Grains

Salt comes with an interface to derive information about the underlying system. This is called the grains interface, because it presents salt with grains of information. Grains are collected for the operating system, domain name, IP address, kernel, OS type, memory, and many other system properties.

The grains interface is made available to Salt modules and components so that the right salt minion commands are automatically available on the right systems.

Grain data is relatively static, though if system information changes (for example, if network settings are changed), or if a new value is assigned to a custom grain, grain data is refreshed.

Note

Grains resolve to lowercase letters. For example, FOO, and foo target the same grain.

Listing Grains

Available grains can be listed by using the 'grains.ls' module:

salt '*' grains.ls

Grains data can be listed by using the 'grains.items' module:

salt '*' grains.items

Grains in the Minion Config

Grains can also be statically assigned within the minion configuration file. Just add the option grains and pass options to it:

grains:
  roles:
    - webserver
    - memcache
  deployment: datacenter4
  cabinet: 13
  cab_u: 14-15

Then status data specific to your servers can be retrieved via Salt, or used inside of the State system for matching. It also makes targeting, in the case of the example above, simply based on specific data about your deployment.

Grains in /etc/salt/grains

If you do not want to place your custom static grains in the minion config file, you can also put them in /etc/salt/grains on the minion. They are configured in the same way as in the above example, only without a top-level grains: key:

roles:
  - webserver
  - memcache
deployment: datacenter4
cabinet: 13
cab_u: 14-15

Note

The content of /etc/salt/grains is ignored if you specify grains in the minion config.

Note

Grains are static, and since they are not often changed, they will need a grains refresh when they are updated. You can do this by calling: salt minion saltutil.refresh_modules

Matching Grains in the Top File

With correctly configured grains on the Minion, the top file used in Pillar or during Highstate can be made very efficient. For example, consider the following configuration:

'roles:webserver':
  - match: grain
  - state0

'roles:memcache':
  - match: grain
  - state1
  - state2

For this example to work, you would need to have defined the grain role for the minions you wish to match.

Writing Grains

The grains are derived by executing all of the "public" functions (i.e. those which do not begin with an underscore) found in the modules located in the Salt's core grains code, followed by those in any custom grains modules. The functions in a grains module must return a Python dict, where the dictionary keys are the names of grains, and each key's value is that value for that grain.

Custom grains modules should be placed in a subdirectory named _grains located under the file_roots specified by the master config file. The default path would be /srv/salt/_grains. Custom grains modules will be distributed to the minions when state.highstate is run, or by executing the saltutil.sync_grains or saltutil.sync_all functions.

Grains modules are easy to write, and (as noted above) only need to return a dictionary. For example:

def yourfunction():
     # initialize a grains dictionary
     grains = {}
     # Some code for logic that sets grains like
     grains['yourcustomgrain'] = True
     grains['anothergrain'] = 'somevalue'
     return grains

The name of the function does not matter and will not factor into the grains data at all; only the keys/values returned become part of the grains.

When to Use a Custom Grain

Before adding new grains, consider what the data is and remember that grains should (for the most part) be static data.

If the data is something that is likely to change, consider using Pillar or an execution module instead. If it's a simple set of key/value pairs, pillar is a good match. If compiling the information requires that system commands be run, then putting this information in an execution module is likely a better idea.

Good candidates for grains are data that is useful for targeting minions in the top file or the Salt CLI. The name and data structure of the grain should be designed to support many platforms, operating systems or applications. Also, keep in mind that Jinja templating in Salt supports referencing pillar data as well as invoking functions from execution modules, so there's no need to place information in grains to make it available to Jinja templates. For example:

...
...
{{ salt['module.function_name']('argument_1', 'argument_2') }}
{{ pillar['my_pillar_key'] }}
...
...

Warning

Custom grains will not be available in the top file until after the first highstate. To make custom grains available on a minion's first highstate, it is recommended to use this example to ensure that the custom grains are synced when the minion starts.

Loading Custom Grains

If you have multiple functions specifying grains that are called from a main function, be sure to prepend grain function names with an underscore. This prevents Salt from including the loaded grains from the grain functions in the final grain data structure. For example, consider this custom grain file:

#!/usr/bin/env python
def _my_custom_grain():
    my_grain = {'foo': 'bar', 'hello': 'world'}
    return my_grain


def main():
    # initialize a grains dictionary
    grains = {}
    grains['my_grains'] = _my_custom_grain()
    return grains

The output of this example renders like so:

# salt-call --local grains.items
local:
    ----------
    <Snipped for brevity>
    my_grains:
        ----------
        foo:
            bar
        hello:
            world

However, if you don't prepend the my_custom_grain function with an underscore, the function will be rendered twice by Salt in the items output: once for the my_custom_grain call itself, and again when it is called in the main function:

# salt-call --local grains.items
local:
----------
    <Snipped for brevity>
    foo:
        bar
    <Snipped for brevity>
    hello:
        world
    <Snipped for brevity>
    my_grains:
        ----------
        foo:
            bar
        hello:
            world

Precedence

Core grains can be overridden by custom grains. As there are several ways of defining custom grains, there is an order of precedence which should be kept in mind when defining them. The order of evaluation is as follows:

  1. Core grains.
  2. Custom grains in /etc/salt/grains.
  3. Custom grains in /etc/salt/minion.
  4. Custom grain modules in _grains directory, synced to minions.

Each successive evaluation overrides the previous ones, so any grains defined by custom grains modules synced to minions that have the same name as a core grain will override that core grain. Similarly, grains from /etc/salt/minion override both core grains and custom grain modules, and grains in _grains will override any grains of the same name.

Examples of Grains

The core module in the grains package is where the main grains are loaded by the Salt minion and provides the principal example of how to write grains:

https://github.com/saltstack/salt/blob/develop/salt/grains/core.py

Syncing Grains

Syncing grains can be done a number of ways, they are automatically synced when state.highstate is called, or (as noted above) the grains can be manually synced and reloaded by calling the saltutil.sync_grains or saltutil.sync_all functions.