Introduction to Jinja Templates for Salt

Introduction to Templating Languages

Jinja is a flexible templating language for Python that can be used to generate any text based format such as HTML, XML, and YAML. Templating languages like Jinja allow you to insert data into a structured format. You can also embed logic or control-flow statements into templates for greater reusability and modularity. Jinja’s template engine is responsible for processing the code within the templates and generating the output to the final text based document.

Templating languages are well known within the context of creating web pages in a Model View Controller architecture. In this scenario the template engine processes source data, like the data found in a database, and a web template that includes a mixture of HTML and the templating language. These two pieces are then used to generate the final web page for users to consume. Templating languages, however, are not limited to web pages. Salt, a popular Python based configuration management software, supports Jinja to allow for abstraction and reuse within Salt state files and regular files.

This guide will provide an overview of the Jinja templating language used primarily within Salt. If you are not yet familiar with Salt concepts, review the Beginner’s Guide to Salt before continuing. While you will not be creating Salt states of your own in this guide, it is also helpful to review the Getting Started with Salt - Basic Installation and Setup guide.

Jinja Basics

This section provides an introductory description of Jinja syntax and concepts along with examples of Jinja and Salt states. For an exhaustive dive into Jinja, consult the official Jinja Template Designer Documentation.

Applications like Salt can define default behaviors for the Jinja templating engine. All examples in this guide use Salt’s default Jinja environment options. These settings can be changed in the Salt master configuration file:

File: /etc/salt/master

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# Default Jinja environment options for all templates except sls templates #jinja_env: # block_start_string: '{%' # block_end_string: '%}' # variable_start_string: '{{' # variable_end_string: '}}' # comment_start_string: '{#' # comment_end_string: '#}' # line_statement_prefix: # line_comment_prefix: # trim_blocks: False # lstrip_blocks: False # newline_sequence: '\n' # keep_trailing_newline: False # Jinja environment options for sls templates #jinja_sls_env: # block_start_string: '{%' # block_end_string: '%}' # variable_start_string: '{{' # variable_end_string: '}}' # comment_start_string: '{#' # comment_end_string: '#}' # line_statement_prefix: # line_comment_prefix: # trim_blocks: False # lstrip_blocks: False

Delimiters

Templating language delimiters are used to denote the boundary between the templating language and another type of data format like HTML or YAML. Jinja uses the following delimiters:

In this example Salt state file, you can differentiate the Jinja syntax from the YAML because of the {% ... %} delimiters surrounding the if/else conditionals:

File: /srv/salt/webserver/init.sls

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{% if grains['group'] == 'admin' %} America/Denver: timezone.system: {% else %} Europe/Minsk: timezone.system: {% endif %}

See the control structures section for more information on conditionals.

Template Variables

Template variables are available via a template’s context dictionary. A template’s context dictionary is created automatically during the different stages of a template’s evaluation. These variables can be accessed using dot notation:

{{ foo.bar }}

Or they can be accessed by subscript syntax:

{{ foo['bar'] }}

Salt provides several context variables that are available by default to any Salt state file or file template:

• Salt: The salt variable provides a powerful set of Salt library functions.

  {{ salt['pw_user.list_groups']('jdoe') }}
  

  You can run salt '*' sys.doc from the Salt master to view a list of all available functions.

• Opts: The opts variable is a dictionary that provides access to the content of a Salt minion’s configuration file:

  {{ opts['log_file'] }}
  

  The location for a minion’s configuration file is /etc/salt/minion.

• Pillar: The pillar variable is a dictionary used to access Salt’s pillar data:

  {{ pillar['my_key'] }}
  

  Although you can access pillar keys and values directly, it is recommended that you use Salt’s pillar.get variable library function, because it allows you to define a default value. This is useful when a value does not exist in the pillar:

  {{ salt['pillar.get']('my_key', 'default_value') }}
  

• Grains: The grains variable is a dictionary and provides access to minions’ grains data:

  {{ grains['shell'] }}
  

  You can also use Salt’s grains.get variable library function to access grain data:

  {{ salt['grains.get']('shell') }}
  

• Saltenv: You can define multiple salt environments for minions in a Salt master’s top file, such as base, prod, dev and test. The saltenv variable provides a way to access the current Salt environment within a Salt state file. This variable is only available within Salt state files.

