Reactor System

Salt's Reactor system gives Salt the ability to trigger actions in response to an event. It is a simple interface to watching Salt's event bus for event tags that match a given pattern and then running one or more commands in response.

This system binds sls files to event tags on the master. These sls files then define reactions. This means that the reactor system has two parts. First, the reactor option needs to be set in the master configuration file. The reactor option allows for event tags to be associated with sls reaction files. Second, these reaction files use highdata (like the state system) to define reactions to be executed.

Event System

A basic understanding of the event system is required to understand reactors. The event system is a local ZeroMQ PUB interface which fires salt events. This event bus is an open system used for sending information notifying Salt and other systems about operations.

The event system fires events with a very specific criteria. Every event has a tag. Event tags allow for fast top-level filtering of events. In addition to the tag, each event has a data structure. This data structure is a dictionary, which contains information about the event.

Mapping Events to Reactor SLS Files

Reactor SLS files and event tags are associated in the master config file. By default this is /etc/salt/master, or /etc/salt/master.d/reactor.conf.

New in version 2014.7.0: Added Reactor support for salt:// file paths.

In the master config section 'reactor:' is a list of event tags to be matched and each event tag has a list of reactor SLS files to be run.

reactor:                            # Master config section "reactor"

  - 'salt/minion/*/start':          # Match tag "salt/minion/*/start"
    - /srv/reactor/start.sls        # Things to do when a minion starts
    - /srv/reactor/monitor.sls      # Other things to do

  - 'salt/cloud/*/destroyed':       # Globs can be used to match tags
    - /srv/reactor/destroy/*.sls    # Globs can be used to match file names

  - 'myco/custom/event/tag':        # React to custom event tags
    - salt://reactor/mycustom.sls   # Reactor files can come from the salt fileserver


In the above example, salt://reactor/mycustom.sls refers to the base environment. To pull this file from a different environment, use the querystring syntax (e.g. salt://reactor/mycustom.sls?saltenv=reactor).

Reactor SLS files are similar to State and Pillar SLS files. They are by default YAML + Jinja templates and are passed familiar context variables. Click here for more detailed information on the variables available in Jinja templating.

Here is the SLS for a simple reaction:

{% if data['id'] == 'mysql1' %}
    - tgt: mysql1
{% endif %}

This simple reactor file uses Jinja to further refine the reaction to be made. If the id in the event data is mysql1 (in other words, if the name of the minion is mysql1) then the following reaction is defined. The same data structure and compiler used for the state system is used for the reactor system. The only difference is that the data is matched up to the salt command API and the runner system. In this example, a command is published to the mysql1 minion with a function of state.apply, which performs a highstate. Similarly, a runner can be called:

{% if data['data']['custom_var'] == 'runit' %}
    - args:
      - mods: orchestrate.runit
{% endif %}

This example will execute the state.orchestrate runner and initiate an execution of the runit orchestrator located at /srv/salt/orchestrate/runit.sls.

Types of Reactions




Runs a remote-execution function on targeted minions


Executes a runner function


Executes a wheel function on the master


Runs a remote-execution function on a masterless minion


The local and caller reaction types will likely be renamed in a future release. These reaction types were named after Salt's internal client interfaces, and are not intuitively named. Both local and caller will continue to work in Reactor SLS files, however.

Where to Put Reactor SLS Files

Reactor SLS files can come both from files local to the master, and from any of backends enabled via the fileserver_backend config option. Files placed in the Salt fileserver can be referenced using a salt:// URL, just like they can in State SLS files.

It is recommended to place reactor and orchestrator SLS files in their own uniquely-named subdirectories such as orch/, orchestrate/, react/, reactor/, etc., to keep them organized.

Writing Reactor SLS

The different reaction types were developed separately and have historically had different methods for passing arguments. For the 2017.7.2 release a new, unified configuration schema has been introduced, which applies to all reaction types.

The old config schema will continue to be supported, and there is no plan to deprecate it at this time.

Local Reactions

A local reaction runs a remote-execution function on the targeted minions.

The old config schema required the positional and keyword arguments to be manually separated by the user under arg and kwarg parameters. However, this is not very user-friendly, as it forces the user to distinguish which type of argument is which, and make sure that positional arguments are ordered properly. Therefore, the new config schema is recommended if the master is running a supported release.

The below two examples are equivalent:

Supported in 2017.7.2 and later

Supported in all releases

    - tgt: 'kernel:Linux'
    - tgt_type: grain
    - args:
      - fun: pkg.installed
      - name: zsh
      - fromrepo: updates
    - tgt: 'kernel:Linux'
    - tgt_type: grain
    - arg:
      - pkg.installed
      - zsh
    - kwarg:
        fromrepo: updates

This reaction would be equivalent to running the following Salt command:

salt -G 'kernel:Linux' state.single pkg.installed name=zsh fromrepo=updates


Any other parameters in the LocalClient().cmd_async() method can be passed at the same indentation level as tgt.


tgt_type is only required when the target expression defined in tgt uses a target type other than a minion ID glob.

