DevOps Classroom Series – 20/Aug/2020

What can be done if the module is not available?

• Scenario: You want to automate certain deployment and couldn’t find the module for the command

• If the module is not found, then ansible gives provision to directly run the linux/windows commands using command modules. But these modules are not idempotent. Ensuring idempotence is your responsibility

• In Ansible we have following command modules Refer Here

  1. command: Execute command on targets
  2. expect: Executes a command & respond to prompts
  3. psexec: Runs commands on a remote Windows host based on PSExec model
  4. raw: Executes a low level command on machines where python is not installed.
  5. shell: Executes the shell command on nodes
  6. script: Executes a local script on remote node after transferring it.
  7. telnet: Executes a low level telnet command

• Lets write a very simple playbook with command modules

---
- name: execute commands
  hosts: all
  tasks:
    - name: execute simple shell command
      shell: 
        cmd: touch test.txt
        chdir: /home/ansible

• Now execute this twice and we can observe there is no idempotance by ansible since we used command modules
• One approach for making this idempotent is check if file exists and if the file doesnot exist then execute the task
• Now lets change the playbook to

---
- name: execute commands
  hosts: all
  tasks:
    - name: check for file existence
      stat:
        path: /home/ansible/test.txt
      register: stat_result
    - name: execute simple shell command
      shell: 
        cmd: touch test.txt
        chdir: /home/ansible
      when: not stat_result.stat.exists
      
        

• Now execute the ansible playbook again
• How to make the commands which donot create a file idempotent

---
- name: execute commands
  hosts: all
  tasks:
    - name: check for file existence
      stat:
        path: /home/ansible/.pingexecution
      register: stat_result
    - name: execute simple shell command
      shell: 
        cmd: ping -c 4 google.com
      when: not stat_result.stat.exists
      notify:
        - store ping execution
  handlers:
    - name: store ping execution
      file:
        path: /home/ansible/.pingexecution
        state: touch

• During the execution of playbook if i need to set the variable value for usage at the host level we have a module called as set_fact

Reusing others work

• Scenario: I have been asked to install mysql on centos and ubuntu nodes.
• Solution: Write your own ansible playbook or since we are not the first people to try mysql execution from ansible we can reuse others work
• Ansible makes it possible to share work(playbooks/roles/collection) done by users to community using Ansible-Galaxy
• Now lets search mysql in ansible galaxy
• We should understand how to reuse work shared by others i.e. understanding roles and collections.