  {{ saltenv }}
  

• SLS: With the sls variable you can obtain the reference value for the current state file (e.g. apache, webserver, etc). This is the same value used in a top file to map minions to state files or via the include option in state files:

  {{ sls }}
  

• Slspath: This variable provides the path to the current state file:

  {{ slspath }}
  

Variable Assignments

You can assign a value to a variable by using the set tag along with the following delimiter and syntax:

{% set var_name = myvalue %}

Follow Python naming conventions when creating variable names. If the variable is assigned at the top level of a template, the assignment is exported and available to be imported by other templates.

Any value generated by a Salt template variable library function can be assigned to a new variable.

{% set username = salt['user.info']('username') %}

Filters

Filters can be applied to any template variable via a | character. Filters are chainable and accept optional arguments within parentheses. When chaining filters, the output of one filter becomes the input of the following filter.

{{ '/etc/salt/' | list_files | join('\n') }}

These chained filters will return a recursive list of all the files in the /etc/salt/ directory. Each list item will be joined with a new line.

  /etc/salt/master
  /etc/salt/proxy
  /etc/salt/minion
  /etc/salt/pillar/top.sls
  /etc/salt/pillar/device1.sls
  

For a complete list of all built in Jinja filters, refer to the Jinja Template Design documentation. Salt’s official documentation includes a list of custom Jinja filters.

Macros

Macros are small, reusable templates that help you to minimize repetition when creating states. Define macros within Jinja templates to represent frequently used constructs and then reuse the macros in state files.

File: /srv/salt/mysql/db_macro.sls

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{% macro mysql_privs(user, grant=select, database, host=localhost) %} {{ user }}_exampledb: mysql_grants.present: - grant: {{ grant }} - database: {{ database }} - user: {{user}} - host: {{ host }} {% endmacro %}

File: db_privs.sls

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{% import "/srv/salt/mysql/db_macro.sls" as db -%} db.mysql_privs('jane','exampledb.*','select,insert,update')

The mysql_privs() macro is defined in the db_macro.sls file. The template is then imported to the db variable in the db_privs.sls state file and is used to create a MySQL grants state for a specific user.

Refer to the Imports and Includes section for more information on importing templates and variables.

Imports and Includes

Imports

Importing in Jinja is similar to importing in Python. You can import an entire template, a specific state, or a macro defined within a file.

{% import '/srv/salt/users.sls' as users %}

This example will import the state file users.sls into the variable users. All states and macros defined within the template will be available using dot notation.

You can also import a specific state or macro from a file.

{% from '/srv/salt/user.sls' import mysql_privs as grants %}

This import targets the macro mysql_privs defined within the user.sls state file and is made available to the current template with the grants variable.

Includes

The {% include %} tag renders the output of another template into the position where the include tag is declared. When using the {% include %} tag the context of the included template is passed to the invoking template.

File: /srv/salt/webserver/webserver_users.sls

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include: - groups {% include 'users.sls' %}

{% include slspath + "/users.sls" %}

Import Context Behavior

By default, an import will not include the context of the imported template, because imports are cached. This can be overridden by adding with context to your import statements.

{% from '/srv/salt/user.sls' import mysql_privs with context %}

Similarly, if you would like to remove the context from an {% include %}, add without context:

{% include 'users.sls' without context %}

Whitespace Control

Jinja provides several mechanisms for whitespace control of its rendered output. By default, Jinja strips single trailing new lines and leaves anything else unchanged, e.g. tabs, spaces, and multiple new lines. You can customize how Salt’s Jinja template engine handles whitespace in the Salt master configuration file. Some of the available environment options for whitespace control are:

• trim_blocks: When set to True, the first newline after a template tag is removed automatically. This is set to False by default in Salt.
• lstrip_blocks: When set to True, Jinja strips tabs and spaces from the beginning of a line to the start of a block. If other characters are present before the start of the block, nothing will be stripped. This is set to False by default in Salt.
• keep_trailing_newline: When set to True, Jinja will keep single trailing newlines. This is set to False by default in Salt.

To avoid running into YAML syntax errors, ensure that you take Jinja’s whitespace rendering behavior into consideration when inserting templating markup into Salt states. Remember, Jinja must produce valid YAML. When using control structures or macros, it may be necessary to strip whitespace from the template block to appropriately render valid YAML.

To preserve the whitespace of contents within template blocks, you can set both the trim_blocks and lstrip_block options to True in the master configuration file. You can also manually enable and disable the white space environment options within each template block. A - character will set the behavior of trim_blocks and lstrip_blocks to False and a + character will set these options to True for the block:

For example, to strip the whitespace after the beginning of the control structure include a - character before the closing %}:

{% for item in [1,2,3,4,5] -%}
    {{ item }}
{% endfor %}

This will output the numbers 12345 without any leading whitespace. Without the - character, the output would preserve the spacing defined within the block.

