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      reclass — recursive external node classification
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reclass is © 2007–2013 martin f. krafft <madduck@madduck.net>
and available under the terms of the Artistic Licence 2.0
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reclass is an "external node classifier" (ENC) as can be used with automation
tools, such as Puppet, Salt, and Ansible.

The purpose of an ENC is to allow a system administrator to maintain an
inventory of nodes to be managed, completely separately from the configuration
of the automation tool. Usually, the external node classifier completely
replaces the tool-specific inventory (such as site.pp for Puppet,
ext_pillar/master_tops for Salt, or /etc/ansible/hosts).

reclass allows you to define your nodes through class inheritance, while
always able to override details of classes further up the tree. Think of
classes as feature sets, as commonalities between nodes, or as tags. Add to
that the ability to nest classes (multiple inheritance is allowed,
well-defined, and encouraged), and piece together your infrastructure from
smaller bits, eliminating redundancy and exposing all important parameters to
a single location, logically organised.

In general, the ENC fulfills two jobs:

  - it provides information about groups of nodes and group memberships
  - it gives access to node-specific information, such as variables

In this document, you will find an overview of the concepts of reclass and the
way it works. Have a look at README.Salt and README.Ansible for information
about integration of reclass with these tools.

Installation
~~~~~~~~~~~~
Before you can use reclass, you need to run make to configure the scripts to
your system. Right now, this only involves setting the full path to the
Python interpreter.

  make

If your Python interpreter is not /usr/bin/python and is also not in your
$PATH, then you need to pass that to make, e.g.

  make PYTHON=/opt/local/bin/python

reclass concepts
~~~~~~~~~~~~~~~~
reclass assumes a node-centric perspective into your inventory. This is
obvious when you query reclass for node-specific information, but it might not
be clear when you ask reclass to provide you with a list of groups. In that
case, reclass loops over all nodes it can find in its database, reads all
information it can find about the nodes, and finally reorders the result to
provide a list of groups with the nodes they contain.

Since the term 'groups' is somewhat ambiguous, it helps to start off with
a short glossary of reclass-specific terminology:

  node:         A node, usually a computer in your infrastructure
  class:        A category, tag, feature, or role that applies to a node
                Classes may be nested, i.e. there can be a class hierarchy
  application:  A specific set of behaviour to apply to members of a class
  parameter:    Node-specific variables, with inheritance throughout the class
                hierarchy.

A class consists of zero or more parent classes, zero or more applications,
and any number of parameters.

A node is almost equivalent to a class, except that it usually does not (but
can) specify applications.

When reclass parses a node (or class) definition and encounters a parent
class, it recurses to this parent class first before reading any data of the
node (or class). When reclass returns from the recursive, depth first walk, it
then merges all information of the current node (or class) into the
information it obtained during the recursion.

Furthermore, a node (or class) may define a list of classes it derives from,
in which case classes defined further down the list will be able to override
classes further up the list.

Information in this context is essentially one of a list of applications or
a list of parameters.

The interaction between the depth-first walk and the delayed merging of data
means that the node (and any class) may override any of the data defined by
any of the parent classes (ancestors). This is in line with the assumption
that more specific definitions ("this specific host") should have a higher
precedence than more general definitions ("all webservers", which includes all
webservers in Munich, which includes "this specific host", for example).

Here's a quick example, showing how parameters accumulate and can get
replaced.

  All unixnodes (i.e. nodes who have the 'unixnodes' class in their ancestry)
  have /etc/motd centrally-managed (through the 'motd' application), and the
  unixnodes class definition provides a generic message-of-the-day to be put
  into this file.

  All debiannodes, which are descendants of unixnodes, should include the
  Debian codename in this message, so the message-of-the-day is overwritten in
  the debiannodes class.

  The node 'quantum.example.org' will have a scheduled downtime this weekend,
  so until Monday, an appropriate message-of-the-day is added to the node
  definition.

  When the 'motd' application runs, it receives the appropriate
  message-of-the-day (from 'quantum.example.org' when run on that node) and
  writes it into /etc/motd.

At this point it should be noted that parameters whose values are lists or
key-value pairs don't get overwritten by children classes or node definitions,
but the information gets merged (recursively) instead.

Similarly to parameters, applications also accumulate during the recursive
walk through the class ancestry. It is possible for a node or child class to
_remove_ an application added by a parent class, by prefixing the application
with '~'.

Finally, reclass happily lets you use multiple inheritance, and ensures that
the resolution of parameters is still well-defined. Here's another example
building upon the one about /etc/motd above:

  'quantum.example.org' (which is back up and therefore its node definition no
  longer contains a message-of-the-day) is at a site in Munich. Therefore, it
  is a child of the class 'hosted@munich'. This class is independent of the
  'unixnode' hierarchy, 'quantum.example.org' derives from both.

  In this example infrastructure, 'hosted@munich' is more specific than
  'debiannodes' because there are plenty of Debian nodes at other sites (and
  some non-Debian nodes in Munich). Therefore, 'quantum.example.org' derives
  from 'hosted@munich' _after_ 'debiannodes'.