The tgt_type argument was named expr_form in releases prior to 2017.7.0.

Runner Reactions

Runner reactions execute runner functions locally on the master.

The old config schema called for passing arguments to the reaction directly under the name of the runner function. However, this can cause unpredictable interactions with the Reactor system's internal arguments. It is also possible to pass positional and keyword arguments under arg and kwarg like above in local reactions, but as noted above this is not very user-friendly. Therefore, the new config schema is recommended if the master is running a supported release.


State ids of reactors for runners and wheels should all be unique. They can overwrite each other when added to the async queue causing lost reactions.

The below two examples are equivalent:

Supported in 2017.7.2 and later

Supported in all releases

    - args:
      - mods: orchestrate.deploy_app
      - pillar:
          event_tag: {{ tag }}
          event_data: {{ data['data']|json }}
    - mods: orchestrate.deploy_app
    - kwarg:
          event_tag: {{ tag }}
          event_data: {{ data['data']|json }}

Assuming that the event tag is foo, and the data passed to the event is {'bar': 'baz'}, then this reaction is equivalent to running the following Salt command:

salt-run state.orchestrate mods=orchestrate.deploy_app pillar='{"event_tag": "foo", "event_data": {"bar": "baz"}}'

Wheel Reactions

Wheel reactions run wheel functions locally on the master.

Like runner reactions, the old config schema called for wheel reactions to have arguments passed directly under the name of the wheel function (or in arg or kwarg parameters).


State ids of reactors for runners and wheels should all be unique. They can overwrite each other when added to the async queue causing lost reactions.

The below two examples are equivalent:

Supported in 2017.7.2 and later

Supported in all releases

    - args:
      - match: {{ data['id'] }}
    - match: {{ data['id'] }}

Caller Reactions

Caller reactions run remote-execution functions on a minion daemon's Reactor system. To run a Reactor on the minion, it is necessary to configure the Reactor Engine in the minion config file, and then setup your watched events in a reactor section in the minion config file as well.


Masterless Minions use this Reactor

This is the only way to run the Reactor if you use masterless minions.

Both the old and new config schemas involve passing arguments under an args parameter. However, the old config schema only supports positional arguments. Therefore, the new config schema is recommended if the masterless minion is running a supported release.

The below two examples are equivalent:

Supported in 2017.7.2 and later

Supported in all releases

    - args:
      - name: /tmp/foo
    - args:
      - /tmp/foo

This reaction is equivalent to running the following Salt command:

salt-call file.touch name=/tmp/foo

Best Practices for Writing Reactor SLS Files

The Reactor works as follows:

  1. The Salt Reactor watches Salt's event bus for new events.

  2. Each event's tag is matched against the list of event tags configured under the reactor section in the Salt Master config.

  3. The SLS files for any matches are rendered into a data structure that represents one or more function calls.

  4. That data structure is given to a pool of worker threads for execution.

Matching and rendering Reactor SLS files is done sequentially in a single process. For that reason, reactor SLS files should contain few individual reactions (one, if at all possible). Also, keep in mind that reactions are fired asynchronously (with the exception of caller) and do not support requisites.

Complex Jinja templating that calls out to slow remote-execution or runner functions slows down the rendering and causes other reactions to pile up behind the current one. The worker pool is designed to handle complex and long-running processes like orchestration jobs.

Therefore, when complex tasks are in order, orchestration is a natural fit. Orchestration SLS files can be more complex, and use requisites. Performing a complex task using orchestration lets the Reactor system fire off the orchestration job and proceed with processing other reactions.

Jinja Context

Reactor SLS files only have access to a minimal Jinja context. grains and pillar are not available. The salt object is available for calling remote-execution or runner functions, but it should be used sparingly and only for quick tasks for the reasons mentioned above.

In addition to the salt object, the following variables are available in the Jinja context:

  • tag - the tag from the event that triggered execution of the Reactor SLS file

  • data - the event's data dictionary

The data dict will contain an id key containing the minion ID, if the event was fired from a minion, and a data key containing the data passed to the event.

Advanced State System Capabilities

Reactor SLS files, by design, do not support requisites, ordering, onlyif/unless conditionals and most other powerful constructs from Salt's State system.

Complex Master-side operations are best performed by Salt's Orchestrate system so using the Reactor to kick off an Orchestrate run is a very common pairing.