Control Structures

Jinja provides control structures common to many programming languages such as loops, conditionals, macros, and blocks. The use of control structures within Salt states allow for fine-grained control of state execution flow.

For Loops

For loops allow you to iterate through a list of items and execute the same code or configuration for each item in the list. Loops provide a way to reduce repetition within Salt states.

File: /srv/salt/users.sls

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{% set groups = ['sudo','wheel', 'admins'] %} include: - groups jane: user.present: - fullname: Jane Doe - shell: /bin/zsh - createhome: True - home: /home/jane - uid: 4001 - groups: {%- for group in groups %} - {{ group }} {%- endfor -%}

The previous for loop will assign the user jane to all the groups in the groups list set at the top of the users.sls file.

Conditionals

A conditional expression evaluates to either True or False and controls the flow of a program based on the result of the evaluated boolean expression. Jinja’s conditional expressions are prefixed with if/elif/else and placed within the {% ... %} delimiter.

File: /srv/salt/users.sls

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{% set users = ['anna','juan','genaro','mirza'] %} {% set admin_users = ['genaro','mirza'] %} {% set admin_groups = ['sudo','wheel', 'admins'] %} {% set org_groups = ['games', 'webserver'] %} include: - groups {% for user in users %} {{ user }}: user.present: - shell: /bin/zsh - createhome: True - home: /home/{{ user }} - groups: {% if user in admin_users %} {%- for admin_group in admin_groups %} - {{ admin_group }} {%- endfor -%} {% else %} {%- for org_group in org_groups %} - {{ org_group }} {% endfor %} {%- endif -%} {% endfor %}

In this example the presence of a user within the admin_users list determines which groups are set for that user in the state. Refer to the Salt Best Practices section for more information on using conditionals and control flow statements within state files.

Template Inheritance

With template inheritance you can define a base template that can be reused by child templates. The child template can override blocks designated by the base template.

Use the {% block block_name %} tag with a block name to define an area of a base template that can be overridden.

File: /srv/salt/users.jinja

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{% block user %}jane{% endblock %}: user.present: - fullname: {% block fullname %}{% endblock %} - shell: /bin/zsh - createhome: True - home: /home/{% block home_dir %} - uid: 4000 - groups: - sudo

This example creates a base user state template. Any value containing a {% block %} tag can be overridden by a child template with its own value.

To use a base template within a child template, use the {% extends "base.sls"%} tag with the location of the base template file.

File: /srv/salt/webserver_users.sls

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{% extends "/srv/salt/users.jinja" %} {% block fullname %}{{ salt['pillar.get']('jane:fullname', '') }}{% endblock %} {% block home_dir %}{{ salt['pillar.get']('jane:home_dir', 'jane') }}{% endblock %}

The webserver_users.sls state file extends the users.jinja template and defines values for the fullname and home_dir blocks. The values are generated using the salt context variable and pillar data. The rest of the state will be rendered as the parent user.jinja template has defined it.

Salt and Jinja Best Practices

If Jinja is overused, its power and versatility can create unmaintainable Salt state files that are difficult to read. Here are some best practices to ensure that you are using Jinja effectively:

• Limit how much Jinja you use within state files. It is best to separate the data from the state that will use the data. This allows you to update your data without having to alter your states.

• Do not overuse conditionals and looping within state files. Overuse will make it difficult to read, understand and maintain your states.

• Use dictionaries of variables and directly serialize them into YAML, instead of trying to create valid YAML within a template. You can include your logic within the dictionary and retrieve the necessary variable within your states.

  The {% load_yaml %} tag will deserialize strings and variables passed to it.

     {% load_yaml as example_yaml %}
         user: jane
         firstname: Jane
         lastname: Doe
     {% endload %}
  
     {{ example_yaml.user }}:
        user.present:
          - fullname: {{ example_yaml.firstname }} {{ example_yaml.lastname }}
          - shell: /bin/zsh
          - createhome: True
          - home: /home/{{ example_yaml.user }}
          - uid: 4001
          - groups:
            - games
  

  Use {% import_yaml %} to import external files of data and make the data available as a Jinja variable.

     {% import_yaml "users.yml" as users %}
  

• Use Salt Pillars to store general or sensitive data as variables. Access these variables inside state files and template files.

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.

• Salt Best Practices
• Salt States Tutorial
• Jinja Template Designer Documentation