  When an electricity outage is expected over the weekend in Munich, the admin
  can change the message-of-the-day in the 'hosted@munich' class, and it will
  apply to all hosts in Munich.

  However, not all hosts in Munich have /etc/motd, because some of them are
  'windowsnodes'. Since the 'windowsnodes' ancestry does not specify the
  'motd' application, those hosts have access to the message-of-the-day in the
  node variables, but the message won't get used…

  … unless, of course, 'windowsnodes' specified a Windows-specific application
  to bring such notices to the attention of the user.

It's also trivial to ensure a certain order of class evaluation. Here's
another example:

  The 'ssh.server' class defines the 'permit_root_login' parameter to 'no'.

  The 'backuppc.client' class defines the parameter to 'without-password',
  because the BackupPC server might need to log in to the host as root.

  Now, what happens if the admin accidentally provides the following two
  classes?

    - backuppc.client
    - ssh.server

  Theoretically, this would mean 'permit_root_login' gets set to 'no'.

  However, since all 'backuppc.client' nodes need 'ssh.server' (at least in
  most setups), the class 'backuppc.client' itself derives from 'ssh.server',
  ensuring that it gets parsed before 'backuppc.client'.

  When reclass returns to the node and encounters the 'ssh.server' class
  defined there, it simply skips it, as it's already been processed.

reclass operations
~~~~~~~~~~~~~~~~~~
While reclass has been built to support different storage backends through
plugins, currently only the 'yaml_fs' storage backend exists. This is a very
simple, yet powerful, YAML-based backend, using flat files on the filesystem
(as suggested by the _fs postfix).

yaml_fs works with two directories, one for node definitions, and another for
class definitions. It is possible to use a single directory for both, but that
could get messy and is therefore not recommended.

Files in those directories are YAML-files, specifying key-value pairs. The
following three keys are read by reclass:

  classes:     a list of parent classes
  appliations: a list of applications to append to the applications defined by
               ancestors. If an application name starts with '~', it would
               remove this application from the list, if it had already been
               added — but it does not prevent a future addition.
               E.g. '~firewalled'
  parameters:  key-value pairs to set defaults in class definitions, override
               existing data, or provide node-specific information in node
               specifications.
               By convention, parameters corresponding to an application
               should be provided as subkey-value pairs, keyed by the name of
               the application, e.g.

                 applications:
                   - ssh.server
                 parameters:
                   ssh.server:
                     permit_root_login: no

reclass starts out reading a node definition file, obtains the list of
classes, then reads the files corresponding to these classes, recursively
reading parent classes, and finally merges the applications list (append
unless

Version control
~~~~~~~~~~~~~~~
I recommend you maintain your reclass inventory database in Git, right from
the start.

Usage
~~~~~
For information on how to use reclass directly, invoke reclass.py with --help
and study the output.

The three options --inventory-base-uri, --nodes-uri, and --classes-uri
together specify the location of the inventory. If the base URI is specified,
then it is prepended to the other two URIs, unless they are absolute URIs. If
these two URIs are not specified, they default to 'nodes' and 'classes'.
Therefore, if your inventory is in '/etc/reclass/nodes' and
'/etc/reclass/classes', all you need to specify is the base URI as
'/etc/reclass'.

More commonly, however, use of reclass will happen indirectly, and through
so-called adapters, e.g. /…/reclass/adapters/salt. The job of an adapter is to
translate between different invocation paradigms, provide a sane set of
default options, and massage the data from reclass into the format expected by
the automation tool in use.

Configuration file
~~~~~~~~~~~~~~~~~~
reclass can read some of its configuration from a file. The file is
a YAML-file and simply defines key-value pairs.

The configuration file can be used to set defaults for all the options that
are otherwise configurable via the command-line interface, so please use the
--help output of reclass for reference. The command-line option '--nodes-uri'
corresponds to the key 'nodes_uri' in the configuration file. For example:

  storage_type: yaml_fs
  pretty_print: True
  output: json
  inventory_base_uri: /etc/reclass
  nodes_uri: ../nodes

reclass first looks in the current directory for the file called
'reclass-config.yml' and if no such file is found, it looks "next to" the
reclass script itself. Adapters implement their own lookup logic.

Contributing to reclass
~~~~~~~~~~~~~~~~~~~~~~~
Conttributions to reclass are very welcome. Since I prefer to keep a somewhat
clean history, I will not merge pull requests. Please send your patches using
git-format-patch and git-send-e-mail to reclass@pobox.madduck.net.

I have added rudimentary unit tests, and it would be nice if you could submit
your changes with appropriate changes to the tests. To run tests, invoke
./run_tests.py in the top-level checkout directory.

If you have larger ideas, I'll be looking forward to discuss them with you.

 -- martin f. krafft <madduck@madduck.net>  Fri, 14 Jun 2013 22:12:05 +0200