NAME
smokeping_extend - Notes on extending Smokeping
OVERVIEW
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.
CHOOSING A BASE CLASS
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.
SKELETON FILE
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.
PROBE DOCUMENTATION
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
PROBE DESCRIPTION
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.
VARIABLES
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 smokeping_examples.
(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
Config::Grammar. 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.
Keys you must provide
_doc
Description of the variable.
_example
An example value. This will be used in the SYNOPSIS section in
the probe manual.
Optional keys
_default
A default value that will be assigned to the variable if none
is specified or inherited.
_re Regular expression upon which the value will be checked.
_re_error
String containing the returned error in case the regular
expression doesn’t match (if not specified, a generic ’syntax
error’ message will be returned).
_sub
A function pointer. It is called for every value, with the
value passed as its first argument. If the function returns a
defined value it is assumed that the test was not successful
and an error is generated with the returned string as content.
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.
INITIALIZATION
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.
PINGING
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!
EXAMPLE CONFIGURATIONS
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.
TIMEOUT HANDLING
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
Copyright 2005 by Niko Tyni.
LICENSE
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.
AUTHOR
Niko Tyni <ntyni@iki.fi>
BUGS
This document makes writing new probes look much harder than it really
is.
SEE ALSO
The other Smokeping documents, especially smokeping_config.