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 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 beacon modules for a current list.


Salt beacons are an event generation mechanism. Beacons leverage the Salt 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:

    - files:
        /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.


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

All beacon configuration is done using list based configuration.

New in version Neon.

Multiple copies of a particular Salt beacon can be configured by including the beacon_module parameter in the beacon configuration.

    - files:
        /etc/important_file: {}
    - beacon_module: inotify
    - files:
        /etc/another_file: {}
    - beacon_module: inotify

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:

    - files:
        /etc/important_file: {}
        /opt: {}
    - interval: 5
    - disable_during_state_run: True
    - averages:
          - 0.0
          - 2.0
          - 0.0
          - 1.5
          - 0.1
          - 1.0
    - interval: 10

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.

    - files:
        /etc/important_file: {}
    - disable_during_state_run: True


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 inotify beacon to monitor a file for changes, and then restores the file to its original contents if a change was made.


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:

important_config: True

Add Beacon Configs to Minion

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

    - files:
            - 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.


The disable_during_state_run: True parameter prevents the inotify beacon from generating reactor events due to salt itself modifying the file.

View Events on the Master

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

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:

 "_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.


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

mkdir -p /srv/reactor

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

    - tgt: {{ data['data']['id'] }}
    - arg:
      - maintain_important_file


In addition to setting 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 idempotent.


The expression {{ data['data']['id'] }} is correct as it matches the event structure shown above.

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.

    - 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:

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


You can have only one top level reactor section, so if one already exists, add this code to the existing section. See here 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:

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 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:

[{"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 via the standard methods, see Modular Systems.