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Useful in formulas and macros, and now featuring a default attribute.
Useful in formulas and macros, and now featuring a default attribute.

If you read some formulas you may have noticed a nice way of managing settings. The grains.filter_by function. Values used in salt states are usually dynamic, coming from grains, from the pillar, from variations depending on the host environment. When you have a big number of variables using only the default jinja tools is sometimes frustrating. This is a must-know feature.

There's a nice example in the Apache formula map.jinja file, which may evolve, so let's show what it looks like:

{% set apache = salt['grains.filter_by']({
    'Debian': {
        'server': 'apache2',
        'service': 'apache2',

        'mod_wsgi': 'libapache2-mod-wsgi',

        'vhostdir': '/etc/apache2/sites-available',
        'confdir': '/etc/apache2/conf.d',
        'logdir': '/var/log/apache2',
        'wwwdir': '/srv',
    'RedHat': {
        'server': 'httpd',
        'service': 'httpd',

        'mod_wsgi': 'mod_wsgi',

        'vhostdir': '/etc/httpd/conf.d',
        'confdir': '/etc/httpd/conf.d',
        'logdir': '/var/log/httpd',
        'wwwdir': '/var/www',
}, merge=salt['pillar.get']('apache:lookup')) %}

This sets an apache jinja variable, a dictionary containing server, service,confdir or wwwdir keys (and some more).

The main idea of salt['grains.filter_by'] is to filter a settings dictionary based on a grain: os_family (here the value may be 'Redhat' or 'Debian').

Usage of this map.jinja file can be seen on the init.sls:

{% from "apache/map.jinja" import apache with context %}

    - installed
    - name: {{ apache.server }}

Now if you have a pretty recent version (> 0.17.2) of Salt-stack this filter_by function has some new very interesting features.

salt.modules.grains.filter_by(lookup_dict, grain='os_family', merge=None, default='default')
  • the lookup dict is your dictionary of settings where you want a filter to be applied, so at the end you will obtain a subtree of this dictionary.
  • the grain is by default the os_family grain, but you can use any defined grain key (salt-call grains.items to see them).
  • the merge argument lets you merge another dictionary on top of the end result, this is very useful, as seen on the apache's formula, to retrieve pillar data overriding the default settings dictionary.
  • and the new argument is default, which lets you specify which key of the lookup dict should be used if you did not have the requested grain or if the requested grain value is not present in this lookup dict.

So this function is now a really good shortcut for a default settings registry mapping.

{% set settings = salt['grains.filter_by']({
    'unset': { 'foo': 'The coconut's tropical', 'bar': 'King Arthur'},
    'prod': { 'foo': 'Bridgekeeper', 'bar': 'We want a shrubbery' },
    'preprod': { 'foo': 'Sir Lancelot', bar: 'Blue. No, yel…'},

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