Metrics Fast ‘n Easy, Part II: actually using RRDTool from Ruby

Continued from part I:

Now that the RRD bundle is installed in Ruby’s default load path, I require RRD and access the convenience methods. The methods basically pass all parameters in as strings, which is fine, but I don’t like thinking of time and values as strings if I can avoid it. So I wrote a wrapper class that allows me to pass in values as typed options, and then casts them to the internal strings.

A couple of notes about creating, updating, and rendering RRD graphs using the built in Ruby binding.

Creating an RRD graph

At create time, the first parameter is the name of the file, minus the .rrb extension (create will puke if you specify the extension). The start time is expressed in seconds, my code below passed it in as a Time object and converts it to seconds. The step time is the minimal amount of time an update can occur at — in other words, if your step is 50 seconds and you try to update at 10 seconds, you get an error.

The DS option defines a dataset as follows:

DS:[name]:[graph type]:[min time to show an error condition]:[min value or unknown]:[max value or unknown]. More explanation of the suitable graph types is found here.

"--start", "#{@start.to_i}",
"--step", "#{@step}",

In the example above, I only create a single dataset. You can create 1..N, although I’m sure N has an upper limit, I haven’t found it specified anywhere. Also, I believe the data set is restricted to < 19 characters in length. The RRA section syntax is as follows:
RRA:AVERAGE | MIN | MAX | LAST:xff:steps:rows
where the collapsing is done by averaging values, or min/maxing values, the xff value specified limits unknown values from being collapsed by establishing a max ratio of unknown values to known values. The steps value specifies the number of datapoints collapsed, and the rows value specifies how many collapsed datapoints to keep. So RRA is where you really get a chance to limit the size of the RRD file.

Updating an RRD Graph

Once the graph has been created, it exists as the file you specified using the name parameter above. You update it with time:value statements: in the code below, I’m updating an array of time:value statements:
# simple update of multiple values
def update(times, values)

for i in 0..times.length-1


In the code above, as for all RRD operations, you specify the name of the RRD file you want to operate on in the first parameter.

Note that you cannot update a graph with a time less than it’s start time or a time that is less then the last time + the step time specified at creation.

Displaying an RRD Graph:

Graph display is the most complex operation with RRD. I’m not going to go into all of the details: some really good examples are found here.

I’ve taken the simplest approach to displaying a graph:

"--title", title,
"--start", start.to_i.to_s,
"--end", finish.to_i.to_s,
"--imgformat", "PNG",

Unlike the update method, the name of the actual desired graph is the first parameter, not the name of the RRD file. The RRD file to load is specified in the DEF line. You can specify multiple DEF values to display dataset from different RRD graphs. You will need to specify the way you want each dataset rendered: in the above example, I define a value a with the DEF statement that I reference in the following LINE statement:

In order to render data, you will need to specify how you want to display it with( as a line, as area under a line, as a tick mark, etc). More details about how to define data sets, including creating datasets via the CDEF statement, are found in the graph data documentation. Details about how to display data are in the rrdgraph method documentation. The format of the DEF, CDEF, LINE statements is RPN, i.

Make sure to specify start and end in a way that shows values as you would like to see them, i.e. make sure your latest value is in the specified start and end range.

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