.. -*- mode: rst -*-

.. _server-plugins-probes-index:

======
Probes
======

At times you need to gather information from a client machine before you
can generate its configuration. For example, if some of your machines
have both a local scratch disk and a system disk while others only have
the system disk, you would want to know this information to correctly
generate an `/etc/auto.master` autofs config file for each type. Here
we will look at how to do this.

First, create a ``Probes`` directory in our toplevel repository
location::

    mkdir /var/lib/bcfg2/Probes

This directory will hold any small scripts we want to use to grab
information from client machines.  These scripts can be in any scripting
language; the shebang line (the ``#!/usr/bin/env some_interpreter_binary``
line at the very top of the script) is used to determine the script's
interpreter.

.. note::

    Bcfg2 uses python mkstemp to create the Probe scripts on the
    client. If your /tmp directory is mounted **noexec**, you will
    likely need to modify the :envvar:`TMPDIR` environment variable so
    that the bcfg2 client creates the temporary files in a directory
    from which it can execute.

.. note::

    .. versionadded:: 1.3.0

    A probe script must exit with a return value of 0.  If it exits
    with a non-0 return value, the client will abort its run.  This
    behavior can be disabled by setting ``exit_on_probe_failure = 0``
    in the ``[client]`` section of ``bcfg2.conf``.

Now we need to figure out what exactly we want to do.  In this case,
we want to hand out an ``/etc/auto.master`` file that looks like::

    /software  /etc/auto.software --timeout 3600
    /home      /etc/auto.home --timeout 3600
    /hometest  /etc/auto.hometest --timeout 3600
    /nfs       /etc/auto.nfs --timeout 3600
    /scratch   /etc/auto.scratch --timeout 3600

for machines that have a scratch disk. For machines without an extra disk,
we want to get rid of that last line::

    /software  /etc/auto.software --timeout 3600
    /home      /etc/auto.home --timeout 3600
    /hometest  /etc/auto.hometest --timeout 3600
    /nfs       /etc/auto.nfs --timeout 3600

So, from the Probes standpoint we want to create a script that counts
the number of SCSI disks in a client machine. To do this, we create a
very simple ``Probes/scratchlocal`` script:

.. code-block:: bash

    grep -c Vendor /proc/scsi/scsi

Running this on a node with *n* disks will return the number *n+1*, as
it also counts the controller as a device. To differentiate between the
two classes of machines we care about, we just need to check the output
of this script for numbers greater than 2. We do this in the template.

.. note::

    This example uses :ref:`server-plugins-generators-cfg-cheetah`,
    but Cheetah templates are **not** required in order for Probes to
    operate properly.

For the template we will want to create a ``Cfg/etc/auto.master``
directory to hold the template of the file in question. Inside of this
template we will need to check the result of the Probe script that got
run and act accordingly. The
``Cfg/etc/auto.master/auto.master.cheetah`` file looks like::

    /software  /etc/auto.software --timeout 3600
    /home      /etc/auto.home --timeout 3600
    /hometest  /etc/auto.hometest --timeout 3600
    /nfs       /etc/auto.nfs --timeout 3600
    #if int($self.metadata.Probes["scratchlocal"]) > 2
    /scratch   /etc/auto.scratch --timeout 3600
    #end if

Any Probe script you run will store its output in
``$self.metadata.Probes["scriptname"]``, so we get to our
`scratchlocal` script's output as seen above.  (See `Handling Probe
Output`_, below, for more information on how this is done.)  Note that
we had to wrap the output in an `int()` call; the script output is
treated as a string, so it needs to be converted before it can be
tested numerically.

With all of these pieces in place, the following series of events will
happen when the client is run:

#. Client runs
#. Server hands down our ``scratchlocal`` probe script
#. Client runs the ``scratchlocal`` probe script and hands its output
   back up to the server
#. Server generates ``/etc/auto.master`` from its template, performing
   any templating substitutions/actions needed in the process.
#. Server hands ``/etc/auto.master`` down to the client
#. Client puts file contents in place.

Now we have a nicely dynamic ``/etc/auto.master`` that can gracefully
handle machines with different numbers of disks. All that's left to do
is to add the ``/etc/auto.master`` to a Bundle:

.. code-block:: xml

    <Path name='/etc/auto.master'/>

Handling Probe Output
=====================

Bcfg2 stores output from probes in the ``Probes`` property of a
client's metadata object.  To access this data in
:ref:`server-plugins-generators-cfg-genshi`, for instance, you could
do::

  ${metadata.Probes['script-name']}

This is not the full output of the probe; any lines that start with
"group:" have been stripped from the output.  The data is a
string-like object that has some interesting and salient features:

* If the data is a valid XML document, then
  ``metadata.Probes['script-name'].xdata`` will be an
  ``lxml.etree._Element`` object representing the XML data.
* If the data is a valid JSON document, and either the Python ``json``
  or ``simplejson`` module is installed, then
  ``metadata.Probes['script-name'].json`` will be a data structure
  representing the JSON data.
* If the data is a valid YAML document, and either the Python ``yaml``
  or ``syck`` module is installed, then
  ``metadata.Probes['script-name'].yaml`` will be a data structure
  representing the YAML data.

If these conditions are not met, then the named properties will be
``None``.  In all other fashions, the probe data objects should act
like strings.

Host- and Group-Specific probes
===============================

Bcfg2 has the ability to alter probes based on client hostname and group
membership. These files work similarly to files in Cfg.

If multiple files with the same basename apply to a client, the most
specific one is used. Only one instance of a probe is served to a given
client, so if a host-specific version and generic version apply, only
the client-specific one will be used.

If you want to to detect information about the client operating system,
the :ref:`server-plugins-probes-ohai` plugin can help.

.. _server-plugins-probes-data-storage:

Data Storage
============

.. versionadded:: 1.3.0

The Probes plugin stores the output of client probes locally on the
Bcfg2 server in order to ensure that probe data and groups are
available on server startup (rather than having to wait until all
probes have run every time the server is restarted) and to
:ref:`bcfg2-info <server-bcfg2-info>` and related tools.  There are
two options for storing this data: ``Probes/probed.xml``, a plain XML
file stored in the Bcfg2 specification; or in a database.

Advantages and disadvantages of using the database:

* The database is easier to query from other machines, for instance if
  you run ``bcfg2-info`` or ``bcfg2-test`` on a machine that is not
  your Bcfg2 server.
* The database allows multiple Bcfg2 servers to share probe data.
* The database is likely to handle probe data writes (which happen on
  every client run) more quickly, since it can only write the probes
  whose data has changed.
* The database is likely to handle probe data reads (which happen only
  on server startup) more slowly, since it must query a database
  rather than the local filesystem.  Once the data has been read in
  initially (from XML file or from the database) it is kept in memory.

To use the database-backed storage model, set ``use_database`` in the
``[probes]`` section of ``bcfg2.conf`` to ``true``.  You will also
need to configure the :ref:`server-database`.

The file-based storage model is the default, although that is likely
to change in future versions of Bcfg2.

Other examples
==============

.. toctree::
   :maxdepth: 1

   current-kernel
   group
   vserver
   grub-serial-order
   manufacturer
   producttype
   serial-console-speed

Other Probing plugins
=====================

.. toctree::

   ohai
   fileprobes