.. _beacons:

=======
Beacons
=======

Beacons let you use the Salt event system to monitor non-Salt processes. The
beacon system allows the minion to hook into a variety of system processes and
continually monitor these processes. When monitored activity occurs in a system
process, an event is sent on the Salt event bus that can be used to trigger a
:ref:`reactor <reactor>`.

Salt beacons can currently monitor and send Salt events for many system
activities, including:

- file system changes
- system load
- service status
- shell activity, such as user login
- network and disk usage

See :ref:`beacon modules <all-salt.beacons>` for a current list.

.. note::
    Salt beacons are an event generation mechanism. Beacons leverage the Salt
    :ref:`reactor <reactor>` system to make changes when beacon events occur.


Configuring Beacons
===================

Salt beacons do not require any changes to the system components that are being
monitored, everything is configured using Salt.

Beacons are typically enabled by placing a ``beacons:`` top level block in
``/etc/salt/minion`` or any file in ``/etc/salt/minion.d/`` such as
``/etc/salt/minion.d/beacons.conf`` or add it to pillars for that minion:

.. code-block:: yaml

    beacons:
      inotify:
        /etc/important_file: {}
        /opt: {}

The beacon system, like many others in Salt, can also be configured via the
minion pillar, grains, or local config file.

.. note::
    The `inotify` beacon only works on OSes that have `inotify` kernel support.
    Currently this excludes FreeBSD, macOS, and Windows.

Beacon Monitoring Interval
--------------------------

Beacons monitor on a 1-second interval by default. To set a different interval,
provide an ``interval`` argument to a beacon. The following beacons run on 5-
and 10-second intervals:

.. code-block:: yaml

    beacons:
      inotify:
        /etc/important_file: {}
        /opt: {}
        interval: 5
        disable_during_state_run: True
      load:
        1m:
          - 0.0
          - 2.0
        5m:
          - 0.0
          - 1.5
        15m:
          - 0.1
          - 1.0
        interval: 10

.. _avoid-beacon-event-loops:

Avoiding Event Loops
--------------------

It is important to carefully consider the possibility of creating a loop
between a reactor and a beacon. For example, one might set up a beacon which
monitors whether a file is read which in turn fires a reactor to run a state
which in turn reads the file and re-fires the beacon.

To avoid these types of scenarios, the ``disable_during_state_run`` argument
may be set. If a state run is in progress, the beacon will not be run on its
regular interval until the minion detects that the state run has completed, at
which point the normal beacon interval will resume.

.. code-block:: yaml

    beacons:
      inotify:
        /etc/important_file: {}
        disable_during_state_run: True

.. _beacon-example:

.. note::
    For beacon writers:  If you need extra stuff to happen, like closing file
    handles for the ``disable_during_state_run`` to actually work, you can add
    a `close()` function to the beacon to run those extra things. See the
    `inotify` beacon.

Beacon Example
==============

This example demonstrates configuring the :py:mod:`~salt.beacons.inotify`
beacon to monitor a file for changes, and then restores the file to its
original contents if a change was made.

.. note::
    The inotify beacon requires Pyinotify on the minion, install it using
    ``salt myminion pkg.install python-inotify``.

Create Watched File
-------------------

Create the file named ``/etc/important_file`` and add some simple content:

.. code-block:: yaml

    important_config: True

Add Beacon Configs to Minion
----------------------------

On the Salt minion, add the following configuration to
``/etc/salt/minion.d/beacons.conf``:

.. code-block:: yaml

    beacons:
      inotify:
        /etc/important_file:
          mask:
            - modify
        disable_during_state_run: True

Save the configuration file and restart the minion service. The beacon is now
set up to notify salt upon modifications made to the file.

.. note::

    The ``disable_during_state_run: True`` parameter :ref:`prevents
    <avoid-beacon-event-loops>` the inotify beacon from generating reactor
    events due to salt itself modifying the file.

.. _beacon-event-bus:

View Events on the Master
-------------------------

On your Salt master, start the event runner using the following command:

.. code-block:: bash

   salt-run state.event pretty=true

This runner displays events as they are received by the master on the Salt
event bus. To test the beacon you set up in the previous section, make and save
a modification to ``/etc/important_file``. You'll see an event similar to the
following on the event bus:

.. code-block:: json

    salt/beacon/larry/inotify//etc/important_file	{
     "_stamp": "2015-09-09T15:59:37.972753",
     "data": {
         "change": "IN_IGNORED",
         "id": "larry",
         "path": "/etc/important_file"
     },
     "tag": "salt/beacon/larry/inotify//etc/important_file"
    }


This indicates that the event is being captured and sent correctly. Now you can
create a reactor to take action when this event occurs.

Create a Reactor
----------------

This reactor reverts the file named ``/etc/important_file`` to the contents
provided by salt each time it is modified.

