=head1 NAME 

rrd-beginners - Beginners guide

=head1 SYNOPSIS

Helping new RRDTool users to understand the basics of RRDTool

=head1 DESCRIPTION

This manual is an attempt to assist beginners in understanding the concepts
of RRDTool. It sheds a light on differences between RRDTool and other
databases. With help of an example, it explains structure of RRDTool
database. This is followed by an overview of the "graph" feature of RRDTool.
At the end, it has sample scripts that illustrates the
usage/wrapping of RRDTool within Shell or Perl scripts.

=head2 What makes RRDTool so special?

RRDTool is GNU licensed software developed by Tobias Oetiker, a system
manager at the Swiss Federal Institute of Technology. Though it is a
database, there are distinct differences between RRDTool database and other
databases as listed below:

=over

=item *

RRDTool stores data; that makes it a back end tool. The RRDTool command set
allows the creation of graphs; that makes it a front end tool as well. Other
databases just stores data and can not create graphs.

=item *

In case of linear databases, new data gets appended at the bottom of
the database table. Thus its size keeps on increasing, whereas size of an RRDTool
database is determined at creation time. Imagine an RRDTool database as the
perimeter of a circle. Data is added along the perimeter. When new data
reaches the starting point, it overwrites existing data. This way, the size of
an RRDTool database always remains constant. The name "Round Robin" stems from this
attribute.

=item *

Other databases store the values as supplied. RRDTool can be configured to
calculate the rate of change from the previous to the current value and
store this information instead.

=item *

Other databases get updated when values are supplied. The RRDTool database
is structured in such a way that it needs data at predefined time
intervals. If it does not get a new value during the interval, it stores an
UNKNOWN value for that interval. So, when using the RRDTool database, it is
imperative to use scripts that runs at regular intervals to ensure a constant
data flow to update the RRDTool database.

=back
  
RRDTool has a lot to do with time. With every data update, it also needs to
know the time when that update occurred. Time is always expressed in
seconds passed since epoch (01-01-1971). RRDTool can be installed on Unix as
well as Windows. It has command set to carry out various
operations on RRD database. This command set can be accessed from the command line,
and from Shell or Perl scripts. The scripts
act as wrappers for accessing data stored in RRDTool database.
  
=head2 Understanding by an example

The structure of an RRD database is different than other linear databases.
Other databases define tables with columns, and many other parameters. These
definitions sometime are very complex, especially in large databases.
RRDTool databases are primarily used for monitoring purposes and
hence are very simple in structure. The parameters
that need to be defined are variables that hold values and archives of those
values. Being time sensitive, a couple of time related parameters are also
defined. Because of its structure, the definition of an RRDTool database also
includes a provision to specify specific actions to take in the absence of
update values. Data Source (DS), heartbeat, Date Source Type (DST), Round
Robin Archive (RRA), and Consolidation Function (CF) are some of the
terminologies related to RRDTool databases.

The structure of a database and the terminology associated with it can be
best explained with an example.

 rrdtool create target.rrd
         --start 1023654125
         --step 300
         DS:mem:GAUGE:600:0:671744
         RRA:AVERAGE:0.5:12:24
         RRA:AVERAGE:0.5:288:31

This example creates a database named F<target.rrd>. Start time (1023654125) is
specified in total number of seconds since epoch (time in seconds since
01-01-1970). While updating the database, update time is also specified.
This update time MUST occur after start time and MUST be in seconds since
epoch.

The step of 300 seconds indicates that database expects new values every
300 seconds. The wrapper script should be scheduled to run every B<step>
seconds so that it updates the database every B<step> seconds.

DS (Data Source) is the actual variable which relates to the parameter on
the device that has to be monitored. Its syntax is

 DS:variable_name:DST:heartbeat:min:max

B<DS> is a key word. C<variable_name> is a name under which the parameter is
saved in database. There can be as many DSs in a database as needed. After
every step interval, a new value of DS is supplied to update the database.
This value is also called as Primary Data Point B<(PDP)>. In our example
mentioned above, a new PDP is generated every 300 seconds.

Note, that if you do NOT supply new datapoints exactly every 300 seconds,
this is not problem, RRDTool will interpolate the data accordingly.

