L8 Idempotentence with Ansible play

$ tree .
.
├── create.yml
├── files
│   └── idempotent.txt
├── idempotent.yml
└── state.yml

1 directory, 4 files

1、Idempotent "prototype" model

---
- hosts: k8s_master
  tasks:
  - name: an idempotent create command
    copy:
      src: files/idempotent.txt
      dest: $HOME/tmp/idempotent.txt
    tags:
      - create
  - name: an idempotent command
    lineinfile:
      dest: $HOME/tmp/idempotent.txt
      regexp: '^(.*)is an(.*)$'
      backrefs: true
      line: '\1is really an\2'
    tags:
      - create
  - name: a non idempotent command
    shell: "echo this is a non-idempotent file >> $HOME/tmp/non-idempotent.txt"
    tags:
      - create

  - name: remove the file we created
    file:
      path: $HOME/tmp/{{ item }}
      state: absent
    with_items:
    - non-idempotent.txt
    - idempotent.txt
    tags:
      - destroy

$ ansible-playbook -i ../inventory.ini idempotent.yml --tags create

lineinfile – Manage lines in text files

• This module ensures a particular line is in a file, or replace an existing line using a back-referenced regular expression.
• This is primarily useful when you want to change a single line in a file only.
• regexp: The regular expression to look for in every line of the file.
• backrefs: true: If set, line can contain backreferences (both positional and named) that will get populated if the regexp matches.
• line: The line to insert/replace into the file.

After run twice and check non-idempotent file

$ cat non-idempotent.txt
this is a non-idempotent file
this is a non-idempotent file

After run twice and check idempotent file

$ cat idempotent.txt
this is really an idempotent file

Remove multiple files inside directory

- name: remove the file we created
    file:
      path: $HOME/tmp/{{ item }}
      state: absent
    with_items:
    - non-idempotent.txt
    - idempotent.txt
    tags:
      - destroy

$ ansible-playbook -i ../inventory.ini idempotent.yml --tags destroy
TASK [remove the file we created] **********************************************************************
changed: [k8s-jx] => (item=non-idempotent.txt)
changed: [k8s-jx] => (item=idempotent.txt)

2、Registering discovered state

- hosts: k8s_master
  vars:
    target: $HOME/tmp/idempotent.txt
  tasks:
  - name: an idempotent create command
    copy:
      src: files/idempotent.txt
      dest: '{{target}}'
    tags:
      - create

  - name: modify the file
    command: sed -ie 's/is an/is really an/' {{target}}
    tags:
      - create

  - name: remove the file
    file:
       path: '{{target}}'
       state: absent
    tags:
      - destroy

  - name: remove the backup file
    file:
       path: $HOME/tmp/idempotent.txte
       state: absent
    tags:
      - destroy

  - name: discover state
    command: grep 'is really an' {{target}}
    register: grep_state
    tags:
      - create
      - destroy
    ignore_errors: true

  - name: show the state of the file
    debug:
      var: grep_state.rc
    tags:
      - create
      - destroy

2-1 sed -ie

• -i[SUFFIX](-ie)

Edit files in place (this makes a backup with file extension SUFFIX, if SUFFIX is supplied).

After run modify the file, generate a new backup file idempotent.txt

2-2 register and debug

register: grep_state

debug:
    var: grep_state.rc

When you execute a task and save the return value in a variable for use in later tasks, you create a registered variable.

2-3 Ignoring Failed Commands

Generally playbooks will stop executing any more steps on a host that has a task fail. Sometimes, though, you want to continue on. To do so, write a task that looks like this:

ignore_errors: true

2-4 grep_state return true after create

ansible-playbook -i ../inventory.ini state.yml --tags create

TASK [show the state of the file] ********************************************************************
ok: [k8s-jx] => {
    "grep_state.rc": "0"
}

2-5 grep_state return false after destroy

$ ansible-playbook -i ../inventory.ini state.yml --tags destroy

TASK [show the state of the file] ********************************************************************
ok: [k8s-jx] => {
    "grep_state.rc": "2"
}

3、Creating an idempotent play

---
- hosts: k8s_master
  vars:
    target: $HOME/tmp/idempotent.txt
  tasks:
  - name: discover state
    command: grep 'is really an' {{target}}
    register: grep_state
    tags:
      - create
      - destroy
    ignore_errors: true

  - name: show the state of the file
    debug:
      var: grep_state.rc

  - name: an idempotent create command
    copy:
      src: files/idempotent.txt
      dest: '{{target}}'
    when: grep_state.rc != 0
    tags:
      - create
      - destroy

  - name: modify the file
    command: sed -ie 's/is/is really an/' {{target}}
    when: grep_state.rc != 0
    tags:
      - create
      - destroy

  - name: remove the file
    file:
      path: '{{target}}'
      state: absent
    tags:
      - destroy

  - name: remove the backup file
    file:
       path: $HOME/tmp/idempotent.txte
       state: absent
    tags:
      - destroy

1. Check the state of file
2. Show the state of the file
3. Create and modify file when grep_state is fasle
4. delete both

So after first run, the task an idempotent create command and modify the file will skip