Jinja2 Templating in Ansible

Lately, I have been playing around with Jinja2 Templating in Ansible. Let me explain the context of that.

With the Morpheus CMP solution, it has an Automation Workflow engine that can be used to run Tasks, or whole sets of Workflows, in a variety of different technologies (scripting languages, Ansible, Chef, Puppet, et al).

To access the variables about your Virtual Machine, say after you launch it, you put tags into your script to reference variables. The tags can have subtle differences in syntax, depending on whether it is a bash script, a Python script, or an Ansible playbook.

This post, is related to Ansible specifically.

If you are needing an explicit specific value, the tag in an Ansible playbook would look as follows:
    - name: "set fact hostname"
      set_fact:
        dnsrecord: |
          {{ morpheus["instance"]["hostname"] | trim }}

Really strange, and confusing, syntax. Not to mention, this pipe to a supposed function called trim.

What language is this? I thought it was groovy, or some kind of groovy scripting language - at first. Then, I thought it was a form of Javascript. Finally, after some web research, I have come to learn that this markup is Ansible's Jinja2 scripting language.

First, I had to understand how Morpheus worked. I realized that I could use a Jinja2 tag to dump the entire object (in JSON) about a launched virtual machine (tons of data actually). Once I understood how Morpheus worked, and the JSON it generates, I was able to go to work snagging values that I needed in my scripts.

But - eventually, my needs (use cases) became more complex. I needed to loop through all of the interfaces of a virtual machine! How do you do THAT??

Well, I discovered that to do more sophisticated logic structures (i.e. like loops), the markup and tagging is different, and the distinctions are important. You can wind up pulling your hair out if you don't understand them.

Let's take an example where we loop through a VM's interfaces with Jinja2.

In this example, we loop through all interfaces of a virtual machine. But - we use an if statement to only grab the first interface's ip address. 

Note: To be optimized, we should break after we get that first ip address, but breaking out of loops is not straightforward in Jinja2, and there are only a handful of interfaces, so we will let the loop continue on, albeit wastefully.

    - name: set fact morpheusips
      set_fact:
         morpheusips: |
           {% for interface in morpheus['instance']['container']['server']['interfaces'] %}
             {% if loop.first %}
                {{ interface['ipAddress'] }}
             {% endif %}
           {% endfor %}

Note that an explicit specific value - has NO percent signs in the tag!

But, the "logic", like for loops, if statements, et al, those DO use percent signs in the tag!

This is extremely important to understand!

Now, the variable we get - morpheusips - is a string, which contains leading and trailing spaces, and has newlines - including an annoying newline at the end of the string which wreaked havoc when I needed to convert this string to an array (using the split function).  

I found myself having to write MORE code, to clean this up, and found more useful Jinja2 functions for doing this kind of string manipulation and conversion (to an array).

    - name: "Replace newlines and tabs with commas so we can split easier"
      set_fact:
         commasep: "{{ morpheusips | regex_replace('[\\r\\n\\t]+',',') | trim }}"

    - name: "Remove comma at the end of the string"
      set_fact:
         notrailcomma: "{{ commasep | regex_replace(',$','') | trim }}"

    - name: "convert the ip delimeter string to a list so we can iterate it"
      set_fact:
         morpheusiplst: "{{ notrailcomma.split(',') }}"

    - name: "Loop and Print variable out for morpheusiplst"
      ansible.builtin.debug:
         var: morpheusiplst

I am NOT a guru, or a SME, on Jinja2 Templating. But, this is a blog to share what I have been poking around with as I get used to it to solve some problems.