Reactor SLS
```````````

On your Salt master, create a file named ``/srv/reactor/revert.sls``.

.. note::

    If the ``/srv/reactor`` directory doesn't exist, create it.

    .. code-block:: bash

        mkdir -p /srv/reactor

Add the following to ``/srv/reactor/revert.sls``:

.. code-block:: yaml

    revert-file:
      local.state.apply:
        - tgt: {{ data['data']['id'] }}
        - arg:
          - maintain_important_file

.. note::

    In addition to :ref:`setting <avoid-beacon-event-loops>`
    ``disable_during_state_run: True`` for an inotify beacon whose reaction is
    to modify the watched file, it is important to ensure the state applied is
    also :term:`idempotent`.

.. note::

    The expression ``{{ data['data']['id'] }}`` :ref:`is correct
    <beacons-and-reactors>` as it matches the event structure :ref:`shown above
    <beacon-event-bus>`.

State SLS
`````````

Create the state sls file referenced by the reactor sls file.  This state file
will be located at ``/srv/salt/maintain_important_file.sls``.

.. code-block:: yaml

    important_file:
      file.managed:
        - name: /etc/important_file
        - contents: |
            important_config: True

Master Config
`````````````

Configure the master to map the inotify beacon event to the ``revert`` reaction
in ``/etc/salt/master.d/reactor.conf``:

.. code-block:: yaml

    reactor:
      - salt/beacon/*/inotify//etc/important_file:
        - /srv/reactor/revert.sls

.. note::
    You can have only one top level ``reactor`` section, so if one already
    exists, add this code to the existing section. See :ref:`here
    <reactor-sls>` to learn more about reactor SLS syntax.


Start the Salt Master in Debug Mode
-----------------------------------

To help with troubleshooting, start the Salt master in debug mode:

.. code-block:: bash

   service salt-master stop
   salt-master -l debug

When debug logging is enabled, event and reactor data are displayed so you can
discover syntax and other issues.

Trigger the Reactor
-------------------

On your minion, make and save another change to ``/etc/important_file``. On the
Salt master, you'll see debug messages that indicate the event was received and
the ``state.apply`` job was sent. When you inspect the file on the minion,
you'll see that the file contents have been restored to ``important_config:
True``.

All beacons are configured using a similar process of enabling the beacon,
writing a reactor SLS (and state SLS if needed), and mapping a beacon event to
the reactor SLS.

Writing Beacon Plugins
======================

Beacon plugins use the standard Salt loader system, meaning that many of the
constructs from other plugin systems holds true, such as the ``__virtual__``
function.

The important function in the Beacon Plugin is the ``beacon`` function. When
the beacon is configured to run, this function will be executed repeatedly by
the minion. The ``beacon`` function therefore cannot block and should be as
lightweight as possible. The ``beacon`` also must return a list of dicts, each
dict in the list will be translated into an event on the master.

Beacons may also choose to implement a ``__validate__`` function which
takes the beacon configuration as an argument and ensures that it
is valid prior to continuing. This function is called automatically
by the Salt loader when a beacon is loaded.

Please see the :py:mod:`~salt.beacons.inotify` beacon as an example.

The `beacon` Function
---------------------

The beacons system will look for a function named `beacon` in the module. If
this function is not present then the beacon will not be fired. This function
is called on a regular basis and defaults to being called on every iteration of
the minion, which can be tens to hundreds of times a second. This means that
the `beacon` function cannot block and should not be CPU or IO intensive.

The beacon function will be passed in the configuration for the executed
beacon. This makes it easy to establish a flexible configuration for each
called beacon. This is also the preferred way to ingest the beacon's
configuration as it allows for the configuration to be dynamically updated
while the minion is running by configuring the beacon in the minion's pillar.

The Beacon Return
-----------------

The information returned from the beacon is expected to follow a predefined
structure. The returned value needs to be a list of dictionaries (standard
python dictionaries are preferred, no ordered dicts are needed).

The dictionaries represent individual events to be fired on the minion and
master event buses. Each dict is a single event. The dict can contain any
arbitrary keys but the 'tag' key will be extracted and added to the tag of the
fired event.

The return data structure would look something like this:

.. code-block:: python

    [{'changes': ['/foo/bar'], 'tag': 'foo'},
     {'changes': ['/foo/baz'], 'tag': 'bar'}]

Calling Execution Modules
-------------------------

Execution modules are still the preferred location for all work and system
interaction to happen in Salt. For this reason the `__salt__` variable is
available inside the beacon.

Please be careful when calling functions in `__salt__`, while this is the
preferred means of executing complicated routines in Salt not all of the
execution modules have been written with beacons in mind. Watch out for
execution modules that may be CPU intense or IO bound. Please feel free to add
new execution modules and functions to back specific beacons.

Distributing Custom Beacons
---------------------------

Custom beacons can be distributed to minions using ``saltutil``, see
:ref:`Dynamic Module Distribution <dynamic-module-distribution>`.