For example:

# /etc/salt/master.d/reactor.conf
# A custom event containing: {"foo": "Foo!", "bar: "bar*", "baz": "Baz!"}
  - my/custom/event:
    - /srv/reactor/some_event.sls
# /srv/reactor/some_event.sls
    - args:
        - mods: orchestrate.do_complex_thing
        - pillar:
            event_tag: {{ tag }}
            event_data: {{ data|json }}
# /srv/salt/orchestrate/do_complex_thing.sls
{% set tag = salt.pillar.get('event_tag') %}
{% set data = salt.pillar.get('event_data') %}

# Pass data from the event to a custom runner function.
# The function expects a 'foo' argument.
    - name: custom_runner.custom_function
    - foo: {{ }}

# Wait for the runner to finish then send an execution to minions.
# Forward some data from the event down to the minion's state run.
    - tgt: {{ }}
    - sls:
      - do_thing_on_minion
    - kwarg:
          baz: {{ data.baz }}
    - require:
      - salt: do_first_thing

Beacons and Reactors

An event initiated by a beacon, when it arrives at the master will be wrapped inside a second event, such that the data object containing the beacon information will be data['data'], rather than data.

For example, to access the id field of the beacon event in a reactor file, you will need to reference {{ data['data']['id'] }} rather than {{ data['id'] }} as for events initiated directly on the event bus.

Similarly, the data dictionary attached to the event would be located in {{ data['data']['data'] }} instead of {{ data['data'] }}.

See the beacon documentation for examples.

Manually Firing an Event

From the Master

Use the event.send runner:

salt-run event.send foo '{orchestrate: refresh}'

From the Minion

To fire an event to the master from a minion, call event.send:

salt-call event.send foo '{orchestrate: refresh}'

To fire an event to the minion's local event bus, call

salt-call '{orchestrate: refresh}' foo

Referencing Data Passed in Events

Assuming any of the above examples, any reactor SLS files triggered by watching the event tag foo will execute with {{ data['data']['orchestrate'] }} equal to 'refresh'.

Getting Information About Events

The best way to see exactly what events have been fired and what data is available in each event is to use the state.event runner.

Example usage:

salt-run state.event pretty=True

Example output:

salt/job/20150213001905721678/new       {
    "_stamp": "2015-02-13T00:19:05.724583",
    "arg": [],
    "fun": "",
    "jid": "20150213001905721678",
    "minions": [
    "tgt": "*",
    "tgt_type": "glob",
    "user": "root"
salt/job/20150213001910749506/ret/jerry {
    "_stamp": "2015-02-13T00:19:11.136730",
    "cmd": "_return",
    "fun": "saltutil.find_job",
    "fun_args": [
    "id": "jerry",
    "jid": "20150213001910749506",
    "retcode": 0,
    "return": {},
    "success": true

Debugging the Reactor

The best window into the Reactor is to run the master in the foreground with debug logging enabled. The output will include when the master sees the event, what the master does in response to that event, and it will also include the rendered SLS file (or any errors generated while rendering the SLS file).

  1. Stop the master.

  2. Start the master manually:

    salt-master -l debug
  3. Look for log entries in the form:

    [DEBUG   ] Gathering reactors for tag foo/bar
    [DEBUG   ] Compiling reactions for tag foo/bar
    [DEBUG   ] Rendered data from file: /path/to/the/reactor_file.sls:
    <... Rendered output appears here. ...>

    The rendered output is the result of the Jinja parsing and is a good way to view the result of referencing Jinja variables. If the result is empty then Jinja produced an empty result and the Reactor will ignore it.

Passing Event Data to Minions or Orchestration as Pillar

An interesting trick to pass data from the Reactor SLS file to state.apply is to pass it as inline Pillar data since both functions take a keyword argument named pillar.

The following example uses Salt's Reactor to listen for the event that is fired when the key for a new minion is accepted on the master using salt-key.


  - 'salt/key':
    - /srv/salt/haproxy/react_new_minion.sls

The Reactor then fires a :state.apply command targeted to the HAProxy servers and passes the ID of the new minion from the event to the state file via inline Pillar.


{% if data['act'] == 'accept' and data['id'].startswith('web') %}
    - tgt: 'haproxy*'
    - args:
      - mods: haproxy.refresh_pool
      - pillar:
          new_minion: {{ data['id'] }}
{% endif %}

The above command is equivalent to the following command at the CLI:

salt 'haproxy*' state.apply haproxy.refresh_pool pillar='{new_minion: minionid}'

This works with Orchestrate files as well:

    - args:
      - mods: orchestrate.some_orchestrate_file
      - pillar:
          stuff: things

Which is equivalent to the following command at the CLI:

salt-run state.orchestrate orchestrate.some_orchestrate_file pillar='{stuff: things}'

Finally, that data is available in the state file using the normal Pillar lookup syntax. The following example is grabbing web server names and IP addresses from Salt Mine. If this state is invoked from the Reactor then the custom Pillar value from above will be available and the new minion will be added to the pool but with the disabled flag so that HAProxy won't yet direct traffic to it.


