[rrd-users] NEWBIE: Is RRDTools suitable for procession non-continuous data?

Alex van den Bogaerdt alex at ergens.op.het.net
Sun Jan 27 18:47:34 CET 2008

On Sun, Jan 27, 2008 at 06:11:57PM +0100, u7l11ey at mail.lrz-muenchen.de wrote:

> I want to log and graphically display data from my solar heating system.
> The system controller sends information about its input and output ports
> (e.g. temperature from solar circuit or power level of a pump) over a
> serial line in a kind of XML format, e.g.
> <OBJVAL>AF=58</OBJVAL> and so on, in a continuous data stream that has
> a new record appended for a parameter only when its value changes. These
> changes happen at very irregular intervals. The value for a temperature
> sensor may change every second, while the status of a valve may stay the
> same for days.

Either you need to remember the current value in your application, fetch
it from somewhere else, or create a database for each parameter.

> What I've learned so far about RRDTools that it expects values ("PDP") for
> its parameters ("DS") at regular intervals, otherwise it will store
> "UNKNOWN" for this parameter.

Mostly correct, but you forgot to mention heartbeat. I think you have
RTFM so I need not explain.

>                               This is obviously not what I need. I thought
> about using a small database (MySQL?) to cache the current value for each
> parameter, but since the whole system has to run on a NSLU2 with uNSLUng
> (or maybe Debian), I have to keep it as simple and processor-saving as
> possible.

One daemon which remembers values, substitutes new values when available,
and sends the entire package to rrdtool.  In case the daemon has no
knowlege (e.g. at startup time) read the last known value from rrdtool.

Or, as I suggested, have more than one database. As long as your updates
come in (at all), you can make this work.

> So can I use RRDTools for my purpose, or should I go on and look elsewhere?

I see no problems which cannot be solved.

Alex van den Bogaerdt

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