Looking for a more elegant way to declare complex variables

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1

Sensu is monitoring services like Nagios and the things it should monitor can be configured with JSON check definition files.

A JSON check definition file can have one or more check definitions. Here is a simplified example:

{
  "checks": {
    "cron": {
      "command": "check-process.rb -p cron",      
      "interval": 60
    }
  }
}

Let’s say I have defined the following template for generating check files:

{
  "checks": {
    {% for check_def in item.check_defs %}
        "{{ check_def.name }}": {
    	  "command": "{{ check_def.command }}",
          "interval": {{ check_def.check_interval }}
        }{{ ',' if not loop.last else '' }}
    {% endfor %}
  }
}

I apply this template a task like this:

- copy:
    content:  "{{ lookup('template', 'check_def.json.j2') | to_nice_json }}"
    dest: "{{ somewhere }}"    
  with_items: "{{ check_sets }}"
  notify: Restart Sensu

The check_sets variable is defined similar to:

  vars:
    services:
      - foo
      - bar
      - baz

  tasks:    
      set_fact:
        check_defs: >
          {{
            check_defs|default([]) + [
              {
                'name' : 'my-check-def-for' + item.name,
                'command' : 'check-process.rb',
                'command_args' : '-p ' + item
              }
            ]
          }}
      with_items: "{{ services }}"

I find this approach to defining check_defs to be ugly for several reasons:

1- It is not declarative (e.g. check_defs|default([]) as well as appending items to the list are not declarative).

2- I don’t like the string concat logic using + and using quotes around hardcoded strings

3- For newcomers, it is not easy to understand what is going on

If I use a static list, I have to repeat some variables/values. For example:

check_defs:
	- name: my-check-def-for-foo
	  command: check-process.rb
	  command_args: -p foo

Here foo is repeated twice.

Also users might diverge from the naming convention. For example by providing:

check_defs:
	- name: foo-check
	  command: check-process.rb
	  command_args: -p foo

Is there a more elegant way to declare the check_defs variable that doesn’t have any of these drawbacks?

Thanks in advance.

2

Hi Behrang,

Comments inline...

Sensu is monitoring services like Nagios and the things it should monitor can be configured with JSON check definition files.

A JSON check definition file can have one or more check definitions. Here is a simplified example:

{
  "checks": {
    "cron": {
      "command": "check-process.rb -p cron",
      "interval": 60
    }
  }
}

Let's say I have defined the following template for generating check files:

{
  "checks": {
    {% for check_def in item.check_defs %}
        "{{ check_def.name }}": {
        "command": "{{ check_def.command }}",
          "interval": {{ check_def.check_interval }}
        }{{ ',' if not loop.last else '' }}
    {% endfor %}
  }
}

<snipped a lot>

I am using Ansible to manage checks.json myself but am doing so with the Sensu module from
http://docs.ansible.com/ansible/latest/sensu_check_module.html as below:

name: Updating checks in - /etc/sensu/conf.d/{{sensu_check_filename|default(check_filename)}}
  sensu_check:
    #path: "{{ sensu_check_filename|default(check_filename) }}"
    path: /etc/sensu/conf.d/checks.json
    name: "{{ item.name }}"
    command: "{{ item.command }}"
    interval: "{{ item.interval|default(check_interval) }}"
    metric: "{{ item.metric | default('no') }}"
    handle: "{{ item.handle | default('yes') }}"
    occurrences: "{{ item.occurrences | default(check_occurrences) }}"
    timeout: "{{ item.timeout | default(check_timeout) }}"
    dependencies: "{{ item.dependencies | default(omit) }}"
    subscribers: "{{ item.subscribers|default(default_subscribers) }}"
    handlers: "{{ item.handlers | default(omit) }}"
    refresh: "{{ item.refresh | default(check_refresh) }}"
    state: "{{ item.state | default('present') }}"
    standalone: "{{ item.standalone | default('no') }}"
    ttl: "{{ item.ttl | default(900) }}"
    aggregate: "{{ item.aggregate | default('no') }}"
    subdue_begin: "{{ item.subdue_begin | default(omit) }}"
    subdue_end: "{{ item.subdue_end | default(omit) }}"
    source: "{{ item.source | default(omit) }}"
    low_flap_threshold: "{{ item.low_flap_threshold | default(omit) }}"
    high_flap_threshold: "{{ item.high_flap_threshold | default(omit) }}"
  notify:
    - restart_sensu_server
    - restart_sensu_client
    - restart_sensu_api
  with_items: "{{ _sensu_check_list|sort }}"

Is there any particular reason why you chose to use a template?

Regards.
@shankerbalan

3

Just a note, this task does x2 transformations it does not need to, if
you use template directly you avoid many problems.

so instead of:
- copy:
    content: "{{ lookup('template', 'check_def.json.j2') | to_nice_json }}"
    dest: "{{ somewhere }}"
  with_items: "{{ check_sets }}"
  notify: Restart Sensu

use:
- template: src=check_def.json.j2 dest={{somewhere}}
  with_items: "{{ check_sets }}"
  notify: Restart Sensu

4

I want to pretty print the generated JSON file. Otherwise I would have used the template module.

5

Oops. I didn’t know this module existed. I am maintaining a set of scripts that was written before 2.0 and this module were released.