{% set new_minion = salt['pillar.get']('new_minion') %}

listen web *:80
    balance source
    {% for server,ip in salt['mine.get']('web*', 'network.interfaces', ['eth0']).items() %}
    {% if server == new_minion %}
    server {{ server }} {{ ip }}:80 disabled
    {% else %}
    server {{ server }} {{ ip }}:80 check
    {% endif %}
    {% endfor %}

A Complete Example

In this example, we're going to assume that we have a group of servers that will come online at random and need to have keys automatically accepted. We'll also add that we don't want all servers being automatically accepted. For this example, we'll assume that all hosts that have an id that starts with 'ink' will be automatically accepted and have state.apply executed. On top of this, we're going to add that a host coming up that was replaced (meaning a new key) will also be accepted.

Our master configuration will be rather simple. All minions that attempt to authenticate will match the tag of salt/auth. When it comes to the minion key being accepted, we get a more refined tag that includes the minion id, which we can use for matching.


  - 'salt/auth':
    - /srv/reactor/auth-pending.sls
  - 'salt/minion/ink*/start':
    - /srv/reactor/auth-complete.sls

In this SLS file, we say that if the key was rejected we will delete the key on the master and then also tell the master to ssh in to the minion and tell it to restart the minion, since a minion process will die if the key is rejected.

We also say that if the key is pending and the id starts with ink we will accept the key. A minion that is waiting on a pending key will retry authentication every ten seconds by default.


{# Ink server failed to authenticate -- remove accepted key #}
{% if not data['result'] and data['id'].startswith('ink') %}
    - args:
      - match: {{ data['id'] }}
    - tgt: salt-master.domain.tld
    - args:
      - cmd: ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no "{{ data['id'] }}" 'sleep 10 && /etc/init.d/salt-minion restart'
{% endif %}

{# Ink server is sending new key -- accept this key #}
{% if 'act' in data and data['act'] == 'pend' and data['id'].startswith('ink') %}
    - args:
      - match: {{ data['id'] }}
{% endif %}

No if statements are needed here because we already limited this action to just Ink servers in the master configuration.


{# When an Ink server connects, run state.apply. #}
    - tgt: {{ data['id'] }}
    - ret: smtp

The above will also return the highstate result data using the smtp_return returner (use virtualname like when using from the command line with --return). The returner needs to be configured on the minion for this to work. See salt.returners.smtp_return documentation for that.

Syncing Custom Types on Minion Start

Salt will sync all custom types (by running a saltutil.sync_all) on every highstate. However, there is a chicken-and-egg issue where, on the initial highstate, a minion will not yet have these custom types synced when the top file is first compiled. This can be worked around with a simple reactor which watches for salt/minion/*/start events, which each minion fires when it first starts up and connects to the master.

On the master, create /srv/reactor/sync_grains.sls with the following contents:

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

And in the master config file, add the following reactor configuration:

  - 'salt/minion/*/start':
    - /srv/reactor/sync_grains.sls

This will cause the master to instruct each minion to sync its custom grains when it starts, making these grains available when the initial highstate is executed.

Other types can be synced by replacing local.saltutil.sync_grains with local.saltutil.sync_modules, local.saltutil.sync_all, or whatever else suits the intended use case.

Also, if it is not desirable that every minion syncs on startup, the * can be replaced with a different glob to narrow down the set of minions which will match that reactor (e.g. salt/minion/appsrv*/start, which would only match minion IDs beginning with appsrv).

Reactor Tuning for Large-Scale Installations

The reactor uses a thread pool implementation that's contained inside salt.utils.process.ThreadPool. It uses Python's stdlib Queue to enqueue jobs which are picked up by standard Python threads. If the queue is full, False is simply returned by the firing method on the thread pool.

As such, there are a few things to say about the selection of proper values for the reactor.

For situations where it is expected that many long-running jobs might be executed by the reactor, reactor_worker_hwm should be increased or even set to 0 to bound it only by available memory. If set to zero, a close eye should be kept on memory consumption.

If many long-running jobs are expected and execution concurrency and performance are a concern, you may also increase the value for reactor_worker_threads. This will control the number of concurrent threads which are pulling jobs from the queue and executing them. Obviously, this bears a relationship to the speed at which the queue itself will fill up. The price to pay for this value is that each thread will contain a copy of Salt code needed to perform the requested action.