B<DST> (Data Source Type) defines type of DS. It can be COUNTER, DERIVE,
ABSOLUTE, GAUGE. A DS declared as COUNTER will save the rate of change of
the value over a step period. This assumes that the value is always
increasing (difference between last two values is more than 0). Traffic
counters on a router is an ideal candidate for using COUNTER as DST. DERIVE
is same as COUNTER but it allows negative values as well. If you want to see
the rate of I<change> in free diskspace on your server, then you might want to
use the DERIVE data type. ABSOLUTE also saves the rate of change but it assumes
that previous value is set to 0. The difference between current and previous
value is always equal to the current value. So, it stores the current value divided
by step interval (300 seconds in our example). GAUGE does not save the rate of
change. It saves the actual value itself. There are no
divisions/calculations. Memory consumption in a server is an ideal
example of gauge. Difference among different types DSTs can be explained
better with following example:

 Values       = 300, 600, 900, 1200
 Step         = 300 seconds
 COUNTER DS   =    1,  1,   1,    1 
 DERIVE DS    =    1,  1,   1,    1
 ABSOLUTE DS  =    1,  2,   3,    4 
 GAUGE DS     = 300, 600, 900, 1200

The next parameter is B<heartbeat>. In our example, heartbeat is 600
seconds. If database does not get a new PDP within 300
seconds, it will wait for another 300 seconds (total 600 seconds).
If it doesnt receive any PDP with in 600 seconds, it will save an UNKNOWN value
into database. This UNKNOWN value is a special feature of RRDTool - it is
much better than to assume a missing value was 0 (zero).
For example, the traffic flow counter on a router
keeps on increasing. Lets say, a value is missed for an interval and 0 is stored
instead of UNKNOWN. Now when next value becomes available, it will calculate
difference between current value and previous value (0) which is not
correct. So, inserting value UNKNOWN makes much more sense here.

The next two parameters are the minimum and maximum value respectively. If variable
to be stored has predictable maximum and minimum value, this should be
specified here. Any update value falling out of this range will be saved as
UNKNOWN.

The next line declares a round robin archive (RRA). The syntax for declaring an RRA is

 RRA:CF:xff:step:rows

RRA is the keyword to declare RRAs. The consolidation function (CF) can be
AVERAGE, MINIMUM, MAXIMUM, and LAST. The concept of the consolidated data point (CDP)
comes into the picture here. A CDP is CFed (averaged, maximum/minimum value or
last value) from I<step> number of PDPs. This RRA will hold I<rows> CDPs.

Lets have a look at the example above. For the first RRA, 12 (steps) PDPs
(DS variables) are AVERAGEed (CF) to form one CDP. 24 (rows) of theses CDPs
are archived. Each PDP occurs at 300 seconds. 12 PDPs represent 12 times 300
seconds which is 1 hour. It means 1 CDP (which is equal to 12 PDPs)
represents data worth 1 hour. 24 such CDPs represent 1 day (1 hour times 24
CDPs). It means, this RRA is an archive for one day. After 24 CDPs, CDP
number 25 will replace the 1st CDP. Second RRA saves 31 CDPs; each CPD
represents an AVERAGE value for a day (288 PDPs, each covering 300 seconds =
24 hours). Therefore this RRA is an archive for one month. A single database
can have many RRAs. If there are multiple DSs, each individual RRA will save
data for all the DSs in the database. For example, if a database has 3 DSs;
and daily, weekly, monthly, and yearly RRAs are declared, then each RRA will
hold data from all 3 data sources.

=head2 Graphical Magic

Another important feature of RRDTool is its ability to create graphs. The
"graph" command uses "fetch" command internally to retrieve values from the
database. With the retrieved values, it draws graphs as defined by the
parameters supplied on the command line. A single graph can show different
DS (Data Sources0) from a database. It is also possible to show the
values from more than one databases into a single graph. Often, it is
necessary to perform some math on the values retrieved from database, before
plotting them. For example, in SNMP replies, memory consumption values are
usually specified in KBytes and traffic flow on interfaces is specified in
Bytes. Graphs for these values will be more senseful if values are
represented in MBytes and mbps. the RRDTool graph command allows to define
such conversions. Apart from mathematical calculations, it is also possible
to perform logical operations such as greater than, less than, and if then
else. If a database contains more than one RRA archive, then a question may
arise - how does RRDTool decide which RRA archive to use for retrieving the
values? RRDTool takes looks at several things when making its choice. First
it makes sure that the RRA covers as much of the graphing time frame as
possible. Second it looks at the resolution of the RRA compared to the
resolution of the graph. It tries to find one which has the same or better
resolution. With the "-r" option you can force RRDTool to assume a different
resolution than the one calculated from the pixel width of the graph.

Values of different variables can be presented in 5 different shapes in a
graph - AREA, LINE1, LINE2, LINE3, and STACK. AREA is represented by a solid
colored area with values as the boundary of this area. LINE1/2/3 (increasing
width) are just plain lines representing the values. STACK is also an area
but it is "stack"ed on AREA or LINE1/2/3. Another important thing to note,
is that variables are plotted in the order they are defined in graph
command. So, care must be taken to define STACK only after defining
AREA/LINE. It is also possible to put formatted comments within the graph.
Detailed instructions be found under graph manual.

