Event-Driven Infrastructure

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Event Reactor

You'll learn how to:

  • Configure Salt reactors
  • Understand reactor SLS file syntax
  • React to an event

Estimated time: 10 minutes 


Salt’s Reactor system gives you the ability to trigger actions in response to any event. Not only can you trigger actions when jobs and tasks complete, you can trigger actions when services go down, users log in, files are changed, and from anywhere that you can send a custom event (end of a build script, cron job, the list goes on).

Reactor Configuration

The reactor is configured by adding a top-level reactor section to /etc/salt/master or to /etc/salt/master.d/reactor.conf (only one reactor section is allowed, so choose only one of these locations).

The following is an example of a reactor configuration that matches minion start events, cloud resource destroyed events, and a custom event string:

reactor:                            # Salt 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   # Put reactor files under file_roots

Reactor configuration is intentionally simple; the only job of the reactor section is to associate an event tag with a reactor SLS file to run. The reactor is single-threaded, so the heavy-lifting should be done in the reactor SLS file (Reactor SLS files are spun off in a separate process and can take as long as needed).

Reactor SLS files have the full power of YAML and Jinja, so you can perform filtering and tests using the event tag and data.

Let’s add a reactor to our demo environment so we can see the system in action. Open the salt-vagrant-demo/saltstack/etc/master file and add a reactor section:

    - 'my/custom/event/tag':
    - salt://reactor/customevent.sls

This tells the Salt master that anytime it sees an event that contains my/custom/event/tag, call the customevent.sls file.

We’ll create the customevent.sls file in the next section after we learn about reactor SLS files.

For now, restart the salt-master service (service salt-master restart) to pick up the change, and then continue.

Reactor SLS Files

You are already familiar with Salt state SLS files, and Salt reactor SLS files have many similarities. Both Salt state and Salt reactor SLS files are written using YAML and Jinja, but since they have some syntax differences and are used for different purposes, they should be kept in separate locations in your Salt fileserver (for example, reactors can go in a /srv/salt/reactors directory).

Since Salt reactor SLS files execute on the Salt master, it is useful to think of them more as entry points into the salt and salt-run commands rather than as entry points into the Salt state system that executes on the Salt minion.

So what exactly can you do with Salt reactors? Let’s go on to the next section to find out.

Types of Reactions

Salt reactors trigger one of the following systems:

  • Remote execution: run an execution module on the targeted minions. This is anything you would do by calling the salt command (including applying a state or highstate).

  • Salt Runners: These are tasks you would start using salt-run. For example, the HTTP runner can trigger a webhook.

  • Wheel: Wheel commands manage your Salt environment, performing tasks such as accepting keys and updating configuration settings. Some of these tasks are exposed by the salt-key utility.

Remote Execution

This interface is a direct connection to the Salt execution modules. If you think about this in terms of how remote execution is triggered from the command line using the salt command, you’ll know that the salt command expects three things:

  • target
  • function
  • arguments

Remote execution in the Salt reactor requires the same information, in YAML format:

<section id>:
    - tgt: <target>
    - arg:

Note that the execution module function that you want to call is prefaced by local for reasons that are not really important to us right now (or maybe ever).

Let’s look at a simple package installation that you might perform from the command-line:

salt 'myminion' pkg.install cowsay

In a reactor SLS, this would look something like the following:

install cowsay on myminion:
    - tgt: 'myminion'
    - arg:
      - cowsay

Getting the hang of this? Just remember that since reactor SLS files run on the master and use execution modules, a minion target needs to be specified upon which to run the execution module. This parameter is called tgt, meaning “target”. Additionally, arguments to be passed to the module are also passed using the arg or kwarg (not kwargs) parameter.

Runner and Wheel Modules

Calling Runner modules and Wheel modules from the Reactor uses a more direct syntax since the function is being executed locally instead of sending a command to a remote system. There are no arg or kwarg parameters (unless the Runner function or Wheel function accepts a parameter with either of those names.)

- prof: centos_6
- instances:
  - web11       # These VM names would be generated via Jinja in a
  - web12       # real-world example.

Here is a wheel example that accepts a minion key automatically (in production you’ll want to add additional integrity checks so you don’t accept rogue minions).

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

More Examples

The following reactor SLS might be used to react to a salt/cloud/*/created event:

new vm alert:
    - tgt: minion
    - kwarg:
        description: "New VM {{ data['name'] }}"
        details: "New VM on {{ data['provider'] }}: {{ data['name'] }}"
        service_key: 1162ee51ed6e46239265c969729c48eb
        profile: my-pagerduty-account

Notice how we are using the event data structure to customize the event message? Most events also include the minion ID as data['id'] where it makes sense, so you can use this to take action directly on the minion that triggered the event. For example, in the install cowsay example above, myminion could be replaced with data['id'].

If you set up your build system to trigger a custom event when a build completes, you could notify a slack channel with the status:

spam slack:
    - tgt: buildserver
    - kwarg:
        channel: "Development"
        api_key: peWcBiMOS9HrZG15peWcBiMOS9HrZG15"
        message: "Build {{ data['build_id'] }} finished with status: {{ data['status'] }}"

The build_id and status would be custom data that you insert when you trigger the event.

Salt States

The state execution module can be used within a reactor SLS file to apply a Salt state or to trigger a highstate. You can also use Jinja to substitute values and perform testing and filtering:

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

Let’s try it out

Now that you understand Salt reactor SLS files, create the salt-vagrant-demo/saltstack/salt/reactor directory, and then create a file called customevent.sls in that directory. Next, add one of the reactor SLS examples from this section, or create your own. Make sure you update the target to target one of your systems, for example minion1.

From the command-line on your Salt master, trigger the following custom event:

sudo salt-call event.send 'my/custom/event/tag' 

This event is picked up by the Salt reactor and triggers the customevent.sls file you created.

This guide has shown you some of the many ways you can use the Salt reactor to automate your infrastructure, the sky is the limit!