Is there something already available server/client?
Certainly, yes.
Here's what I've found on the subject... There are many options, as you might expect. And certainly more than those I've found and listed here..
Munin
As explained here:
Munin the tool surveys all your computers and remembers what it saw.
It presents all the information in in graphs through a web interface.
Its emphasis is on plug and play capabilities. After completing a
installation a high number of monitoring plugins will be playing with
no more effort. Using Munin you can easily monitor the performance of
your computers, networks, SANs, and quite possibly applications as
well. It makes it easy to determine “what’s different today” when a
performance problem crops up.
Munin is available from apt. Personally, I'd try Munin first.
Nagios2
As explained here:
Nagios is an open source host, service and network monitoring program.
It is composed by the main service and a set of plugins which makes it
very flexible as they can be developed by anyone. Nagios provides a
web interface for the user to monitor but it also is capable of
sending emails or even messages to a pager in case of problems.
You can install it via apt:
sudo apt-get install nagios2 nagios-plugins nagios-images
There are some additional configuration steps to follow, please refer to the above link.
Alternatively...
Logwatch
As explained here:
Logwatch is a tool that will monitor your server's logs and email the
administrator a digest on a daily basis.
Logwatch is also available from apt:
sudo apt-get install logwatch
Note that you'll also need to setup sendmail or something so it can email you reports.
If I were to make my own server monitoring web application, I may use logwatch as a part of a backend for that...
Cacti
As explained here:
Cacti is a complete network graphing solution designed to harness the
power of RRDTool's data storage and graphing functionality. Cacti
provides a fast poller, advanced graph templating, multiple data
acquisition methods, and user management features out of the box. All
of this is wrapped in an intuitive, easy to use interface that makes
sense for LAN-sized installations up to complex networks with hundreds
of devices.