=head2 Wrapping RRDTool within Shell/Perl script

After understanding RRDTool, it is now a time to actually use RRDTool in
scripts. Tasks involved in network management are data collection, data
storage, and data retrieval. In the following example,
the previously created target.rrd database is used. Data collection and data
storage is done using Shell scrip. Data retrieval
and report generation is done using Perl script. These
scripts are as shown below:

=head3 Shell script (collects data, updates database)

 #!/bin/sh
 a=0
 while [ "$a" == 0 ]; do
 snmpwalk -c public 192.168.1.250 hrSWRunPerfMem > snmp_reply
     total_mem=`awk 'BEGIN {tot_mem=0}
                           { if ($NF == "KBytes")
                             {tot_mem=tot_mem+$(NF-1)}
                           }
                     END {print tot_mem}' snmp_reply`
     # I can use N as a replacement for the current time
     rrdtool update target.rrd N:$total_mem
     # sleep until the next 300 seconds are full
     perl -e 'sleep 300 - time % 300'
 done # end of while loop

=head3 Perl script (retrieves data from database and generates graphs and statistics)

 #!/usr/bin/perl -w
 #This script fetch data from target.rrd, creates graph of memory consumption
 on target (Dual P3 Processor 1 GHz, 656 MB RAM)

 #calling RRD perl module
 use lib qw( /usr/local/rrdtool-1.0.41/lib/perl ../lib/perl );
 use RRDs;
 my $cur_time = time();                # setting current time
 my $end_time = $cur_time - 86400;     # setting end time to 24 hours behind current time
 my $start_time = $end_time - 2592000; # setting start time to 30 days from end time

 #fetching average values from RRD database between start and end time
 my ($start,$step,$ds_names,$data) = 
     RRDs::fetch("target.rrd", "AVERAGE", 
                 "-r", "600", "-s", "$start_time", "-e", "$end_time");
 #saving fetched values in 2-dimensional array
 my $rows = 0;
 my $columns = 0;
 my $time_variable = $start;
 foreach $line (@$data) {
   $vals[$rows][$columns] = $time_variable;
   $time_variable = $time_variable + $step;
   foreach $val (@$line) {
           $vals[$rows][++$columns] = $val;}
   $rows++;
   $columns = 0;
 }
 my $tot_time = 0;
 my $count = 0;
 #saving values from 2-dimensional into 1-dimensional array
 for $i ( 0 .. $#vals ) {
     $tot_mem[$count] = $vals[$i][1];
     $count++;
 }
 my $tot_mem_sum = 0;
 #calculating total of all values
 for $i ( 0 .. ($count-1) ) {
     $tot_mem_sum = $tot_mem_sum + $tot_mem[$i];
 }
 #calculating average of array
 my $tot_mem_ave = $tot_mem_sum/($count);
 #creating graph
 RRDs::graph ("/images/mem_$count.gif",   \
             "--title= Memory Usage",    \
             "--vertical-label=Memory Consumption (MB)", \
             "--start=$start_time",      \
             "--end=$end_time",          \
             "--color=BACK#CCCCCC",      \
             "--color=CANVAS#CCFFFF",    \
             "--color=SHADEB#9999CC",    \
             "--height=125",             \
             "--upper-limit=656", 	 \
             "--lower-limit=0",          \
             "--rigid",                  \
             "--base=1024",              \
             "DEF:tot_mem=target.rrd:mem:AVERAGE", \
             "CDEF:correct_tot_mem=tot_mem,0,671744,LIMIT,UN,0,tot_mem,IF,1024,/",\
             "CDEF:machine_mem=tot_mem,656,+,tot_mem,-",\
             "COMMENT:Memory Consumption between $start_time",\
             "COMMENT:    and $end_time                     ",\
             "HRULE:656#000000:Maximum Available Memory - 656 MB",\
             "AREA:machine_mem#CCFFFF:Memory Unused",   \
             "AREA:correct_tot_mem#6699CC:Total memory consumed in MB");
 my $err=RRDs::error;
 if ($err) {print "problem generating the graph: $err\n";}
 #printing the output
 print "Average memory consumption is ";
 printf "%5.2f",$tot_mem_ave/1024;
 print " MB. Graphical representation can be found at /images/mem_$count.gif.";

=head1 AUTHOR

Ketan Patel E<lt>k2pattu@yahoo.comE<gt>

