Templates and variables

Separating Code and Data

Chapter 7

CONCEPTS

```
Static Content Explosion
```
```
Code vs Data
```
```
Templates : Jinja2
```
```
Variables and Facts
```
```
Variable Precedence
```

httpd.dev.conf

httpd.stg.conf

httpd.prod.conf

port = 80

port = 8080

port = 9000

Static

Configs

port = {{port}}

dynamic

configs

httpd.conf.j2

port = 80

/etc/httpd/httpd.conf

apache_port = 80

roles/apache/default

apache_port = 8080

group_vars/staging

port = 80

templates

vars

port = {{ port }}
user = {{ user }}
conn = {{ conn }}

code vs data

apache_port = 80
apache_user = httpd
conn = 1024

code

templates

tasks

vars

facts

data

templates

```
Way to generate dynamic configurations
```
```
Ansible uses Jinja2 template engine
```

Along with Variables, can be used to create flexible roles

config => template

Jinja2

```
Python based template language 
```

Compiled just in time into python byte code

```
Fast
```

Commonly used by Python based Automation Tools such as Ansible, Fabric, Salt

http://jinja.pocoo.org/docs/dev/

Jinja2

Contains regular text with dynamic fragments, marked with special tags

Dynamic fragments are either variables or python code, which is executed at the run time, creating the resulting text files.

Files are created using .j2 extensions and stored in templates directory inside roles

https://blog.codecentric.de/en/2014/08/jinja2-better-ansible-playbooks-templates/

{{ var }}

{% code %}

Jinja2 tags

{{ var }}

{% code %}

{{ mysql['config']['port'] }}

mysql.config.port

{% if mysql.config.port is defined %}
do something
{% endif %}

[mysqld]


{% if mysql.config.port is defined %}
port		 = {{ mysql['config']['port'] }}
{% endif %}

{% if mysql.config.pid is defined %}
datadir		 = {{ mysql['config']['datadir'] }}
{% endif %}

Jinja2 example

Variables

```
Provide the properties
```
```
Can be defined from multiple places
```

Get merged at the run time, and injected into templates/tasks

```
Precedence Rules Apply
```

variable types

User

Defined

automatic

Facts

vars

user defined vars

```
Role Default Vars
```
```
Inventory Vars :  hostvars, groupvars
```
```
Playbook Vars
```
```
Role Vars 
```
```
Extra Vars at the runtime (-e switch)
```

accessing vars

port

port1

mysql_port

mysql: 
  port: 3306 
  user: mysql
  server: true

{{ port }}

{{ port1 }}

{{ mysql_port }}

{{ mysql[port] }}

{{ mysql.port  }}

defining vars

variable precedence

-e switch

role vars

playbook vars

host vars

group vars

role defaults