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A Beginner's Guide to Salt

Updated Originally authored by Linode
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Salt (also referred to as SaltStack) is a Python-based configuration management and orchestration system. Salt uses a master/client model in which a dedicated Salt master server manages one or more Salt minion servers. Two of Salt’s primary jobs are:

  • Remotely executing commands across a set of minions

  • Applying Salt states to a set of minions (referred to generally as configuration management)

This guide will introduce the core concepts that Salt employs to fulfill these jobs.

Masters and Minions

The Salt master is a server that acts as a command-and-control center for its minions, and it is where Salt’s remote execution commands are run from. For example, this command reports the current disk usage for each of the minions that the master controls:

salt '*' disk.usage

Many other commands are available. This installs NGINX on the minion named webserver1:

salt 'webserver1' pkg.install nginx

Salt minions are your servers that actually run your applications and services. Each minion has an ID assigned to it (which can be automatically generated from the minion’s hostname), and the Salt master can refer to this ID to target commands to specific minions.

Note
When using Salt, you should configure and manage your minion servers from the master as much as possible, instead of logging into them directly via SSH or another protocol.

To enable all of these functions, the Salt master server runs a daemon named salt-master, and the Salt minion servers run a daemon named salt-minion.

Authentication

Communication between the master and minions is performed over the ZeroMQ transport protocol, and all communication is encrypted with public/private keypairs. A keypair is generated by a minion when Salt is first installed on it, after which the minion will send its public key to the master. You will need to accept the minion’s key from the master; communication can then proceed between the two.

Remote Execution

Salt offers a very wide array of remote execution modules. An execution module is a collection of related functions that you can run on your minions from the master. For example:

salt 'webserver1' npm.install gulp

In this command npm is the module and install is the function. This command installs the Gulp Node.js package via the Node Package Manager (NPM). Other functions in the npm module handle uninstalling NPM packages, listing installed NPM packages, and related tasks.

The execution modules that Salt makes available represent system administration tasks that you would otherwise perform in a shell, including but not limited to:

cmd.run

The cmd.run function is used to run arbitrary commands on your minions from the master:

salt '*' cmd.run 'ls -l /etc'

This would return the contents of /etc on each minion.

Note
Where possible, it’s better to use execution modules than to “shell out” with cmd.run.

States, Formulas, and the Top File

The previous section described how to use remote execution to perform specific actions on a minion. With remote execution, you could administer a minion by entering a series of such commands.

Salt offers another way to configure a minion in which you declare the state that a minion should be in. This kind of configuration is called a Salt state, and the methodology is referred to generally as configuration management.

The distinction between the two styles is subtle; to illustrate, here’s how installing NGINX is interpreted in each methodology:

  • Remote execution: “Install NGINX on the minion”

  • Configuration management: “NGINX should be installed on the minion”

Salt states are defined in state files. Once you have recorded your states, you then apply them to a minion. Salt analyzes the state file and determines what it needs to do to make sure that the minion satisfies the state’s declarations.

Note
This sometimes results in the same command that would be run via remote execution, but sometimes it doesn’t. In the NGINX example, if Salt sees that NGINX was already installed previously, it won’t invoke the package manager again when the state is applied.

Anatomy of a State

Here’s an example state file which ensures that: rsync and curl are installed; NGINX is installed; and NGINX is run and enabled to run at boot:

File: /srv/salt/webserver_setup.sls
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network_utilities:
  pkg.installed:
    - pkgs:
      - rsync
      - curl

nginx_pkg:
  pkg.installed:
    - name: nginx

nginx_service:
  service.running:
    - name: nginx
    - enable: True
    - require:
      - pkg: nginx_pkg

State files end with the extension .sls (SaLt State). State files can have one or more state declarations, which are the top-level sections of the file (network_utilities, nginx_pkg, and nginx_service in the above example). State declarations IDs are arbitrary, so you can name them however you prefer.

Note

If you were to name the ID to be the same as the relevant installed package, then you do not need to specify the - name option, as it will be inferred from the ID. For example, this snippet also installs NGINX:

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nginx:
  pkg.installed

The same name/ID inference convention is true for other Salt modules.

State declarations contain state modules. State modules are distinct from execution modules but often perform similar jobs. For example, a pkg state module exists with functions analogous to the pkg execution module, as with the pkg.installed state function and the pkg.install execution function. As with execution modules, Salt provides a wide array of state modules for you to use.

Note
State declarations are not necessarily applied in the order they appear in a state file, but you can specify that a declaration depends on another one using the require option. This is the case in the above example; Salt will not attempt to run and enable NGINX until it is installed.

State files are really just collections of dictionaries, lists, strings, and numbers that are then interpreted by Salt. By default, Salt uses the YAML syntax for representing states.

State files are often kept on the Salt master’s filesystem, but they can also be stored in other fileserver locations, like a Git repository (in particular, GitHub).

Applying a State to a Minion

To apply a state to a minion, use the state.apply function from the master:

salt `webserver1` state.apply webserver_setup

This command applies the example webserver_setup.sls state to a minion named webserver1. When applying the state, the .sls suffix is not mentioned. All of the state declarations in the state file are applied.

Salt Formulas

Formulas are just collections of states that together configure an application or system component on a minion. Formulas are usually organized across several different .sls files. Splitting a formula’s states up across different files can make it easier to organize your work. State declarations can include and reference declarations across other files.

Formulas that are sufficiently generic are often shared on GitHub to be used by others. The SaltStack organization maintains a collection of popular formulas. Salt’s documentation has a guide on using a formula hosted on GitHub.

