Advanced use of jinja2 for Ansible Templates

Take a practical dive into advanced use of the Jinja2 templating language as implemented in the Ansible core template module.

1. ADVANCED JINJA2 FOR TEMPLATES REVIEW JINJA2 ORIGIN STORY COMPARING TO DIY, JINJA 101, AND A DIVE INTO SOME LESS COMMONLY USED PATTERNS Keith Resar @KeithResar
2. @KeithResar Keith Resar: Bio Wear many hats @KeithResar Keith.Resar@RedHat.com Coder Open Source Contributor and Advocate Infrastructure Architect
3. TEMPLATING THE HARD WAY, FROM AN AMATUER FOCUSSED ON DIY
4. <b>From:</b> Pro-Speller &lt;noreply@pro-speller.com&gt; <br><b>Date:</b> %{{exec("%s" % ((datetime.today() - timedelta(minutes = 47)).strftime("%A, %B %e, %Y at %l:%M %p")))}}% <br><b>To:</b> Andrea Jacobs &lt;Andrea.Jacobs@pro-speller.com&gt; <br><b>Subject:</b> Your Scan Results - %{{url_f}}% <br> THE ILL ADVISED DIY TEMPLATE
5. THE ILL ADVISED DIY TEMPLATE %{{INNER_TPL}}% <tr style="%{{style( ('miss_tr',) )}}%"> <td colspan="2" style="%{{style( ('miss_td',) )}}%"> <div style="%{{style( ('word_div',) )}}%"> <span style="%{{style( ('word_span',) )}}%">%{{INNER.WORD}}%</span> &nbsp;: &nbsp; <span>%{{INNER.SUGGESTIONS}}%</span> </div> <div style="%{{style( ('suggestions_div',) )}}%"> <span>%{{INNER.CONTEXT}}%</span> </div> </td> </td> %{{ENDINNER_TPL}}%
6. PERL BOOTSTRAPPING > perl -p -i -e \ 's/^PermitRootLogin .*/PermitRootLogin no' \ sshd_config
7. JINJA2, AN ORIGIN STORY
8. APPLICATION LOGIC IS FOR THE CONTROLLER, BUT DON'T MAKE THE TEMPLATE DESIGNER'S LIFE DIFFICULT BY RESTRICTING FUNCTIONALITY TOO MUCH. https://github.com/pallets/jinja#philosophy
9. {% extends "layout.html" %} {% block body %} <ul> {% for user in users %} <li><a href="{{ user.url }}">{{ user.username }}</a></li> {% endfor %} </ul> {% endblock %} PURE, BEAUTIFUL JINJA
10. PURE, BEAUTIFUL JINJA Sandboxed Execution Mode Every aspect of the template execution is monitored and explicitly whitelisted or blacklisted, whatever is preferred. This makes it possible to execute untrusted templates. Template Inheritance Makes it possible to use the same or a similar layout for all templates. Easy to Debug With a debug system that integrates template compile and runtime errors into the standard Python traceback system. Configurable Syntax For instance you can reconfigure Jinja2 to better fit output formats such as LaTeX or JavaScript. (skipped the features more relevant to high-volume usage such as in Django)
11. JINJA2 TEMPLATE MODULE 101
12. TEMPLATE FILE NAMING CONVENTIONS Accepts any text file type text, xml, html, etc Convention is to name files with .j2 extension http.conf.j2, sshd_config.j2 Convention to place in the templates/ directory templates/http.conf.j2, templates/sshd_config.j2
13. DEFAULT DELIMITERS {{ ... }} {% ... %} {# ... #}
14. EXPRESSIONS AS PYTHON Accessing Ansible variables like {{ ansible_hostname }} {{ ansible_date_time.epoch }} We can do math like {{ 1+3 }} Standard data types, like my list: {{ ('a','b','c') }} We can call standard methods associated with our types: {{ "lorem ipsum".upper() }} {{ ['a','b','c'].pop() }}
15. EXPRESSIONS JINJA STYLE Inline Conditionals: {{ '[%s]' % page.title if page.title }} Built-in Filters: {{ my_var | default('my_var undefined') }} Maths: abs, int, float, round, sum Str: capitalize, length, center, escape, lower, regex Lists: join, last, sort, shuffle, json_query
16. STATEMENTS Loops: <ul> {% for href, caption in my_list }} <li><a href="{{ href }}">{{ caption }}</a></li> {% endfor %} </ul> Conditionals: {% if kenny.sick %} Kenny is sick. {% elif kenny.dead %} You killed Kenny! You bastard!!! {% else %} Kenny looks okay --- so far {% endif %}
17. COMMENTS {# Note: Nothing in the comment will be included in the template output {% for user in users %} ... {% endfor %} #}
18. WHEN THINGS GO WRONG fatal: [localhost]: FAILED! => {"changed": false, "failed": true, "msg": "AnsibleUndefinedVariable: 'my_var' is undefined"} fatal: [localhost]: FAILED! => {"changed": false, "failed": true, "msg": "AnsibleError: template error while templating string: expected token 'end of print statement', got '...' String: <...>"}
19. MAKING WAVES WITH THE TEMPLATE MODULE, THANKS JINJA2!
20. OVERRIDING BEHAVIOR Include as first line in the template file: #jinja2:variable_start_string:'[%' , variable_end_string:'%]', trim_blocks: False
21. NESTING TEMPLATES {% include "file1.j2" %} {% include ['file1.j2', 'file2.j2'] %}
22. INTERESTING LOOPS Alternation: {% for row in rows %} <li class="{{ loop.cycle('odd', 'even') }}">{{ row }}</li> {% endfor %} Empty Lists: <ul> {% for user in users %} <li>{{ user.username|e }}</li> {% else %} <li><em>no users found</em></li> {% endfor %} </ul>
23. SORTING AND GROUPING FILTERS Sort by Attribute: {% for item in iterable|sort(attribute='date') %} ... {% endfor %} Group by Attribute: {% for group in persons|groupby('gender') %} <li>{{ group.grouper }}<ul> {% for person in group.list %} <li>{{ person.first_name }}</li> {% endfor %}</ul></li> {% endfor %}
24. RESOURCES Ansible Template Module http://docs.ansible.com/.../template_module.html jinja2 Template Designer Documentation http://jinja.pocoo.org/docs/2.9/templates/ ANSIBLE MINNEAPOLIS MEETUP https://www.meetup.com/Ansible-Minneapolis/
25. @KeithResar @KeithResar THANKS!