Grains¶

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

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:

'node_type:web':
  - match: grain
  - webserver

'node_type:postgres':
  - match: grain
  - database

'node_type:redis':
  - match: grain
  - redis

'node_type:lb':
  - match: grain
  - lb

For this example to work, you would need to have defined the grain node_type for the minions you wish to match. This simple example is nice, but too much of the code is similar. To go one step further, Jinja templating can be used to simplify the the top file.

{% set node_type = salt['grains.get']('node_type', '') %}

{% if node_type %}
  'node_type:{{ self }}':
    - match: grain
    - {{ self }}
{% endif %}

Using Jinja templating, only one match entry needs to be defined.

Writing Grains¶

Grains are easy to write. The grains interface is derived by executing all of the "public" functions found in the modules located in the grains package or the custom grains directory. The functions in the modules of the grains must return a Python dict, where the keys in the dict are the names of the grains and the values are the values.

Custom grains should be placed in a _grains directory located under the file_roots specified by the master config file. They will be distributed to the minions when state.highstate is run, or by executing the saltutil.sync_grains or saltutil.sync_all functions.

Grains are easy to write, and only need to return a dictionary. A common approach would be code something similar to the following:

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

Before adding a grain to Salt, consider what the grain is and remember that grains need to be static data. If the data is something that is likely to change, consider using Pillar instead.

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

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/grains override both core grains and custom grain modules, and grains in /etc/salt/minion 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.