The definition of what constitutes a formula is somewhat loose, and the specific structure of a formula is not mandated by Salt.

The Top File

In addition to manually applying states to minions, Salt provides a way for you to automatically map which states should be applied to different minions. This map is called the top file.

Here’s a simple top file:

File: /srv/salt/top.sls
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base:
  '*':
    - universal_setup

  'webserver1':
    - webserver_setup

base refers to the Salt environment. You can specify more than one environment corresponding to different phases of your work; for example: development, QA, production, etc. base is the default.

Groups of minions are specified under the environment, and states are listed for each set of minions. The above example top file says that a universal_setup state should be applied to all minions ('*'), and the webserver_setup state should be applied to the webserver1 minion.

If you run the state.apply function with no arguments, then Salt will inspect the top file and apply all states within it according to the mapping you’ve created:

salt '*' state.apply
Note
This action is colloquially known as a highstate.

Benefits of States and Configuration Management

Defining your configurations in states eases system administration:

  • Setting up states minimizes human error, as you will not need to enter commands manually one-by-one.

  • Applying a state to minion multiple times generally does not result in any changes beyond the first application. Salt understands when a state has already been implemented on a minion and will not perform unnecessary actions.

  • If you update a state file and apply it to a minion, Salt will detect and only apply the changes, which makes updating your systems more efficient.

  • A state can be reused and applied to more than one minion, which will result in identical configurations across different servers.

  • State files can be entered into a version control system, which helps you track changes to your systems over time.

Targeting Minions

You can match against your minions’ IDs using shell style globbing. This works at either the command line or in the top file.

These examples would apply the webserver_setup state to all minions whose ID begins with webserver (e.g. webserver1, webserver2, etc):

  • CLI:

    salt 'webserver*' state.apply webserver_setup

  • Top file:

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    base:
  'webserver*':
    - webserver_setup

Regular Expressions and lists can also be used to match against minion IDs.

Grains

Salt’s grains system provides access to information that is generated by and stored on a minion. Examples include a minion’s operating system, domain name, IP address, and so on. You can also specify custom grain data on a minion, as outlined in Salt’s documentation.

You can use grain data to target minions from the command line. This command installs httpd on all minions running CentOS:

salt -G 'os:CentOS' pkg.install httpd

You can also use grains in a top file:

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base:
  'os:CentOS':
    - match: grain
    - centos_setup

Grain information generally isn’t very dynamic, but it can change occasionally, and Salt will refresh its grain data when it does. To view your minions’ grain data:

salt '*' grains.items

Storing Data and Secrets in Pillar

Salt’s pillar feature takes data defined on the Salt master and distributes it to minions. A primary use for pillar is to store secrets, such as account credentials. Pillar is also a useful place to store non-secret data that you wouldn’t want to record directly in your state files.

Note
In addition to storing pillar data on the master, you can also keep it in other locations, like in a Git repository or Hashicorp’s Vault.

Let’s say that you want to create system users on a minion and assign different shells to each of them. If you were to code this information into a state file, you would need a new declaration for each user. If you store the data in pillar instead, you can then just create one state declaration and inject the pillar data into it using Salt’s Jinja templating feature.

Note
Salt Pillar is sometimes confused with Salt Grains, as they both keep data that is used in states and remote execution. The data that grains maintains originates from the minions, while the data in pillar originates on the master (or another backend) and is delivered to the minions.

Anatomy of Pillar Data

Pillar data is kept in .sls files which are written in the same YAML syntax as states:

File: /srv/pillar/user_info.sls
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users:
  joe:
    shell: /bin/zsh
  amy:
    shell: /bin/bash
  sam
    shell: /bin/fish

As with state files, a top file (separate from your states’ top file) maps pillar data to minions:

File: /srv/pillar/top.sls
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base:
  'webserver1':
    - user_info

Jinja Templates

To inject pillar data into your states, use Jinja’s template syntax. While Salt uses the YAML syntax for state and pillar files, the files are first interpreted as Jinja templates (by default).

This example state file uses the pillar data from the previous section to create system users and set the shell for each:

File: /srv/salt/user_setup.sls
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{% for user_name, user_info in pillar['users'].iteritems() %}
{{ user_name }}:
  user.present:
    - shell: {{ user_info['shell'] }}
{% endfor %}

Salt will compile the state file into something that looks like this before it is applied to the minion:

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joe:
  user.present:
    - shell: /bin/zsh

amy:
  user.present:
    - shell: /bin/bash

sam:
  user.present:
    - shell: /bin/fish

You can also use Jinja to interact with grain data in your states. This example state will install Apache and adjust the name for the package according to the operating system:

File: /srv/salt/webserver_setup.sls
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install_apache:
  pkg.installed:
    {% if grains['os'] == 'CentOS' %}
    - name: httpd
    {% else %}
    - name: apache
    {% endif %}
Note
In addition to Salt’s documentation on Jinja, the official Jinja documentation also details the template syntax.

Beacons

The beacon system is a way of monitoring a variety of system processes on Salt minions. There are a number of beacon modules available.

Beacons can trigger reactors which can then help implement a change or troubleshoot an issue. For example, if a service’s response times out, the reactor system can restart the service.

Getting Started with Salt

Now that you’re familiar with some of Salt’s basic terminology and components, move on to our guide Getting Started with Salt - Basic Installation and Setup to set up a configuration to start running commands and provisioning minion servers.

The SaltStack documentation also contains a page of best practices to be mindful of when working with Salt. You should review this page and implement those practices into your own workflow whenever possible.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

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