PreOS
=====

Description
-----------
With cdist you can install and configure new machines. You can use cdist to
create PreOS, minimal OS whose purpose is to boot a new machine.
After PreOS is booted, the machine is ready for installing the desired OS and
afterwards it is ready for configuration.

PreOS creation
--------------
With cdist you can create PreOS.
Currently supported PreOS-es include:

* debian
* ubuntu
* devuan

PreOS is created using the ``cdist preos`` command.
This command has subcommands that determine the desired PreOS.

For example, to create an ubuntu PreOS:

.. code-block:: sh

    $ cdist preos ubuntu /preos/preos-ubuntu -B -C \
        -k ~/.ssh/id_rsa.pub -p /preos/pxe-ubuntu \
        -t "/usr/bin/curl 192.168.111.5:3000/install/"

For more info about the available options see the cdist manual page.

This will bootstrap (``-B``) ubuntu PreOS in ``/preos/preos-ubuntu`` directory, it
will be configured (``-C``) using default built-in initial manifest and with
specified ssh authorized key (``-k``) and with specified trigger command (``-t``).
After bootstrapping and configuration PXE
boot directory will be created (``-p``) in ``/preos/pxe-ubuntu``.

After PreOS is created, new machines can be booted using the created PXE
(after proper dhcp and tftp settings).

Since PreOS is configured with ssh authorized key it can be accessed through
ssh, i.e. it can be further installed and configured with cdist.

When installing and configuring new machines using cdist's PreOS concept
cdist can use triggering for host installation/configuration, which is described
in the previous chapter.

When new machine is booted with PreOS then trigger command is executed.
Machine will connect to cdist trigger server. If the request is, for example,
for installation then cdist trigger server will start install command for the
client host using parameters specified at trigger server startup.

Implementing new PreOS sub-command
----------------------------------
preos command is implemented as a plugin system. This plugin system scans for
preos subcommands in the ``cdist/preos/`` distribution directory and also in
``~/.cdist/preos/`` directory if it exists.

preos subcommand is a module or a class that satisfies the following:

* it has the attribute ``_cdist_preos`` set to ``True``
* it defines a function/method ``commandline``.

For a module-based preos subcommand, the ``commandline`` function accepts a
module object as its first argument and the list of command line
arguments (``sys.argv[2:]``).

For a class-based preos subcommand ``commandline`` method should be
static-method and must accept a class as its first argument and the
list of command line arguments (``sys.argv[2:]``).

If preos scanning finds a module/class that has ``_cdist_preos`` set
to ``True`` and a function/method ``commandline`` then this module/class is
registered to preos subcommands. The name of the command is set to ``_preos_name``
attribute if defined in the module/class, defaulting to the module/class name in lowercase.
When a registered preos subcommand is specified, ``commandline``
will be called with the first argument set to module/class and the second
argument set to ``sys.argv[2:]``.

Example of writing new dummy preos sub-command
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Module-based preos:
^^^^^^^^^^^^^^^^^^^

#. Create directory ``~/.cdist/preos/`` if it does not exist
#. Create ``~/.cdist/preos/netbsd.py`` with the following contents:

.. code-block:: python

    _preos_name = 'netbsd'
    _cdist_preos = True

    def commandline(cls, args):
        print("NetBSD PreOS: {}".format(args))

When you try to run this new preos you will get:

.. code-block:: sh

    $ cdist preos -L
    Available PreOS-es:
        - debian
        - devuan
        - netbsd
        - ubuntu
    $ cdist preos netbsd
    NetBSD PreOS: []

Class based preos:
^^^^^^^^^^^^^^^^^^

#. Create directory ``~/.cdist/preos/`` if it does not exist
#. Create ``~/.cdist/preos/freebsd.py`` with the following contents:

.. code-block:: python

    class FreeBSD(object):
        _cdist_preos = True

        @classmethod
        def commandline(cls, args):
            print("FreeBSD dummy preos: {}".format(args))

When you try to run this new preos you will get:

.. code-block:: sh

    $ cdist preos -h
    Available PreOS-es:
        - debian
        - devuan
        - freebsd
        - ubuntu
    $ cdist preos freebsd
    FreeBSD dummy preos: []

In the ``commandline`` function/method you have all the freedom to actually create
a PreOS.

Simple tipical use case for using PreOS and trigger
---------------------------------------------------
Tipical use case for using PreOS and trigger command include the following steps.

#. Create PreOS PXE with ssh key and trigger command for installation.

    .. code-block:: sh

        $ cdist preos ubuntu /preos/ubuntu -b -C \
            -k ~/.ssh/id_rsa.pub -p /preos/pxe \
            -t "/usr/bin/curl 192.168.111.5:3000/install/"

#. Configure dhcp server and tftp server.

#. On cdist host (192.168.111.5 from above) start trigger command (it will use
   default init manifest for installation).

    .. code-block:: sh

        $ cdist trigger -b -v

#. After all is set up start new machines (PXE boot).

#. New machine boots and executes trigger command, i.e. triggers installation.

#. Cdist trigger server starts installing host that has triggered it.

#. After cdist install is finished new host is installed.