smokeping_extend - Notes on extending Smokeping
This document is intended to guide prospective authors in writing new Smokeping probes. It mostly describes the interface between Smokeping and its probe modules. If it seems too complicated to understand, look at the existing modules for examples.
Comments and proposed changes or additions are welcome. Please send them to the smokeping-users mailing list. Patches against the POD source of this document are most appreciated.
The first thing you should decide is which base class you should use for your probe. For most (if not all) uses it's a choice between Smokeping::probes::base and Smokeping::probes::basefork. The former is intended for probes that can measure their targets all in one go, while the latter is for probing them one at a time, possibly in several concurrent subprocesses.
At the moment, the only probes that use Smokeping::probes::base
are the FPing
derivatives. All the others use Smokeping::probes::basefork
, and chances are
you should too. This document will thus concentrate on the latter case.
The Smokeping::probes::skel module is a non-functional probe that is intended
to make a good basis for a new probe module. Copy the file,
lib/probes/skel.pm
, to a new name and just fill out the blanks :)
Note that the names of real probe modules must start with a capital letter.
The probe documentation is generated from the source code with the
smokeping arguments --man
or --makepod
. The embedded
POD documentation should point to this real documentation, so
that curious users of the perldoc
command see what's going on.
All the current probes do this.
You should provide the method pod_hash
that returns a reference to
a hash with keys corresponding to the section names you want in the
manpage. The supported section names are
name
, overview
, description
, authors
, notes
, bugs
, and
see_also
. If you don't need a particular section, just leave it out.
The special sections synopsis
and variables
are automatically
generated from the description of your variables. See below.
Note that if you use 'here documents' ('<<') that have POD markup inside, you should escape the markup so that it doesn't show up in the embedded POD documentation. Most probes do it like this:
my $e = "="; my $doc = <<DOC; ${e}head1 SECTION TITLE DOC
The probe should offer the ProbeDesc
method that returns a short
description of what it does. This will be used in the graphs produced
by the web frontend.
All Smokeping probes must define their variables by implementing a
probevars
method for probe-specific variables and a targetvars
method for target-specific variables. If you don't know the difference
between these yet, see the smokeping_examples manpage.
(The probes that are derived from Smokeping::probes::base
don't support
target-specific variables, so they only use the probevars
method.)
The base classes offer these methods too to provide the variables that
are common to all the probes (eg. the probe-specific step
variable
and the target-specific pings
variable. If you don't want to add
anything to the base class variables (perhaps because all your variables
are of a target-specific nature, so you don't need new probe-specific
variables at all), you can leave the corresponding method out and it
will be inherited from the base class.
When you do supply your own probevars
or targetvars
method, you should
combine your variables with those coming from the superclass. There is a
convenience method called _makevars
that does this, and the common idiom is
sub probevars { my $class = shift; return $class->_makevars($class->SUPER::probevars, { # your variables go here } }
The variables are declared in a syntax that comes from the module used
for parsing the configuration file, Config::Grammar
. Each variable
should be a hash that uses the ``special variable keys'' documented in
the Config::Grammar manpage. See Smokeping::probes::skel
and the other
probes for examples.
For reference, here are the keys the hash should have. Much of this
is taken straight from the Config::Grammar
manual.
The probevars
and targetvars
methods should return hash references
that contain the variable names as keys and the hashes described above
as values. In addition the Config::Grammar
special section key
_mandatory
is supported and should contain a reference to a list of
mandatory variables. The _makevars
method is aware of this special
key and merges the mandatory lists in its arguments. Note that no other
Config::Grammar
special section keys are supported.
If you must do something at probe initialization time, like check that
the external program you're going to use behaves as you expect, you should
do it in the new
method. You should probably also take care that
you don't run the tests needlessly while in CGI mode. The usual way to
do this is to test for $ENV{SERVER_SOFTWARE}. See the Smokeping::probes::skel
module for an example.
All the real action happens in the pingone
method (or, for
Smokeping::probes::base
-derived probes, in the ping
method.) The arguments
for pingone
are $self
, the module instance (since this is a method)
and $target
, the target to be probed.
You can access the probe-specific variables here via the
$self->{properties}
hash and the target-specific ones via the
$target->{vars}
hash. You get the number of pings needed for
the target via the pings
method: my $count = $self->pings($target)
.
You should return a sorted array of the latency times measured. If a ping fails, don't put anything in the array.
That's it, you're done!
If you would like to provide a documented example configuration for your probe (in addition to the automatically generated SYNOPSIS section in the probe manual), you can do so by adding it to the Smokeping::Examples module. Look for the 'examples' subroutine and add your example there.
Future versions of Smokeping might provide a way to embed examples in the probe modules too. The author's motivation for implementing this would be greatly increased by even a single demand for it, so please speak up if you think you'd use it.
If you deal with timeouts (for example because your program offers a parameter for specifying the timeout for the pings), you should know a few things.
First, there's timeout logic in Smokeping::probes::basefork
that kills the probe
when the timeout is reached. By default the timeout is (# of pings *
5 seconds) + 1 second. If you expect that your pings can take longer,
you should modify the default value of the probe-specific variable timeout
.
This would be done like this:
sub probevars { my $class = shift; my $h = $class->SUPER::probevars; $h->{timeout}{_default} = 10; # override the superclass default return $class->_makevars($h, { # your variables go here } }
If you want to provide a target-specific timeout
setting, you should
delete the probe-specific variable and be sure to provide a default for
your target-specific one. See eg. Smokeping::probes::AnotherDNS
for an example of
how this is done.
Providing a target-specific timeout
will make the timeout in
Smokeping::probes::basefork
be (# of pings * the maximum timeout of all targets)
+ 1 second. The 1 second is added so that the own timeout logic of the
probe has time to kick in even in the worst case (ie. all pings are lost)
before Smokeping::probes::basefork
starts killing the processes.
Copyright 2005 by Niko Tyni.
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
Niko Tyni <ntyni@iki.fi>
This document makes writing new probes look much harder than it really is.
The other Smokeping documents, especially the smokeping_config manpage.