Grains

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# 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
  Static bits of information that a minion collects about the system when the minion first starts.

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.

It is important to remember that grains are bits of information loaded when the salt minion starts, so this information is static. This means that the information in grains is unchanging, therefore the nature of the data is static. So grains information are things like the running kernel, or the operating system.

Match all CentOS minions:

    salt -G 'os:CentOS' test.ping

Match all minions with 64-bit CPUs and return number of available cores:

    salt -G 'cpuarch:x86_64' grains.item num_cpus

## 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.

## 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 your `file_roots`. Before adding a grain to Salt, consider what the grain is and remember that grains need to be static data.

### 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 the grains can be synced and reloaded by calling the saltutil.sync_grains or saltutil.sync_all functions.

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