the daemon (restrict to localhost, if monitoring from the same server), you are virtually giving all the important data to whoever wish to gain unauthorized access to the server.
Try to restrict access to read-only, there is hardly need to grant write access to monitoring software.
It's easy to find the OIDs (object identifiers) of the data you wish to monitor; i.e., to allow viewing general system information such as RAM usage, grant access to .1.3.6.1.4.1 hierarchy.
Most of the data you could use are numeric; thus, the SNMP-based monitors available at IP Host Network Monitor software can be used to create very precise monitors able to reflect the level of resource usage without creating sever-based scripts and communicating with them.
Write access
Note that certain variables (OIDs) can be writable, thus allowing to control, to some extent, the device your monitoring software is connected to via SNMP.Note only that SNMP is supported by many devices, such as routers, and it can be used, say, to programmatically restrict or even close access to them/set usage limits base upon parameters being monitored. Say, you can restrict or limit transfer speed for ethernet cards if a data transferred cross a limit. However, it is strongly advised that the network monitoring software does never modify any device's settings it monitors.
